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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	Apr. 30, 2018
Entity Registrant Name	TWILIO INC
Entity Central Index Key	0001447669
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Common Class A
Entity Common Stock, Shares Outstanding		71,849,359
Common Class B
Entity Common Stock, Shares Outstanding		23,926,240

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 117,737	$ 115,286
Short-term marketable securities	190,271	175,587
Accounts receivable, net	57,310	43,113
Prepaid expenses and other current assets	16,791	19,279
Total current assets	382,109	353,265
Restricted cash	5,502	5,502
Property and equipment, net	53,694	50,541
Intangible assets, net	18,981	20,064
Goodwill	18,269	17,851
Other long-term assets	4,240	2,559
Total assets	482,795	449,782
Current liabilities:
Accounts payable	17,924	11,116
Accrued expenses and other current liabilities	76,595	53,614
Customer deposits	7,442
Deferred revenue	7,343	13,797
Total current liabilities	109,304	78,527
Long-term liabilities	10,951	11,409
Total liabilities	120,255	89,936
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock
Class A and Class B common stock	95	94
Additional paid-in capital	633,460	608,165
Accumulated other comprehensive income	2,439	2,025
Accumulated deficit	(273,454)	(250,438)
Total stockholders' equity	362,540	359,846
Total liabilities and stockholders' equity	$ 482,795	$ 449,782

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Consolidated Statements of Operations
Revenue	$ 129,116	$ 87,372
Cost of revenue	59,582	37,286
Gross profit	69,534	50,086
Operating expenses:
Research and development	37,576	26,522
Sales and marketing	32,822	21,116
General and administrative	23,393	17,203
Total operating expenses	93,791	64,841
Loss from operations	(24,257)	(14,755)
Other income (expenses), net	665	498
Loss before (provision) benefit for income taxes	(23,592)	(14,257)
(Provision) benefit for income taxes	(137)	30
Net loss attributable to common stockholders	$ (23,729)	$ (14,227)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.25)	$ (0.16)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	94,673,557	88,612,804

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Consolidated Statements of Comprehensive Loss
Net loss	$ (23,729)	$ (14,227)
Other comprehensive income (loss):
Unrealized loss on marketable securities	(317)	(88)
Foreign currency translation	731	(102)
Total other comprehensive income (loss)	414	(190)
Comprehensive loss attributable to common stockholders	$ (23,315)	$ (14,417)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (23,729)	$ (14,227)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization	5,631	3,955
Amortization of bond premium	28	63
Stock-based compensation	17,540	9,385
Provision for doubtful accounts	375	72
Write-off of internal-use software	182	26
Gain on lease termination		(295)
Changes in operating assets and liabilities:
Accounts receivable	(14,612)	(1,950)
Prepaid expenses and other current assets	2,512	(972)
Other long-term assets	(1,169)	(178)
Accounts payable	6,703	149
Accrued expenses and other current liabilities	22,789	5,221
Customer deposits	7,441
Deferred revenue	(6,256)	885
Long-term liabilities	(499)	306
Net cash provided by operating activities	16,936	2,440
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(42,693)	(170,769)
Maturities of marketable securities	27,600
Capitalized software development costs	(4,795)	(3,649)
Purchases of property and equipment	(940)	(4,971)
Purchases of intangible assets	(112)	(8)
Acquisition, net of cash acquired		(22,621)
Net cash used in investing activities	(20,940)	(202,018)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments of costs related to public offerings		(238)
Proceeds from exercises of stock options	6,678	12,735
Value of equity awards withheld for tax liabilities	(371)	(155)
Net cash provided by financing activities	6,307	12,342
Effect of exchange rate changes on cash, cash equivalents and restricted cash	148	11
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	2,451	(187,225)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH-Beginning of period	120,788	314,280
CASH, CASH EQUIVALENTS AND RESTRICTED CASH -End of period	123,239	127,055
Cash paid for income taxes	18	42
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Purchases of property, equipment and intangible assets, accrued but not paid	473	1,184
Stock-based compensation capitalized in software development costs	$ 1,428	709
Costs related to public offerings, accrued but not paid		$ 192

Organization and Description of Business	3 Months Ended
Organization and Description of Business	Mar. 31, 2018
Organization and Description of Business
Organization and Description of Business	1. Organization and Description of Business Twilio Inc. (the “Company”) was incorporated in the state of Delaware on March 13, 2008. The Company is the leader in the Cloud Communications Platform category and enables developers to build, scale and operate real-time communications within their software applications via simple-to-use Application Programming Interfaces (“API”). The power, flexibility, and reliability offered by the Company’s software building blocks empower entities of virtually every shape and size to build world-class engagement into their customer experience. The Company’s headquarters are located in San Francisco, California and the Company has subsidiaries in the United Kingdom, Estonia, Ireland, Colombia, Germany, Hong Kong, Singapore, Bermuda, Spain, Sweden and Australia.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

(a)Basis of Presentation

The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. Certain information and note disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. Therefore, these condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes included in our Annual Report on Form 10-K filed with the SEC on March 1, 2018 (“Annual Report”).

The condensed consolidated balance sheet as of December 31, 2017, included herein, was derived from the audited financial statements as of that date, but may not include all disclosures including certain notes required by U.S. GAAP on an annual reporting basis.

In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all normal recurring adjustments necessary to present fairly the financial position, results of operations, comprehensive loss and cash flows for the interim periods, but are not necessarily indicative of the results of operations to be anticipated for the full year 2018 or any future period.

(b)Principles of Consolidation

The condensed consolidated financial statements include the Company and its wholly owned subsidiaries. All significant intercompany balances and transactions have been eliminated.

(c)Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. These estimates are used for, but not limited to, revenue allowances and returns; recoverability of long-lived and intangible assets; capitalization and useful life of the Company’s capitalized internal-use software development costs; fair value of acquired intangible assets and goodwill; accruals and contingencies. Estimates are based on historical experience and on various assumptions that the Company believes are reasonable under current circumstances. However, future events are subject to change and best estimates and judgments may require further adjustments; therefore, actual results could differ materially from those estimates. Management periodically evaluates such estimates and they are adjusted prospectively based upon such periodic evaluation.

(d)Concentration of Credit Risk

Financial instruments that potentially expose the Company to a concentration of credit risk consist primarily of cash, cash equivalents, marketable securities, restricted cash and accounts receivable. The Company maintains cash, cash equivalents, marketable securities and restricted cash with financial institutions that management believes are financially sound and have minimal credit risk exposure although the balances will exceed insured limits.

The Company sells its services to a wide variety of customers. If the financial condition or results of operations of any significant customers deteriorate substantially, operating results could be adversely affected. To reduce credit risk, management performs ongoing credit evaluations of the financial condition of significant customers. The Company does not require collateral from its credit customers and maintains reserves for estimated credit losses on customer accounts when considered necessary. Actual credit losses may differ from the Company’s estimates. During the three months ended March 31, 2018, there was no customer organization that accounted for more than 10% of the Company’s total revenue. During the three months ended March 31, 2017, one customer organization represented approximately 12% of the Company’s total revenue.

As of March 31, 2018, one customer organization represented 16% of the Company’s gross accounts receivable, and as of December 31, 2017, no customer organization represented more than 10% of the Company’s gross accounts receivable.

(e)Significant Accounting Policies

Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) 606, “Revenue from Contracts with Customers”, which replaced the existing revenue recognition guidance, ASC 605, and outlines a single set of comprehensive principles for recognizing revenue under U.S. GAAP. Among other things, ASC 606 requires entities to assess the products or services promised in contracts with customers at contract inception to determine the appropriate unit at which to record revenue, which is referred to as a performance obligation. Revenue is recognized when control of the promised products or services is transferred to customers at an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those products or services.

The Company adopted ASC 606 using the modified retrospective method with cumulative catch-up adjustment to the opening retained earnings as of January 1, 2018. Results for reporting periods beginning after December 31, 2017 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with the Company’s historical accounting policies prior to adoption. In adopting the standard, the Company elected to apply the new guidance only to those contracts which were not completed as of the date of the adoption.

The impact of adopting the new standard on the Company’s consolidated financial statements was insignificant. The Company recorded a net cumulative catch-up adjustment to the beginning retained earnings as of January 1, 2018, of $0.7 million.

The primary impact relates to the deferral of incremental commission costs of obtaining new contracts. Under ASC 605, the Company deferred only direct and incremental commission costs to obtain a contract and amortized those costs on a straight-line basis over the term of the related subscription contract. Under the new standard, the Company defers all incremental commission costs to obtain the contract and amortizes these costs on a straight-line basis over the expected term of benefit of the underlying asset, which was determined to be five years.

The impact on the Company’s revenue recognition policies was insignificant. Prior to the adoption of ASC 606, the Company recognized the majority of its revenue according to the usage by its customers in the period in which that usage occurred. ASC 606 continues to support the recognition of revenue over time, and on a usage basis, for the majority of the Company’s contracts due to continuous transfer of control to the customer. The impact on the Company’s balance sheet presentation includes presenting customer refundable prepayments as customer deposit liabilities, whereas under ASC 605 these were included in deferred revenues.

There was not a significant tax impact to the Company’s consolidated statements of operations and consolidated balance sheet relating to the adoption of the new standard as there is a full valuation allowance due to the Company’s history of continued losses.

Revenue Recognition

Revenue is recognized upon transfer of control of promised products or services to customers in an amount that reflects the consideration the Company expects to receive in exchange for those products or services. The Company enters into contracts that can include various combinations of products and services, which are generally capable of being distinct and accounted for as separate performance obligations. Revenue is recognized net of allowances for credits and any taxes collected from customers, which are subsequently remitted to governmental authorities.

The Company determines revenue recognition through the following steps:

Identification of the contract, or contracts, with a customer

Identification of the performance obligations in the contract

Determination of the transaction price

Allocation of the transaction price to the performance obligations in the contract

Recognition of revenue when, or as, the Company satisfies a performance obligation

Nature of Products and Services

The Company’s revenue is primarily derived from usage-based fees earned from customers accessing the Company’s enterprise cloud computing services. Platform access is considered a monthly series comprising one performance obligation and usage-based fees are recognized as revenue in the period in which the usage occurs. In the three months ended March 31, 2018 and 2017, the revenue from usage-based fees represented 84% and 83% of total revenue, respectively.

Subscription-based fees are derived from certain term-based contracts, such as with the sales of short codes and support. Term-based contracts revenue is recognized on a ratable basis over the contractual term of the arrangement beginning on the date that the service is made available to the customer. In the three months ended March 31, 2018 and 2017, the revenue from term-based fees represented 16% and 17% of total revenue, respectively.

No significant judgments are required in determining whether products and services are considered distinct performance obligations and should be accounted for separately versus together, or to determine the stand-alone selling price (“SSP”).

Refer to Note 9, Revenue by Geographic Area, for additional disaggregation.

The Company’s arrangements do not contain general rights of return. However, credits may be issued on a case-by-case basis. The contracts do not provide customers with the right to take possession of the software supporting the applications. Amounts that have been invoiced are recorded in accounts receivable and in revenue or deferred revenue depending on whether the revenue recognition criteria have been met.

The reserve for sales credits is included in accounts receivable and is calculated based on historical trends and any specific risks identified in processing transactions. Changes in the reserve are recorded against revenue.

Deferred Revenue and Customer Deposits

Deferred revenue is recorded when cash payments are received in advance of future usage on non-cancellable contracts. Customer refundable prepayments are recorded as customer deposits. During the three months ended March 31, 2018, the Company recognized $3.6 million of revenue that was included in the deferred revenue balance, as adjusted for ASC 606 at the beginning of the period.

Deferred Sales Commissions

The Company records an asset for the incremental costs of obtaining a contract with a customer, for example, sales commissions that are earned upon execution of contracts. The Company uses the portfolio method to recognize the amortization expense related to these capitalized costs related to initial contracts, upsells and renewals and such expense is recognized over the estimated period of benefit of the capitalized commissions, which is determined to be five years. Total net capitalized costs as of March 31, 2018 were $4.1 million and are included in prepaid expenses and other current assets and other long-term assets in the accompanying condensed consolidated balance sheet. Amortization of these assets was $0.2 million during the three months ended March 31, 2018 and is included in sales and marketing expense in the accompanying condensed consolidated statement of operations.

Other than adoption of ASC 606, there were no changes to our significant accounting policies as described in our Annual Report.

(f)Restricted Cash

In November 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-18, “Statement of Cash Flows (Topic 230) — Restricted Cash”. This standard provides guidance on the presentation of restricted cash and restricted cash equivalents in the statement of cash flows. Restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period amounts shown on the statements of cash flows. The amendments of this ASU should be applied using a retrospective transition method and are effective for reporting periods beginning after December 15, 2017. The Company adopted ASU 2016-18 in the first quarter of 2018 and applied the guidance retrospectively to the prior period’s condensed consolidated statement of cash flows with the following impact (in thousands):

	Three Months Ended March 31, 2017
	As Originally Reported		As Adjusted
Cash, cash equivalents and restricted cash — beginning of period	$	305,665	$	314,280
Cash, cash equivalents and restricted cash — end of period	$	118,440	$	127,055

Other than the revised statement of cash flows presentation of restricted cash, the adoption of ASU 2016-18 did not have an impact on the Company’s financial position and results of operations.

(g)Recently Issued Accounting Guidance, Not yet Adopted

In January 2017, the FASB issued ASU 2017-04, “Simplifying the Test for Goodwill Impairment”, which removes the second step of the goodwill impairment test that requires a hypothetical purchase price allocation. A goodwill impairment will now be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. This guidance is effective prospectively for interim and annual reporting periods beginning after December 15, 2019. The Company will adopt this guidance upon its effective date. The Company does not expect the adoption of this guidance to have a material impact on the Company’s financial position, results of operations or cash flows.

In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments”, which changes the impairment model for most financial assets. The new model uses a forward-looking expected loss method, which will generally result in earlier recognition of allowances for losses. ASU 2016-13 is effective for annual and interim periods beginning after December 15, 2019 and early adoption is permitted for annual and interim periods beginning after December 15, 2018. The Company is evaluating the impact of this guidance on its consolidated financial statements and related disclosures.

In February 2016, the FASB issued ASU No. 2016-02, “Leases.” The standard will affect all entities that lease assets and will require lessees to recognize a lease liability and a right-of-use asset for all leases (except for short-term leases that have a duration of less than one year) as of the date on which the lessor makes the underlying asset available to the lessee. For lessors, accounting for leases is substantially the same as in prior periods. For public companies, the new standard is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. For leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, lessees and lessors must apply a modified retrospective transition approach. While the Company expects the adoption of this standard to result in an increase to the reported assets and liabilities, the Company has not yet determined the full impact that the adoption of this standard will have on its consolidated financial statements and related disclosures.

Fair Value Measurements

3 Months Ended

Mar. 31, 2018

Fair Value Measurements

Fair Value Measurements

3. Fair Value Measurements

The following tables provide the financial assets measured at fair value on a recurring basis as of March 31, 2018 and December 31, 2017 (in thousands):

	Amortized Cost or Carrying		Gross Unrealized		Gross Unrealized Losses Less Than			Gross Unrealized Losses More Than			Fair Value Hierarchy as of March 31, 2018						Aggregate
	Value		Gains		12 Months			12 Months			Level 1		Level 2		Level 3		Fair Value
Financial Assets:
Cash and cash equivalents:
Money market funds	$	93,103	$	—	$	—		$	—		$	93,103	$	—	$	—	$	93,103

Total included in cash and cash equivalents	93,103		—		—			—			93,103		—		—		93,103

Marketable securities:
U.S. Treasury securities	59,973		—		(127		)	(73		)	59,773		—		—		59,773
Corporate debt securities	131,213		—		(715		)	—			—		130,498		—		130,498

Total marketable securities	191,186		—		(842		)	(73		)	59,773		130,498		—		190,271

Total financial assets	$	284,289	$	—	$	(842	)	$	(73	)	$	152,876	$	130,498	—		$	283,374

	Amortized Cost or Carrying		Gross Unrealized		Gross Unrealized Losses Less Than			Gross Unrealized Losses More Than		Fair Value Hierarchy as of December 31, 2017						Aggregate
	Value		Gains		12 Months			12 Months		Level 1		Level 2		Level 3		Fair Value
Financial Assets:
Cash and cash equivalents:
Money market funds	$	95,432	$	—	$	—		$	—	$	95,432	$	—	$	—	$	95,432

Total included in cash and cash equivalents	95,432		—		—			—		95,432		—		—		95,432

Marketable securities:
U.S. Treasury securities	59,962		—		(216		)	—		59,746		—		—		59,746
Corporate debt securities	116,223		—		(382		)	—		—		115,841		—		115,841

Total marketable securities	176,185		—		(598		)	—		59,746		115,841		—		175,587

Total financial assets	$	271,617	$	—	$	(598	)	$	—	$	155,178	$	115,841	$	—	$	271,019

As the Company views these securities as available to support current operations, it has classified all available for sale securities as short-term. As of March 31, 2018, for fixed income securities that were in unrealized loss positions, the Company has determined that (i) it does not have the intent to sell any of these investments, and (ii) it is not more likely than not that it will be required to sell any of these investments before recovery of the entire amortized cost basis. In addition, as of March 31, 2018, the Company anticipates that it will recover the entire amortized cost basis of such fixed income securities and has determined that no other-than-temporary impairments associated with credit losses were required to be recognized during the three month ended March 31, 2018. Interest earned on marketable securities in the three months ended March 31, 2018 and 2017, was $0.7 million and $0.5 million, respectively, and is recorded as other income (expense), net, in the accompanying condensed consolidated statements of operations.

The following table summarizes the contractual maturities of marketable securities as of March 31, 2018 and December 31, 2017 (in thousands):

	March 31, 2018				December 31, 2017
	Amortized Cost		Aggregate Fair Value		Amortized Cost		Aggregate Fair Value
Financial Assets:
Less than one year	$	147,714	$	147,156	$	108,584	$	108,360
One to two years	43,472		43,115		67,601		67,227

Total	$	191,186	$	190,271	$	176,185	$	175,587

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment

3 Months Ended

Mar. 31, 2018

Property and Equipment

Property and Equipment

4. Property and Equipment

Property and equipment consisted of the following (in thousands):

	As of March 31,			As of December 31,
	2018			2017
Capitalized internal-use software development costs	$	55,328		$	49,177
Leasehold improvements	14,422			14,246
Office equipment	10,410			9,652
Furniture and fixtures	2,108			1,976
Software	1,765			1,675

Total property and equipment	84,033			76,726
Less: accumulated depreciation and amortization	(30,339		)	(26,185		)

Total property and equipment, net	$	53,694		$	50,541

Depreciation and amortization expense was $4.2 million and $2.8 million for the three months ended March 31, 2018 and 2017, respectively.

The Company capitalized $6.4 million and $4.3 million in internal-use software development costs in the three months ended March 31, 2018 and 2017, respectively, of which $1.4 million and $0.7 million, respectively, was stock-based compensation expense. Amortization of capitalized software development costs was $2.7 million and $1.8 million in the three months ended March 31, 2018 and 2017, respectively.

Business Combinations

3 Months Ended

Mar. 31, 2018

Business Combinations

Business Combinations

5. Business Combinations

In February 2017, the Company completed its acquisition of Beepsend AB, a messaging provider based in Sweden, specializing in messaging and SMS solutions, for a total purchase price of $23.0 million, paid in cash, of which $5.0 million was held in escrow with a term of 18 months.

Additionally, the Company deposited $2.0 million into a separate escrow that was conditioned upon future service conditions and was recorded ratably into the compensation expense as the services were rendered. As of March 31, 2018, the remaining balance in the escrow was $0.3 million.

The acquisition was accounted for as a business combination and the total purchase price was allocated to the net tangible and intangible assets and liabilities based on their fair values on the acquisition date and the excess was recorded as goodwill. The acquired entity’s results of operations have been included in the condensed consolidated financial statements of the Company from the date of acquisition.

The following table presents the purchase price allocation, as adjusted, recorded in the Company’s condensed consolidated balance sheet as of March 31, 2017 (in thousands):

	Total
Net tangible liabilities	$	(3,575	)
Goodwill(1)	12,837
Intangible assets(2)	13,700

Total purchase price	$	22,962

(1)

The goodwill is primarily attributable to the future cash flows to be realized from the acquired technology platform, existing customer and supplier relationships as well as operational synergies. Goodwill is deductible for tax purposes.

(2)

Identifiable finite-lived intangible assets were comprised of the following (in thousands):

	Total		Estimated life (in years)
Developed technology	$	5,000	4
Customer relationships	6,100		7 - 8
Supplier relationships	2,600		5

Total intangible assets acquired	$	13,700

The Company acquired a net deferred tax liability of $2.6 million in this business combination that is included in long-term liabilities in the accompanying condensed consolidated balance sheets.

The estimated fair value of the intangible assets acquired was determined by the Company, and the Company considered or relied in part upon a valuation report of a third-party expert. The Company used income approaches to estimate the fair values of the identifiable intangible assets.

The Company incurred costs related to this acquisition of $0.7 million, of which $0.3 million and $0.4 million were incurred during fiscal years 2017 and 2016, respectively. All acquisition-related costs were expensed as incurred and have been recorded in general and administrative expenses in the accompanying condensed consolidated statements of operations.

Pro forma results of operations for this acquisition are not presented as the financial impact to the Company’s condensed consolidated financial statements is immaterial.

Goodwill and Intangible Assets

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets

Goodwill and Intangible Assets

6. Goodwill and Intangible Assets

Goodwill

Goodwill balance as of March 31, 2018 and December 31, 2017 was as follows (in thousands):

	Total
Balance as of December 31, 2017	$	17,851
Effect of exchange rate	418

Balance as of March 31, 2018	$	18,269

Intangible assets

Intangible assets consisted of the following (in thousands):

	As of March 31, 2018
	Gross		Accumulated Amortization			Net
Amortizable intangible assets:
Developed technology	$	15,040	$	(6,529	)	$	8,511
Customer relationships	7,280		(1,229		)	6,051
Supplier relationships	2,932		(631		)	2,301
Trade name	60		(60		)	—
Patent	1,947		(124		)	1,823

Total amortizable intangible assets	27,259		(8,573		)	18,686

Non-amortizable intangible assets:
Domain names	32		—			32
Trademarks	263		—			263

Total	$	27,554	$	(8,573	)	$	18,981

	As of December 31, 2017
	Gross		Accumulated Amortization			Net
Amortizable intangible assets:
Developed technology	$	14,941	$	(5,476	)	$	9,465
Customer relationships	7,159		(1,006		)	6,153
Supplier relationships	2,881		(500		)	2,381
Trade name	60		(60		)	—
Patent	1,878		(108		)	1,770

Total amortizable intangible assets	26,919		(7,150		)	19,769

Non-amortizable intangible assets:
Domain names	32		—			32
Trademarks	263		—			263

Total	$	27,214	$	(7,150	)	$	20,064

Amortization expense was $1.4 million and $1.2 million for the three months ended March 31, 2018 and December 31, 2017, respectively.

Total estimated future amortization expense was as follows (in thousands):

	As of March 31, 2018
2018 (remaining nine months)	$	5,646
2019	5,088
2020	2,658
2021	1,525
2022	929
Thereafter	2,840

Total	$	18,686

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

Accrued Expenses and Other Liabilities

3 Months Ended

Mar. 31, 2018

Accrued Expenses and Other Liabilities

Accrued Expenses and Other Liabilities

7. Accrued Expenses and Other Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	As of March 31,		As of December 31,
	2018		2017
Accrued payroll and related	$	10,042	$	4,898
Accrued bonus and commission	5,721		4,777
Accrued cost of revenue	18,690		10,876
Sales and other taxes payable	21,227		20,877
ESPP contributions	3,345		1,338
Deferred rent	1,179		1,048
Accrued other expense	16,391		9,800

Total accrued expenses and other current liabilities	$	76,595	$	53,614

Long-term liabilities consisted of the following (in thousands):

	As of March 31,		As of December 31,
	2018		2017
Deferred rent	$	8,217	$	8,480
Deferred tax liability, net	2,262		2,452
Accrued other expense	472		477

Total long-term liabilities	$	10,951	$	11,409

Supplemental Balance Sheet Information

3 Months Ended

Mar. 31, 2018

Supplemental Balance Sheet Information

Supplemental Balance Sheet Information

8. Supplemental Balance Sheet Information

A roll-forward of the Company’s reserves for the three months ended March 31, 2018 and 2017 is as follows (in thousands):

(a)Allowance for doubtful accounts:

	Three Months Ended March 31,
	2018			2017
Balance, beginning of period	$	1,033		$	1,076
Additions	375			72
Write-offs	(4		)	(378		)

Balance, end of period	$	1,404		$	770

b)Sales credit reserve:

	Three Months Ended March 31,
	2018			2017
Balance, beginning of period	$	1,761		$	544
Additions	1,107			551
Deductions against reserve	(1,166		)	(277		)

Balance, end of period	$	1,702		$	818

Revenue by Geographic Area

3 Months Ended

Mar. 31, 2018

Revenue by Geographic Area

Revenue by Geographic Area

9. Revenue by Geographic Area

Revenue by geographic area is based on the IP address at the time of registration. The following table sets forth revenue by geographic area (dollars in thousands):

	Three Months Ended March 31,
	2018			2017
Revenue by geographic area:
United States	$	98,635		$	70,098
International	30,481			17,274

Total	$	129,116		$	87,372

Percentage of revenue by geographic area:
United States	76		%	80		%
International	24		%	20		%

Long-lived assets outside the United States were not significant.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies

Commitments and Contingencies

10. Commitments and Contingencies

(a)Lease Commitments

The Company entered into various non-cancelable operating lease agreements for its facilities that expire over the next six years. Certain operating leases contain provisions under which monthly rent escalates over time. When lease agreements contain escalating rent clauses or free rent periods, the Company recognizes rent expense on a straight-line basis over the term of the lease.

Rent expense was $2.1 million and $1.9 million for the three months ended March 31, 2018 and 2017, respectively.

Future minimum lease payments under non-cancelable operating leases were as follows (in thousands):

Year Ending December 31:	As of March 31, 2018
2018 (remaining nine months)	$	6,607
2019	7,952
2020	7,112
2021	7,033
2022	5,864
Thereafter	10,189

Total minimum lease payments	$	44,757

Additionally, in the three months ended March 31, 2018, the Company entered into a 22 month non-cancellable agreement with a cloud services vendor for a total commitment of $4.5 million. Except for this new commitment, there have been no material changes to the Company’s principle commitments described in the Company’s most recent Annual Report.

(b)Legal Matters

On April 30, 2015, Telesign Corporation, or Telesign, filed a lawsuit against the Company in the United States District Court, Central District of California (“Telesign I”). Telesign alleges that the Company is infringing three U.S. patents that it holds: U.S. Patent No. 8,462,920 (“920”), U.S. Patent No. 8,687,038 (“038”) and U.S. Patent No. 7,945,034 (“034”). The patent infringement allegations in the lawsuit relate to the Company’s Account Security products, its two-factor authentication use case and an API tool to find information about a phone number. The Company petitioned the U.S. Patent and Trademark Office (“U.S. PTO”) for inter partes review of the patents at issue. On July 8, 2016, the PTO denied the Company’s petition for inter partes review of the ‘920 and ‘038 patents, and on June 26, 2017, it upheld the patentability of the ‘034 patent.

On March 28, 2016, Telesign filed a second lawsuit against the Company in the United States District Court, Central District of California (“Telesign II”), alleging infringement of U.S. Patent No. 9,300,792 (‘792”) held by Telesign. The ‘792 patent is in the same patent family as the ‘920 and ‘038 patents asserted in Telesign I. On March 8, 2017, in response to a petition by the Company, the PTO issued an order instituting the inter partes review for the ‘792 patent. On March 6, 2018, the PTO found all claims challenged by Twilio in the inter partes review unpatentable. On March 15, 2017, Twilio filed a motion to consolidate and stay related cases pending the conclusion of the ‘792 patent inter partes review, which the court granted. The Central District of California court lifted the stay on April 13, 2018. With respect to each of the patents asserted in the now-consolidated Telesign I and Telesign II cases, the complaints seek, among other things, to enjoin the Company from allegedly infringing the patents, along with damages for lost profits. The Telesign I/II litigation is currently ongoing.

On December 1, 2016, the Company filed a patent infringement lawsuit against Telesign in the United States District Court, Northern District of California, alleging indirect infringement of United States Patent No. 8,306,021 (“021”), United States Patent No. 8,837,465 (“465”), United States Patent No. 8,755,376 (“376”), United States Patent No. 8,736,051 (“051”), United States Patent No. 8,737,962 (“962”), United States Patent No. 9,270,833 (“833”), and United States Patent No. 9,226,217 (“217”). Telesign filed a motion to dismiss the complaint on January 25, 2017. In two orders, issued on March 31, 2017 and April 17, 2017, the Court granted Telesign’s motion to dismiss with respect to the ‘962, ‘833, ‘051 and ‘217 patents, but denied Telesign’s motion to dismiss as to the ‘021, ‘465 and ‘376 patents. On August 23, 2017, Telesign petitioned the U.S. Patent and Trademark Office (“U.S. PTO”) for inter partes review of the ‘021, ‘465, and ‘376 patents. On March 9, 2018, the PTO denied Telesign’s petition for inter partes review of the ‘021 patent and granted Telesign’s petitions for inter partes review of the ‘465 and ‘376 patents. The Northern District of California litigation is currently stayed pending resolution of the inter partes reviews of the ‘465 and ‘376 patents. The Company is seeking a judgment of infringement, a judgment of willful infringement, monetary and injunctive relief, enhanced damages, and an award of costs and expenses against Telesign.

On February 18, 2016, a putative class action complaint was filed in the Alameda County Superior Court in California, entitled Angela Flowers v. Twilio Inc. The complaint alleges that the Company’s products permit the interception, recording and disclosure of communications at a customer’s request and are in violation of the California Invasion of Privacy Act. The complaint seeks injunctive relief as well as monetary damages. On May 27, 2016, the Company filed a demurrer to the complaint. On August 2, 2016, the court issued an order denying the demurrer in part and granted it in part, with leave to amend by August 18, 2016 to address any claims under California’s Unfair Competition Law. The plaintiff opted not to amend the complaint. Following a period of discovery, the plaintiff filed a motion for class certification on September 20, 2017. On January 2, 2018, the court issued an order granting in part and denying in part the plaintiff’s class certification motion. The court certified two classes of individuals who, during specified time periods, allegedly sent or received certain communications involving the accounts of three of the Company’s customers that were recorded. The court has not yet finalized a schedule for notice to potential class members, additional discovery, summary judgment motions, or trial.

The Company intends to vigorously defend itself against these lawsuits and believes it has meritorious defenses to each matter in which it is a defendant. It is too early in these matters to reasonably predict the probability of the outcomes or to estimate ranges of possible losses.

In addition to the litigation matters discussed above, from time to time, the Company is a party to legal action and subject to claims that arise in the ordinary course of business. The claims are investigated as they arise and loss estimates are accrued, when probable and reasonably estimable. While it is not feasible to predict or determine the ultimate outcome of these matters, the Company believes that these legal proceedings will not have a material adverse effect on its financial position or results of operations.

Legal fees and other costs related to litigation and other legal proceedings are expensed as incurred and are included in general and administrative expenses in the accompanying condensed consolidated statements of operations.

(c)Indemnification Agreements

The Company has signed indemnification agreements with all of its board members and executive officers. The agreements indemnify the board members and executive officers from claims and expenses on actions brought against the individuals separately or jointly with the Company for certain indemnifiable events. Indemnifiable Events generally mean any event or occurrence related to the fact that the board member or the executive officer was or is acting in his or her capacity as a board member or an executive officer for the Company or was or is acting or representing the interests of the Company.

In the ordinary course of business, the Company enters into contractual arrangements under which it agrees to provide indemnification of varying scope and terms to business partners and other parties with respect to certain matters, including, but not limited to, losses arising out of the breach of such agreements, intellectual property infringement claims made by third parties and other liabilities relating to or arising from the Company’s various products, or its acts or omissions. In these circumstances, payment may be conditional on the other party making a claim pursuant to the procedures specified in the particular contract. Further, the Company’s obligations under these agreements may be limited in terms of time and/or amount, and in some instances, the Company may have recourse against third parties for certain payments. The terms of such obligations may vary.

As of March 31, 2018 and December 31, 2017, no amounts were accrued.

(d)Other Taxes

The Company conducts operations in many tax jurisdictions throughout the United States. In many of these jurisdictions, non-income-based taxes, such as sales and use and telecommunications taxes are assessed on the Company’s operations. Prior to March 2017, the Company had not billed nor collected these taxes from its customers and, in accordance with U.S. GAAP, recorded a provision for its tax exposure in these jurisdictions when it was both probable that a liability had been incurred and the amount of the exposure could be reasonably estimated. These estimates included several key assumptions including, but not limited to, the taxability of the Company’s services, the jurisdictions in which its management believes it has nexus, and the sourcing of revenues to those jurisdictions. Starting in March 2017, the Company began collecting these taxes from customers in certain jurisdictions, and since then, has expanded the number of jurisdictions where these taxes are being collected. Effective January 2018, the Company began to collect taxes in one additional jurisdiction and accordingly, the Company is no longer recording a provision for its exposure in that jurisdiction. The Company expects to continue to expand the number of jurisdictions where these taxes will be collected in the future. Simultaneously, the Company was and continues to be in discussions with certain states regarding its prior state sales and other taxes, if any, that the Company may owe.

During 2017, the Company revised its estimates of its tax exposure based on settlements reached with various states indicating that certain revisions to the key assumptions including, but not limited to, the sourcing of revenue and the taxability of the Company’s services were appropriate in the current period. In the year ended December 31, 2017, the total impact of these changes on the net loss attributable to common stockholders was a reduction of $13.4 million. As of March 31, 2018 and December 31, 2017, the liability recorded for these taxes was $21.2 million and $20.9 million, respectively.

In the event other jurisdictions challenge management’s assumptions and analysis, the actual exposure could differ materially from the current estimates.

Stockholders' Equity

3 Months Ended

Mar. 31, 2018

Stockholders' Equity

Stockholders' Equity

11. Stockholders’ Equity

(a)Preferred Stock

As of March 31, 2018 and December 31, 2017, the Company had authorized 100,000,000 shares of preferred stock, par value $0.001, of which no shares were issued and outstanding.

(b)Common Stock

As of March 31, 2018 and December 31, 2017, the Company had authorized 1,000,000,000 shares of Class A common stock and 100,000,000 shares of Class B common stock, each par value $0.001 per share. As of March 31, 2018, 71,748,415 shares of Class A common stock and 23,943,253 shares of Class B common stock were issued and outstanding. As of December 31, 2017, 69,906,550 shares of Class A common stock and 24,063,246 shares of Class B common stock were issued and outstanding.

The Company had reserved shares of common stock for issuance as follows:

	As of March 31,	As of December 31,
	2018	2017
Stock options issued and outstanding	10,392,199	10,710,427
Nonvested restricted stock units issued and outstanding	7,106,203	5,665,459
Class A common stock reserved for Twilio.org	635,014	635,014
Stock-based awards available for grant under 2016 Plan	12,054,291	10,200,189
Class A common stock committed under 2016 ESPP	249,730	235,372

Total	30,437,437	27,446,461

Stock-Based Compensation

3 Months Ended

Mar. 31, 2018

Stock-Based Compensation

Stock-Based Compensation

12. Stock-Based Compensation

2008 Stock Option Plan

The Company granted options under its 2008 Stock Option Plan (the “2008 Plan”), as amended and restated, until June 22, 2016, when the plan was terminated in connection with the Company’s IPO. Accordingly, no shares are available for future issuance under the 2008 Plan. The 2008 Plan continues to govern outstanding equity awards granted thereunder.

2016 Stock Option Plan

The Company’s 2016 Stock Option and Incentive Plan (the “2016 Plan”) became effective on June 21, 2016. The 2016 Plan provides for the grant of ISOs, NSOs, restricted stock, RSUs, stock appreciation rights, unrestricted stock awards, performance share awards, dividend equivalent rights and cash-based awards to employees, directors and consultants of the Company. A total of 11,500,000 shares of the Company’s Class A common stock were initially reserved for issuance under the 2016 Plan. These available shares automatically increase each January 1, beginning on January 1, 2017, by 5% of the number of shares of the Company’s Class A and Class B common stock outstanding on the immediately preceding December 31, or such lesser number of shares as determined by the Company’s compensation committee. On January 1, 2018, the shares available for grant under the 2016 Plan were automatically increased by 4,698,490 shares.

Under the 2016 Plan, the stock options are granted at a price per share not less than 100% of the fair market value per share of the underlying common stock on the date of grant. Under both plans, stock options generally expire 10 years from the date of grant and vest over periods determined by the board of directors. The vesting period for new-hire options and restricted stock units is generally a four-year term from the date of grant, at a rate of 25% after one year, then monthly or quarterly, respectively, on a straight-line basis thereafter. In July 2017, the Company began granting restricted stock units to existing employees that vest in equal quarterly installments over a four year service period.

2016 Employee Stock Purchase Plan

The Company’s Employee Stock Purchase Plan (“2016 ESPP”) became effective on June 21, 2016. A total of 2,400,000 shares of the Company’s Class A common stock were initially reserved for issuance under the 2016 ESPP. These available shares will automatically increase each January 1, beginning on January 1, 2017, by the lesser of 1,800,000 shares of the common stock, 1% of the number of shares of the Company’s Class A and Class B common stock outstanding on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. On January 1, 2018, the shares available for grant under the 2016 Plan were automatically increased by 939,698 shares.

The 2016 ESPP allows eligible employees to purchase shares of the Company’s Class A common stock at a discount of up to 15% through payroll deductions of their eligible compensation, subject to any plan limitations. Except for the initial offering period, the 2016 ESPP provides for separate six-month offering periods beginning in May and November of each fiscal year, starting in May 2017.

On each purchase date, eligible employees will purchase the Company’s stock at a price per share equal to 85% of the lesser of (i) the fair market value of the Company’s Class A common stock on the offering date or (ii) the fair market value of the Company’s Class A common stock on the purchase date.

In the three months ended March 31, 2018 and 2017, no shares of Class A common stock were purchased under the 2016 ESPP and 249,730 shares are expected to be purchased in the second quarter of 2018. As of March 31, 2018, total unrecognized compensation cost related to the 2016 ESPP was $0.4 million, which will be amortized over a weighted-average period of 0.1 years.

Stock-based awards activity under the 2008 Plan and 2016 Plan was as follows:

Stock Options

	Number of options outstanding		Weighted- average exercise price (per share)		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)
Outstanding options as of December 31, 2017	10,155,427		$	10.31	7.12	$	145,763
Granted	1,003,733		33.01
Exercised	(1,190,387	)	5.61
Forfeited and cancelled	(131,574	)	8.83

Outstanding options as of March 31, 2018	9,837,199		$	13.21	7.34	$	245,614

Options vested and exercisable as of March 31, 2018	5,054,924		$	7.71	6.37	$	154,024

Aggregate intrinsic value represents the difference between the fair value of the Company’s Class A common stock as reported on the New York Stock Exchange and the exercise price of outstanding “in-the-money” options. The aggregate intrinsic value of stock options exercised was $31.1 million and $78.2 million during the three months ended March 31, 2018 and 2017, respectively.

The total estimated grant date fair value of options vested was $7.8 million and $4.1 million during the three months ended March 31, 2018 and 2017, respectively. The weighted-average grant-date fair value of options granted was $15.24 and $13.72 during the three months ended March 31, 2018 and 2017, respectively.

On February 28, 2017, the Company granted a total of 555,000 shares of performance-based stock options in three distinct awards to an employee with grant date fair values of $13.48, $10.26 and $8.41 per share for a total grant value of $5.9 million. The first half of each award vests upon satisfaction of a performance condition and the remainder vests thereafter in equal monthly installments over a 24-month period. The achievement window expires after 4.3 years from the date of grant and the stock options expire seven years after the date of grant. The stock options are amortized over a derived service period, as adjusted, of 3.1 years, 4.6 years and 5.3 years, respectively. The stock options value and the derived service period were estimated using the Monte-Carlo simulation model. The following table summarizes the details of the performance options:

	Number of awards outstanding		Weighted- average exercise price (per share)		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)
Outstanding options as of December 31, 2017	555,000		$	31.72	6.0	$	—
Granted	—		—
Exercised	—		—
Forfeited and cancelled	—		—

Outstanding options as of March 31, 2018	555,000		$	31.72	5.9	$	3,585

Options vested and exercisable as of March 31, 2018	92,500	*	$	31.72	5.9	$	598

*Vesting of these shares is triggered by the filing of the Company’s Quarterly Report on Form 10-Q.

As of March 31, 2018, total unrecognized compensation cost related to nonvested stock options was $44.5 million, which will be amortized over a weighted-average period of 2.3 years.

Restricted Stock Units

	Number of awards outstanding		Weighted- average grant date fair value (per share)		Aggregate intrinsic value (in thousands)
Nonvested RSUs as of December 31, 2017	5,665,459		$	29.29	$	133,648
Granted	2,160,719		28.37
Vested	(544,217	)	29.32
Forfeited and cancelled	(175,758	)	28.69

Nonvested RSUs as of March 31, 2018	7,106,203		$	29.02	$	271,223

As of March 31, 2018, total unrecognized compensation cost related to nonvested RSUs was $190.9 million, which will be amortized over a weighted-average period of 3.3 years.

Valuation Assumptions

The fair value of employee stock options was estimated on the date of grant using the following assumptions in the Black-Scholes option pricing model:

	Three Months Ended March 31,
	2018			2017
Employee Stock Options
Fair value of common stock	$	33.01		$31.72 - $31.96
Expected term (in years)	6.08			6.08
Expected volatility	44.09		%	47.56	%
Risk-free interest rate	2.74		%	2.07	%
Dividend rate	0		%	0	%

The following assumptions were used in the Monte Carlo simulation model to estimate the fair value and the derived service period of the performance options:

Asset volatility	40		%
Equity volatility	45		%
Discount rate	14		%
Stock price at grant date	$	31.72

In May 2017, the FASB issued ASU 2017-09, “Compensation-Stock Compensation (Topic 718), Scope of Modification Accounting”, ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. ASU 2017-09 allows companies to make certain changes to awards, such as vesting conditions, without accounting for them as modifications. It does not change the accounting for modifications. ASU 2017-09 should be applied prospectively to awards modified on or after the adoption date. The Company adopted ASU 2017-09 in the first quarter of 2018. The adoption of this guidance did not have an impact on the Company’s financial position, results of operations or cash flows.

Stock-Based Compensation Expense

The Company recorded total stock-based compensation expense as follows (in thousands):

	Three Months Ended March 31,
	2018		2017
Cost of revenue	$	222	$	138
Research and development	7,872		4,484
Sales and marketing	3,859		1,995
General and administrative	5,587		2,768

Total	$	17,540	$	9,385

Net Loss Per Share Attributable to Common Stockholders

3 Months Ended

Mar. 31, 2018

Net Loss Per Share Attributable to Common Stockholders

Net Loss Per Share Attributable to Common Stockholders

13. Net Loss Per Share Attributable to Common Stockholders

Basic and diluted net loss per common share is presented in conformity with the two-class method required for participating securities.

Class A and Class B common stock are the only outstanding equity in the Company. The rights of the holders of Class A and Class B common stock are identical, except with respect to voting and conversion. Each share of Class A common stock is entitled to one vote per share and each share of Class B common stock is entitled to ten votes per share. Shares of Class B common stock may be converted into Class A common stock at any time at the option of the stockholder, and are automatically converted into Class A common stock upon sale or transfer, subject to certain limited exceptions.

Basic net loss per share attributable to common stockholders is computed using the weighted-average number of common shares outstanding during the period. Diluted net loss per share attributable to common stockholders is computed using the weighted-average number of common shares and, if dilutive, potential common shares outstanding during the period. The dilutive effect of these potential common shares is reflected in diluted earnings per share by application of the treasury stock method.

The following table sets forth the calculation of basic and diluted net loss per share attributable to common stockholders during the periods presented (in thousands, except share and per share data):

	Three Months Ended March 31,
	2018			2017
Net loss attributable to common stockholders	$	(23,729	)	$	(14,227	)

Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted	94,673,557			88,612,804

Net loss per share attributable to common stockholders, basic and diluted	$	(0.25	)	$	(0.16	)

The following outstanding shares of common stock equivalents were excluded from the calculation of the diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive:

	Three Months Ended March 31,
	2018	2017
Stock options issued and outstanding	10,392,199	13,545,334
Nonvested restricted stock units issued and outstanding	7,106,203	2,692,636
Class A common stock reserved for Twilio.org	635,014	680,397
Class A common stock committed under 2016 ESPP	249,730	594,218
Unvested shares subject to repurchase	767	38,886

Total	18,383,913	17,551,471

Transactions with Investors	3 Months Ended
Transactions with Investors	Mar. 31, 2018
Transactions with Investors
Transactions with Investors	14. Transactions With Investors In 2015, two of the Company’s vendors participated in the Company’s Series E convertible preferred stock financing and owned approximately 1.9% and 1.0%, respectively, of the Company’s capital stock, on an as-if converted basis, as of each of March 31, 2018, and December 31, 2017. During the three months ended March 31, 2018 and 2017, the amounts of software services the Company purchased from the first vendor were $6.3 million and $4.6 million, respectively. The net amount due to this vendor as of March 31, 2018 was $2.8 million. The amount of services the Company purchased from the second vendor was $0.2 million and $0.2 million in the three months ended March 31, 2018 and 2017, respectively. The net amount due to this vendor as of March 31, 2018 was insignificant.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Taxes
Income Taxes	15. Income Taxes In October 2016, the FASB issued ASU 2016-16, ‘‘Intra-Entity Transfers Other Than Inventory’’, which requires entities to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The Company adopted ASU 2016-16 in the first quarter 2018. Adoption of the ASU did not have any impact on the Company’s financial statements. The Company determines its income tax provision or benefit for interim periods using an estimate of its annual effective tax rate, adjusted for discrete items occurring in the quarter. The primary difference between the Company’s effective tax rate and the federal statutory rate relates to the net operating losses in jurisdictions with a valuation allowance or a zero tax rate. The Company recorded a (provision) benefit for income taxes of $(0.1) million and $0.03 million for the three months ended March 31, 2018 and 2017, respectively. The provision for income taxes consists primarily of income taxes and withholding taxes in foreign jurisdictions in which the Company conducts business. The Company’s U.S. operations have been in a loss position and the Company maintains a full valuation allowance against its U.S. deferred tax assets. On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act makes broad and complex changes to the U.S. tax code including, but not limited to, (1) reducing the U.S. federal corporate tax rate from 35 percent to 21 percent; (2) requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries; (3) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; (4) requiring a current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations; (5) eliminating the corporate alternative minimum tax (“AMT”) and changing how existing AMT credits can be realized; (6) creating the base erosion anti-abuse tax (“BEAT”), a new minimum tax; (7) creating a new limitation on deductible interest expense; and (8) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017. The SEC staff issued SAB 118, which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting under ASC 740. The Company has not completed its accounting assessment for the effects of the Tax Act as of March 31, 2018; however, the final accounting assessment will occur no later than one year from the date the Tax Act was enacted. The Company has also considered and estimated a number of provisions of the Tax Act effective January 1, 2018 and, based on the initial assessment, the Company has determined that the Tax Act did not have a material effect on its consolidated financial statements for the three months ended March 31, 2018.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Summary of Significant Accounting Policies

Basis of Presentation

(a)Basis of Presentation

Principles of Consolidation

(b)Principles of Consolidation

The condensed consolidated financial statements include the Company and its wholly owned subsidiaries. All significant intercompany balances and transactions have been eliminated.

Use of Estimates

(c)Use of Estimates

Concentration of Credit Risk

(d)Concentration of Credit Risk

Revenue Recognition

(e)Significant Accounting Policies

Revenue Recognition

The Company determines revenue recognition through the following steps:

Identification of the contract, or contracts, with a customer

Identification of the performance obligations in the contract

Determination of the transaction price

Allocation of the transaction price to the performance obligations in the contract

Recognition of revenue when, or as, the Company satisfies a performance obligation

Nature of Products and Services

Refer to Note 9, Revenue by Geographic Area, for additional disaggregation.

Deferred Revenue and Customer Deposits

Deferred Sales Commissions

Other than adoption of ASC 606, there were no changes to our significant accounting policies as described in our Annual Report.

Restricted Cash

(f)Restricted Cash

	Three Months Ended March 31, 2017
	As Originally Reported		As Adjusted
Cash, cash equivalents and restricted cash — beginning of period	$	305,665	$	314,280
Cash, cash equivalents and restricted cash — end of period	$	118,440	$	127,055

Other than the revised statement of cash flows presentation of restricted cash, the adoption of ASU 2016-18 did not have an impact on the Company’s financial position and results of operations.

Recently Issued Accounting Guidance, Not yet Adopted

(g)Recently Issued Accounting Guidance, Not yet Adopted

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

ASU 2016-18 - Restricted Cash

Summary of Significant Accounting Policies

Schedule of impact from adoption of guidance

The Company adopted ASU 2016-18 in the first quarter of 2018 and applied the guidance retrospectively to the prior period’s condensed consolidated statement of cash flows with the following impact (in thousands):

	Three Months Ended March 31, 2017
	As Originally Reported		As Adjusted
Cash, cash equivalents and restricted cash — beginning of period	$	305,665	$	314,280
Cash, cash equivalents and restricted cash — end of period	$	118,440	$	127,055

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Measurements

Schedule of financial assets measured at fair value on a recurring basis

The following tables provide the financial assets measured at fair value on a recurring basis as of March 31, 2018 and December 31, 2017 (in thousands):

	Amortized Cost or Carrying		Gross Unrealized		Gross Unrealized Losses Less Than			Gross Unrealized Losses More Than			Fair Value Hierarchy as of March 31, 2018						Aggregate
	Value		Gains		12 Months			12 Months			Level 1		Level 2		Level 3		Fair Value
Financial Assets:
Cash and cash equivalents:
Money market funds	$	93,103	$	—	$	—		$	—		$	93,103	$	—	$	—	$	93,103

Total included in cash and cash equivalents	93,103		—		—			—			93,103		—		—		93,103

Marketable securities:
U.S. Treasury securities	59,973		—		(127		)	(73		)	59,773		—		—		59,773
Corporate debt securities	131,213		—		(715		)	—			—		130,498		—		130,498

Total marketable securities	191,186		—		(842		)	(73		)	59,773		130,498		—		190,271

Total financial assets	$	284,289	$	—	$	(842	)	$	(73	)	$	152,876	$	130,498	—		$	283,374

	Amortized Cost or Carrying		Gross Unrealized		Gross Unrealized Losses Less Than			Gross Unrealized Losses More Than		Fair Value Hierarchy as of December 31, 2017						Aggregate
	Value		Gains		12 Months			12 Months		Level 1		Level 2		Level 3		Fair Value
Financial Assets:
Cash and cash equivalents:
Money market funds	$	95,432	$	—	$	—		$	—	$	95,432	$	—	$	—	$	95,432

Total included in cash and cash equivalents	95,432		—		—			—		95,432		—		—		95,432

Marketable securities:
U.S. Treasury securities	59,962		—		(216		)	—		59,746		—		—		59,746
Corporate debt securities	116,223		—		(382		)	—		—		115,841		—		115,841

Total marketable securities	176,185		—		(598		)	—		59,746		115,841		—		175,587

Total financial assets	$	271,617	$	—	$	(598	)	$	—	$	155,178	$	115,841	$	—	$	271,019

Schedule of contractual maturities of marketable securities

The following table summarizes the contractual maturities of marketable securities as of March 31, 2018 and December 31, 2017 (in thousands):

	March 31, 2018				December 31, 2017
	Amortized Cost		Aggregate Fair Value		Amortized Cost		Aggregate Fair Value
Financial Assets:
Less than one year	$	147,714	$	147,156	$	108,584	$	108,360
One to two years	43,472		43,115		67,601		67,227

Total	$	191,186	$	190,271	$	176,185	$	175,587

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2018

Property and Equipment

Schedule of property and equipment

Property and equipment consisted of the following (in thousands):

	As of March 31,			As of December 31,
	2018			2017
Capitalized internal-use software development costs	$	55,328		$	49,177
Leasehold improvements	14,422			14,246
Office equipment	10,410			9,652
Furniture and fixtures	2,108			1,976
Software	1,765			1,675

Total property and equipment	84,033			76,726
Less: accumulated depreciation and amortization	(30,339		)	(26,185		)

Total property and equipment, net	$	53,694		$	50,541

Business Combinations (Tables) - Beepsend AB, a messaging provider based in Sweden

3 Months Ended

Mar. 31, 2018

Business Combinations

Schedule of final purchase price allocation

The following table presents the purchase price allocation, as adjusted, recorded in the Company’s condensed consolidated balance sheet as of March 31, 2017 (in thousands):

	Total
Net tangible liabilities	$	(3,575	)
Goodwill(1)	12,837
Intangible assets(2)	13,700

Total purchase price	$	22,962

(1)

(2)

Identifiable finite-lived intangible assets were comprised of the following (in thousands):

	Total		Estimated life (in years)
Developed technology	$	5,000	4
Customer relationships	6,100		7 - 8
Supplier relationships	2,600		5

Total intangible assets acquired	$	13,700

Schedule of identifiable finite-lived intangible assets

Identifiable finite-lived intangible assets were comprised of the following (in thousands):

	Total		Estimated life (in years)
Developed technology	$	5,000	4
Customer relationships	6,100		7 - 8
Supplier relationships	2,600		5

Total intangible assets acquired	$	13,700

Goodwill and Intangible Assets (Tables)

3 Months Ended

Mar. 31, 2018

Goodwill and Intangible Assets

Schedule of goodwill balance

Goodwill balance as of March 31, 2018 and December 31, 2017 was as follows (in thousands):

	Total
Balance as of December 31, 2017	$	17,851
Effect of exchange rate	418

Balance as of March 31, 2018	$	18,269

Schedule of intangible assets

Intangible assets consisted of the following (in thousands):

	As of March 31, 2018
	Gross		Accumulated Amortization			Net
Amortizable intangible assets:
Developed technology	$	15,040	$	(6,529	)	$	8,511
Customer relationships	7,280		(1,229		)	6,051
Supplier relationships	2,932		(631		)	2,301
Trade name	60		(60		)	—
Patent	1,947		(124		)	1,823

Total amortizable intangible assets	27,259		(8,573		)	18,686

Non-amortizable intangible assets:
Domain names	32		—			32
Trademarks	263		—			263

Total	$	27,554	$	(8,573	)	$	18,981

	As of December 31, 2017
	Gross		Accumulated Amortization			Net
Amortizable intangible assets:
Developed technology	$	14,941	$	(5,476	)	$	9,465
Customer relationships	7,159		(1,006		)	6,153
Supplier relationships	2,881		(500		)	2,381
Trade name	60		(60		)	—
Patent	1,878		(108		)	1,770

Total amortizable intangible assets	26,919		(7,150		)	19,769

Non-amortizable intangible assets:
Domain names	32		—			32
Trademarks	263		—			263

Total	$	27,214	$	(7,150	)	$	20,064

Schedule of total estimated future amortization expense

Total estimated future amortization expense was as follows (in thousands):

	As of March 31, 2018
2018 (remaining nine months)	$	5,646
2019	5,088
2020	2,658
2021	1,525
2022	929
Thereafter	2,840

Total	$	18,686

Accrued Expenses and Other Liabilities (Tables)

3 Months Ended

Mar. 31, 2018

Accrued Expenses and Other Liabilities

Schedule of accrued expenses and other current liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	As of March 31,		As of December 31,
	2018		2017
Accrued payroll and related	$	10,042	$	4,898
Accrued bonus and commission	5,721		4,777
Accrued cost of revenue	18,690		10,876
Sales and other taxes payable	21,227		20,877
ESPP contributions	3,345		1,338
Deferred rent	1,179		1,048
Accrued other expense	16,391		9,800

Total accrued expenses and other current liabilities	$	76,595	$	53,614

Schedule of long-term liabilities

Long-term liabilities consisted of the following (in thousands):

	As of March 31,		As of December 31,
	2018		2017
Deferred rent	$	8,217	$	8,480
Deferred tax liability, net	2,262		2,452
Accrued other expense	472		477

Total long-term liabilities	$	10,951	$	11,409

Supplemental Balance Sheet Information (Tables)

3 Months Ended

Mar. 31, 2018

Supplemental Balance Sheet Information

Schedule of the allowance for doubtful accounts

A roll-forward of the Company’s reserves for the three months ended March 31, 2018 and 2017 is as follows (in thousands):

(a)Allowance for doubtful accounts:

	Three Months Ended March 31,
	2018			2017
Balance, beginning of period	$	1,033		$	1,076
Additions	375			72
Write-offs	(4		)	(378		)

Balance, end of period	$	1,404		$	770

Schedule of the sales credit reserve

A roll-forward of the Company’s reserves for the three months ended March 31, 2018 and 2017 is as follows (in thousands):

b)Sales credit reserve:

	Three Months Ended March 31,
	2018			2017
Balance, beginning of period	$	1,761		$	544
Additions	1,107			551
Deductions against reserve	(1,166		)	(277		)

Balance, end of period	$	1,702		$	818

Revenue by Geographic Area (Tables)

3 Months Ended

Mar. 31, 2018

Revenue by Geographic Area

Schedule of revenue by geographic area

Revenue by geographic area is based on the IP address at the time of registration. The following table sets forth revenue by geographic area (dollars in thousands):

	Three Months Ended March 31,
	2018			2017
Revenue by geographic area:
United States	$	98,635		$	70,098
International	30,481			17,274

Total	$	129,116		$	87,372

Percentage of revenue by geographic area:
United States	76		%	80		%
International	24		%	20		%

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies

Schedule of future minimum lease payments under non-cancelable operating leases

Future minimum lease payments under non-cancelable operating leases were as follows (in thousands):

Year Ending December 31:	As of March 31, 2018
2018 (remaining nine months)	$	6,607
2019	7,952
2020	7,112
2021	7,033
2022	5,864
Thereafter	10,189

Total minimum lease payments	$	44,757

Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2018

Stockholders' Equity

Schedule of reserved shares of common stock for issuance

	As of March 31,	As of December 31,
	2018	2017
Stock options issued and outstanding	10,392,199	10,710,427
Nonvested restricted stock units issued and outstanding	7,106,203	5,665,459
Class A common stock reserved for Twilio.org	635,014	635,014
Stock-based awards available for grant under 2016 Plan	12,054,291	10,200,189
Class A common stock committed under 2016 ESPP	249,730	235,372

Total	30,437,437	27,446,461

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Stock-Based Compensation

Schedule of restricted stock unit activity

	Number of awards outstanding		Weighted- average grant date fair value (per share)		Aggregate intrinsic value (in thousands)
Nonvested RSUs as of December 31, 2017	5,665,459		$	29.29	$	133,648
Granted	2,160,719		28.37
Vested	(544,217	)	29.32
Forfeited and cancelled	(175,758	)	28.69

Nonvested RSUs as of March 31, 2018	7,106,203		$	29.02	$	271,223

Schedule of stock based compensation expense

The Company recorded total stock-based compensation expense as follows (in thousands):

	Three Months Ended March 31,
	2018		2017
Cost of revenue	$	222	$	138
Research and development	7,872		4,484
Sales and marketing	3,859		1,995
General and administrative	5,587		2,768

Total	$	17,540	$	9,385

Employee and Nonemployee Stock Options

Stock-Based Compensation

Schedule of stock options activity

	Number of options outstanding		Weighted- average exercise price (per share)		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)
Outstanding options as of December 31, 2017	10,155,427		$	10.31	7.12	$	145,763
Granted	1,003,733		33.01
Exercised	(1,190,387	)	5.61
Forfeited and cancelled	(131,574	)	8.83

Outstanding options as of March 31, 2018	9,837,199		$	13.21	7.34	$	245,614

Options vested and exercisable as of March 31, 2018	5,054,924		$	7.71	6.37	$	154,024

Employee Stock Options

Stock-Based Compensation

Schedule of valuation assumptions

	Three Months Ended March 31,
	2018			2017
Employee Stock Options
Fair value of common stock	$	33.01		$31.72 - $31.96
Expected term (in years)	6.08			6.08
Expected volatility	44.09		%	47.56	%
Risk-free interest rate	2.74		%	2.07	%
Dividend rate	0		%	0	%

Performance-based stock options

Stock-Based Compensation

Schedule of stock options activity

	Number of awards outstanding		Weighted- average exercise price (per share)		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)
Outstanding options as of December 31, 2017	555,000		$	31.72	6.0	$	—
Granted	—		—
Exercised	—		—
Forfeited and cancelled	—		—

Outstanding options as of March 31, 2018	555,000		$	31.72	5.9	$	3,585

Options vested and exercisable as of March 31, 2018	92,500	*	$	31.72	5.9	$	598

*Vesting of these shares is triggered by the filing of the Company’s Quarterly Report on Form 10-Q.

Schedule of valuation assumptions

Asset volatility	40		%
Equity volatility	45		%
Discount rate	14		%
Stock price at grant date	$	31.72

Net Loss Per Share Attributable to Common Stockholders (Tables)

3 Months Ended

Mar. 31, 2018

Net Loss Per Share Attributable to Common Stockholders

Schedule of the calculation of basic and diluted net loss per share attributable to common stockholders

	Three Months Ended March 31,
	2018			2017
Net loss attributable to common stockholders	$	(23,729	)	$	(14,227	)

Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted	94,673,557			88,612,804

Net loss per share attributable to common stockholders, basic and diluted	$	(0.25	)	$	(0.16	)

Schedule of common stock equivalents excluded from the computation of the diluted net loss per share attributable to common stockholders

	Three Months Ended March 31,
	2018	2017
Stock options issued and outstanding	10,392,199	13,545,334
Nonvested restricted stock units issued and outstanding	7,106,203	2,692,636
Class A common stock reserved for Twilio.org	635,014	680,397
Class A common stock committed under 2016 ESPP	249,730	594,218
Unvested shares subject to repurchase	767	38,886

Total	18,383,913	17,551,471

Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - customer	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue \| Customer Concentration Risk
Concentration of Credit Risk
Number of customers	0	1
Revenue \| Customer Concentration Risk \| Customer One
Concentration of Credit Risk
Concentration risk (as a percent)		12.00%
Accounts Receivable \| Credit Concentration Risk
Concentration of Credit Risk
Number of customers	1		0
Accounts Receivable \| Credit Concentration Risk \| Customer One
Concentration of Credit Risk
Concentration risk (as a percent)	16.00%

Summary of Significant Accounting Policies - Impact of Adopting ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Adoption of revenue recognition guidance
Retained earnings	$ (273,454)		$ (250,438)
Incremental commission costs of obtaining new contracts
Adoption of revenue recognition guidance
Amortization period for deferred incremental commission costs of obtaining new contracts	5 years
ASU 2014-09 - Revenue from Contracts with Customers \| Impact of adopting ASC 606
Adoption of revenue recognition guidance
Retained earnings		$ 700

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue Recognition
Revenue recognized out of adjusted deferred revenue balance	$ 3.6
Usage-based fees
Revenue Recognition
Percent of revenue	84.00%	83.00%
Subscription-based fees
Revenue Recognition
Percent of revenue	16.00%	17.00%

Summary of Significant Accounting Policies - Deferred Sales Commissions (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Capitalized Contract Cost [Line Items]
Total net capitalized costs	$ 4.1
Amortization of capitalized costs of obtaining a contract	$ 0.2
Incremental commission costs of obtaining new contracts
Capitalized Contract Cost [Line Items]
Amortization period for deferred incremental commission costs of obtaining new contracts	5 years

Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash
Cash, cash equivalents and restricted cash	$ 123,239	$ 120,788	$ 127,055	$ 314,280
As Originally Reported
Restricted Cash
Cash, cash equivalents and restricted cash			$ 118,440	$ 305,665

Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements
Marketable securities, Amortized Cost	$ 191,186	$ 176,185
Marketable securities, Aggregate Fair Value	190,271	175,587
Recurring
Fair Value Measurements
Marketable securities, Amortized Cost	191,186	176,185
Marketable securities, Gross Unrealized Losses Less Than 12 Months	(842)	(598)
Marketable securities, Gross Unrealized Losses More Than 12 Months	(73)
Total financial assets, Amortized Cost or Carrying Value	284,289	271,617
Recurring \| U.S. Treasury securities
Fair Value Measurements
Marketable securities, Amortized Cost	59,973	59,962
Marketable securities, Gross Unrealized Losses Less Than 12 Months	(127)	(216)
Marketable securities, Gross Unrealized Losses More Than 12 Months	(73)
Recurring \| Corporate debt securities
Fair Value Measurements
Marketable securities, Amortized Cost	131,213	116,223
Marketable securities, Gross Unrealized Losses Less Than 12 Months	(715)	(382)
Recurring \| Carrying Value
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Recurring \| Carrying Value \| Money market funds
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Recurring \| Aggregate Fair Value
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Marketable securities, Aggregate Fair Value	190,271	175,587
Total financial assets	283,374	271,019
Recurring \| Aggregate Fair Value \| U.S. Treasury securities
Fair Value Measurements
Marketable securities, Aggregate Fair Value	59,773	59,746
Recurring \| Aggregate Fair Value \| Corporate debt securities
Fair Value Measurements
Marketable securities, Aggregate Fair Value	130,498	115,841
Recurring \| Aggregate Fair Value \| Money market funds
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Recurring \| Aggregate Fair Value \| Level 1
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Marketable securities, Aggregate Fair Value	59,773	59,746
Total financial assets	152,876	155,178
Recurring \| Aggregate Fair Value \| Level 1 \| U.S. Treasury securities
Fair Value Measurements
Marketable securities, Aggregate Fair Value	59,773	59,746
Recurring \| Aggregate Fair Value \| Level 1 \| Money market funds
Fair Value Measurements
Cash and cash equivalents	93,103	95,432
Recurring \| Aggregate Fair Value \| Level 2
Fair Value Measurements
Marketable securities, Aggregate Fair Value	130,498	115,841
Total financial assets	130,498	115,841
Recurring \| Aggregate Fair Value \| Level 2 \| Corporate debt securities
Fair Value Measurements
Marketable securities, Aggregate Fair Value	$ 130,498	$ 115,841

Fair Value Measurements - Additional Disclosures (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value Measurements
Other-than-temporary impairments associated with credit losses	$ 0
Interest earned on marketable securities	$ 700	$ 500

Fair Value Measurements - Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements
Less than one year, Amortized Cost	$ 147,714	$ 108,584
One to two years, Amortized Cost	43,472	67,601
Total Amortized Cost	191,186	176,185
Less than one year, Aggregate Fair Value	147,156	108,360
One to two years, Aggregate Fair Value	43,115	67,227
Total Aggregate Fair Value	$ 190,271	$ 175,587

Property and Equipment - Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property and Equipment
Total property and equipment	$ 84,033	$ 76,726
Less: accumulated depreciation and amortization	(30,339)	(26,185)
Total property and equipment, net	53,694	50,541
Capitalized internal-use software development costs
Property and Equipment
Total property and equipment	55,328	49,177
Leasehold improvements
Property and Equipment
Total property and equipment	14,422	14,246
Office equipment
Property and Equipment
Total property and equipment	10,410	9,652
Furniture and fixtures
Property and Equipment
Total property and equipment	2,108	1,976
Software
Property and Equipment
Total property and equipment	$ 1,765	$ 1,675

Property and Equipment - Depreciation and Amortization Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property and Equipment
Depreciation and amortization	$ 4.2	$ 2.8

Property and Equipment - Capitalized Software Development Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property and Equipment
Capitalized software development costs	$ 6,400	$ 4,300
Stock-based compensation capitalized in software development costs	1,428	709
Amortization of capitalized software development costs	$ 2,700	$ 1,800

Business Combinations - Consideration (Details) - Beepsend AB, a messaging provider based in Sweden - USD ($) $ in Millions	1 Months Ended
	Feb. 28, 2017	Mar. 31, 2018
Acquisition
Total purchase price	$ 23.0
Purchase price paid in cash	23.0
Amount of purchase price placed into an escrow account	$ 5.0
Escrow effective period	18 months
Amount deposited into a separate escrow conditioned upon future service conditions	$ 2.0
Remaining balance in employee escrow		$ 0.3

Business Combinations - Purchase Price Allocation (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Feb. 28, 2017
Acquisition
Goodwill	$ 18,269	$ 17,851
Beepsend AB, a messaging provider based in Sweden
Acquisition
Net tangible liabilities			$ (3,575)
Goodwill			12,837
Intangible assets			13,700
Total purchase price			$ 22,962

Business Combinations - Identifiable Finite-lived Intangible Assets (Details) - Beepsend AB, a messaging provider based in Sweden $ in Thousands	1 Months Ended
	Feb. 28, 2017 USD ($)
Acquisition
Total intangible assets acquired	$ 13,700
Net deferred tax liability	2,600
Developed technology
Acquisition
Total intangible assets acquired	$ 5,000
Estimated life (in years)	4 years
Customer relationships
Acquisition
Total intangible assets acquired	$ 6,100
Customer relationships \| Minimum
Acquisition
Estimated life (in years)	7 years
Customer relationships \| Maximum
Acquisition
Estimated life (in years)	8 years
Supplier relationships
Acquisition
Total intangible assets acquired	$ 2,600
Estimated life (in years)	5 years

Business Combinations - Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended		24 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Beepsend AB, a messaging provider based in Sweden \| General and administrative
Business Combinations
Acquisition related costs	$ 0.3	$ 0.4	$ 0.7

Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Goodwill
Balance at the beginning of the period	$ 17,851
Effect of exchange rate	418
Balance at the end of the period	$ 18,269

Goodwill and Intangible Assets - Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Intangible Assets
Gross	$ 27,259	$ 26,919
Accumulated Amortization	(8,573)	(7,150)
Net	18,686	19,769
Developed technology
Intangible Assets
Gross	15,040	14,941
Accumulated Amortization	(6,529)	(5,476)
Net	8,511	9,465
Customer relationships
Intangible Assets
Gross	7,280	7,159
Accumulated Amortization	(1,229)	(1,006)
Net	6,051	6,153
Supplier relationships
Intangible Assets
Gross	2,932	2,881
Accumulated Amortization	(631)	(500)
Net	2,301	2,381
Trade name
Intangible Assets
Gross	60	60
Accumulated Amortization	(60)	(60)
Patent
Intangible Assets
Gross	1,947	1,878
Accumulated Amortization	(124)	(108)
Net	$ 1,823	$ 1,770

Goodwill and Intangible Assets - Non-amortizable Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Domain names
Intangible Assets
Non-amortizable intangible assets	$ 32	$ 32
Trademarks
Intangible Assets
Non-amortizable intangible assets	$ 263	$ 263

Goodwill and Intangible Assets - Total Intangible Assets, Gross (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Intangible Assets
Amortizable intangible assets, gross	$ 27,259	$ 26,919
Total	27,554	27,214	$ 26,919
Domain names
Intangible Assets
Non-amortizable intangible assets	32	32
Trademarks
Intangible Assets
Non-amortizable intangible assets	$ 263	$ 263

Goodwill and Intangible Assets - Total Intangible Assets, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Intangible Assets
Amortizable intangible assets, net	$ 18,686	$ 19,769
Total	18,981	20,064
Domain names
Intangible Assets
Non-amortizable intangible assets	32	32
Trademarks
Intangible Assets
Non-amortizable intangible assets	$ 263	$ 263

Goodwill and Intangible Assets - Total Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Goodwill and Intangible Assets
Gross	$ 27,554	$ 27,214	$ 26,919
Accumulated Amortization	(8,573)	(7,150)
Net	$ 18,981	$ 20,064

Goodwill and Intangible Assets - Amortization Expense (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets
Amortization expense	$ 1.4	$ 1.2

Goodwill and Intangible Assets - Total Estimated Future Amortization Expense (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Intangible Assets
2018 (remaining nine months)	$ 5,646
2019	5,088
2020	2,658
2021	1,525
2022	929
Thereafter	2,840
Net	$ 18,686	$ 19,769

Accrued Expenses and Other Liabilities - Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Expenses and Other Liabilities
Accrued payroll and related	$ 10,042	$ 4,898
Accrued bonus and commission	5,721	4,777
Accrued cost of revenue	18,690	10,876
Sales and other taxes payable	21,227	20,877
ESPP contributions	3,345	1,338
Deferred rent	1,179	1,048
Accrued other expense	16,391	9,800
Total accrued expenses and other current liabilities	$ 76,595	$ 53,614

Accrued Expenses and Other Liabilities - Long-term Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Expenses and Other Liabilities
Deferred rent	$ 8,217	$ 8,480
Deferred tax liability, net	2,262	2,452
Accrued other expense	472	477
Total long-term liabilities	$ 10,951	$ 11,409

Supplemental Balance Sheet Information - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Allowance for doubtful accounts
Balance, beginning of period	$ 1,033	$ 1,076
Additions	375	72
Write-offs	(4)	(378)
Balance, end of period	$ 1,404	$ 770

Supplemental Balance Sheet Information - Sales Credit Reserve (Details) - Sales credit reserve - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Sales credit reserve
Balance, beginning of period	$ 1,761	$ 544
Additions	1,107	551
Deductions against reserve	(1,166)	(277)
Balance, end of period	$ 1,702	$ 818

Revenue by Geographic Area - Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue by geographic area
Revenue	$ 129,116	$ 87,372
United States
Revenue by geographic area
Revenue	98,635	70,098
International
Revenue by geographic area
Revenue	$ 30,481	$ 17,274

Revenue by Geographic Area - Percentage of Revenue by Geographic Area (Details) - Revenue - Geographic Concentration Risk	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
United States
Percentage of revenue by geographic area
Percentage of revenue (as a percent)	76.00%	80.00%
International
Percentage of revenue by geographic area
Percentage of revenue (as a percent)	24.00%	20.00%

Commitments and Contingencies - Lease Commitments (Details)	3 Months Ended
	Mar. 31, 2018
Various Facilities Leases \| Maximum
Lease Commitments
Non-cancelable operating lease agreements, facilities, remaining term (in years)	6 years

Commitments and Contingencies - Rent Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Commitments and Contingencies
Rent expense	$ 2.1	$ 1.9

Commitments and Contingencies - Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Future minimum lease payments
2018 (remaining nine months)	$ 6,607
2019	7,952
2020	7,112
2021	7,033
2022	5,864
Thereafter	10,189
Total minimum lease payments	$ 44,757

Commitments and Contingencies - Other Commitments (Details) - Cloud services vendor $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Non-cancellable commitment
Term of non-cancellable agreement	22 months
Total commitment	$ 4.5

Commitments and Contingencies - Legal Matters (Details) - Pending Litigation - Putative class action complaint, Angela Flowers v. Twilio Inc.	Jan. 02, 2018 item
Legal Matters
Number of classes of individuals who allegedly sent or received certain communications	2
Number of customers' accounts involved in the complaint	3

Commitments and Contingencies - Indemnification Agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Indemnification Agreement
Indemnification Agreements
Amount accrued	$ 0	$ 0

Commitments and Contingencies - Other taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Other taxes
Liability for uncertain tax positions	$ 21,200		$ 20,900
Net loss attributable to common stockholders	$ (23,729)	$ (14,227)
Change in estimate for non-income-based tax exposure - revisions in key assumptions
Other taxes
Net loss attributable to common stockholders			$ 13,400

Stockholders' Equity - Preferred Stock (Details) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Preferred Stock
Preferred stock, authorized (in shares)	100,000,000	100,000,000
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0

Stockholders' Equity - Common Stock (Details) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Common Class A
Common Stock
Common stock, authorized (in shares)	1,000,000,000	1,000,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, issued (in shares)	71,748,415	69,906,550
Common stock, outstanding (in shares)	71,748,415	69,906,550
Common Class B
Common Stock
Common stock, authorized (in shares)	100,000,000	100,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, issued (in shares)	23,943,253	24,063,246
Common stock, outstanding (in shares)	23,943,253	24,063,246

Stockholders' Equity - Common Stock Shares Reserved (Details) - shares	Mar. 31, 2018	Dec. 31, 2017
Stockholders' Equity
Reserved shares of common stock (in shares)	30,437,437	27,446,461
2016 Stock Option and Incentive Plan
Stockholders' Equity
Stock - based awards available for grant (in shares)	12,054,291	10,200,189
Common Class A
Stockholders' Equity
Common stock reserved for Twilio.org (in shares)	635,014	635,014
Stock Options
Stockholders' Equity
Stock options issued and outstanding (in shares)	10,392,199	10,710,427
Restricted Stock Units (RSUs)
Stockholders' Equity
Nonvested restricted stock units issued and outstanding (in shares)	7,106,203	5,665,459
Employee Stock \| Common Class A
Stockholders' Equity
Common stock committed under 2016 ESPP (in shares)	249,730	235,372

Stock-Based Compensation - 2008 Stock Option Plan (Details)	Mar. 31, 2018 shares
2008 Stock Option Plan
Stock Based Compensation
Shares available for future issuance (in shares)	0

Stock-Based Compensation - 2016 Stock Option Plan (Details) - shares		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Jun. 21, 2016
Employee and Nonemployee Stock Options
Stock Based Compensation
Expiration term		10 years
Employee and Nonemployee Stock Options \| New Hires
Stock Based Compensation
Vesting period		4 years
Employee and Nonemployee Stock Options \| First vesting \| New Hires
Stock Based Compensation
Vesting period		1 year
Percentage of vesting rights		25.00%
Restricted Stock Units (RSUs)
Stock Based Compensation
Vesting period		4 years
Restricted Stock Units (RSUs) \| First vesting \| New Hires
Stock Based Compensation
Vesting period		1 year
Percentage of vesting rights		25.00%
2016 Stock Option and Incentive Plan
Stock Based Compensation
Maximum automatic annual increase as a percentage of outstanding common shares		5.00%
Automatic increase in shares available for grant (in shares)	4,698,490
2016 Stock Option and Incentive Plan \| Common Class A
Stock Based Compensation
Shares reserved for issuance (in shares)			11,500,000
2016 Stock Option and Incentive Plan \| Employee and Nonemployee Stock Options
Stock Based Compensation
Minimum grant price as a percentage of fair market value per share of the underlying common stock on the date of grant (as a percent)		100.00%

Stock-Based Compensation - 2016 Employee Stock Purchase Plan (Details) - Employee Stock - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Jun. 21, 2016
Stock Based Compensation
Maximum automatic annual increase (in shares)			1,800,000
Maximum automatic annual increase as a percentage of outstanding common shares			1.00%
Automatic increase in shares available for grant (in shares)	939,698
Unrecognized compensation cost, other than options			$ 0.4
Weighted-average period (in years)			1 month 6 days
Common Class A
Stock Based Compensation
Shares reserved for issuance (in shares)					2,400,000
Discount from market price, offering date (as a percent)			15.00%
Discount from market price, purchase date (as a percent)			15.00%
Purchase price, percentage of fair market value (as a percent)			85.00%
Shares purchased (in shares)			0	0
Common Class A \| Forecast
Stock Based Compensation
Shares purchased (in shares)		249,730

Stock-Based Compensation - Stock Option Activity (Details) - Employee and Nonemployee Stock Options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Number of options outstanding
Outstanding options as of the beginning of the period (in shares)	10,155,427
Granted (in shares)	1,003,733
Exercised (in shares)	(1,190,387)
Forfeited and cancelled (in shares)	(131,574)
Outstanding options as of the end of the period (in shares)	9,837,199	10,155,427
Weighted-average exercise price (per share)
Outstanding options as of the beginning of the period (in dollars per share)	$ 10.31
Granted (in dollars per share)	33.01
Exercised (in dollars per share)	5.61
Forfeited and cancelled (in dollars per share)	8.83
Outstanding options as of the end of the period (in dollars per share)	$ 13.21	$ 10.31
Weighted-average remaining contractual term and aggregate intrinsic value
Weighted-average remaining contractual term (in years)	7 years 4 months 2 days	7 years 1 month 13 days
Aggregate intrinsic value	$ 245,614	$ 145,763
Options vested and exercisable - weighted-average remaining contractual term (in years)	6 years 4 months 13 days
Options vested and exercisable and options vested and expected to vest
Options vested and exercisable - number of options outstanding (in shares)	5,054,924
Options vested and exercisable - weighted-average exercise price (in dollars per share)	$ 7.71
Options vested and exercisable - weighted-average remaining contractual term (in years)	6 years 4 months 13 days
Options vested and exercisable - aggregate intrinsic value	$ 154,024

Stock-Based Compensation - Stock Options - Additional Information (Details) - Employee and Nonemployee Stock Options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock Based Compensation
Aggregate intrinsic value of stock options exercised	$ 31.1	$ 78.2
Grant date fair value of options vested	$ 7.8	$ 4.1
Weighted-average grant date fair value of options granted (in dollars per share)	$ 15.24	$ 13.72

Stock-Based Compensation - Performance-Based Stock Options (Details) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
	Feb. 28, 2017 USD ($) item $ / shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares
Performance-based stock options
Stock-Based Compensation
Number of distinct awards \| item	3
Total grant value \| $	$ 5,900
Vesting period upon satisfaction of performance condition	24 months
Performance condition achievement window	4 years 3 months 18 days
Expiration term	7 years
Number of options outstanding
Outstanding options as of the beginning of the period (in shares) \| shares		555,000
Outstanding options as of the end of the period (in shares) \| shares		555,000	555,000
Weighted-average exercise price (per share)
Outstanding options as of the beginning of the period (in dollars per share)		$ 31.72
Outstanding options as of the end of the period (in dollars per share)		$ 31.72	$ 31.72
Weighted-average remaining contractual term
Weighted-average remaining contractual term (in years)		5 years 10 months 24 days	6 years
Aggregate intrinsic value \| $		$ 3,585
Options vested and exercisable
Options vested and exercisable - number of options outstanding (in shares) \| shares		92,500
Options vested and exercisable - weighted-average exercise price (in dollars per share)		$ 31.72
Options vested and exercisable - weighted-average remaining contractual term (in years)		5 years 10 months 24 days
Options vested and exercisable - aggregate intrinsic value \| $		$ 598
Performance-based stock options, $13.48 grant date fair value
Stock-Based Compensation
Grant date fair value (in dollars per share)	$ 13.48
Derived service period as adjusted	3 years 1 month 6 days
Performance-based stock options, $10.26 grant date fair value
Stock-Based Compensation
Grant date fair value (in dollars per share)	$ 10.26
Derived service period as adjusted	4 years 7 months 6 days
Performance-based stock options, $8.41 grant date fair value
Stock-Based Compensation
Grant date fair value (in dollars per share)	$ 8.41
Derived service period as adjusted	5 years 3 months 18 days

Stock-Based Compensation - Stock Options - Unrecognized Compensation Cost (Details) - Employee and Nonemployee Stock Options $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Stock Based Compensation
Unrecognized compensation cost, options	$ 44.5
Weighted-average period (in years)	2 years 3 months 18 days

Stock-Based Compensation - Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Number of units outstanding
Nonvested RSUs at the beginning of the period (in shares)	5,665,459
Granted (in shares)	2,160,719
Vested (in shares)	(544,217)
Forfeited and cancelled (in shares)	(175,758)
Nonvested RSUs at the end of the period (in shares)	7,106,203
Weighted-average grant date fair value (per share)
Nonvested RSUs at the beginning of the period (in dollars per share)	$ 29.29
Granted (in dollars per share)	28.37
Vested (in dollars per share)	29.32
Forfeited and cancelled (in dollars per share)	28.69
Nonvested RSUs at the end of the period (in dollars per share)	$ 29.02
Aggregate intrinsic value
Aggregate intrinsic value	$ 271,223	$ 133,648

Stock-Based Compensation - Restricted Stock Units - Unrecognized Compensation Cost (Details) - Restricted Stock Units (RSUs) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Stock Based Compensation
Unrecognized compensation cost, other than options	$ 190.9
Weighted-average period (in years)	3 years 3 months 18 days

Stock-Based Compensation - Valuation Assumptions (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Stock Options
Valuation Assumptions
Fair value of common stock (in dollars per share)	$ 33.01
Expected term (in years)	6 years 29 days	6 years 29 days
Expected volatility (as a percent)	44.09%	47.56%
Risk-free interest rate (as a percent)	2.74%	2.07%
Dividend rate (as a percent)	0.00%	0.00%
Performance-based stock options
Valuation Assumptions
Asset volatility (as a percent)	40.00%
Equity volatility (as a percent)	45.00%
Discount rate (as a percent)	14.00%
Stock price at grant date (in dollars per share)	$ 31.72
Minimum \| Employee Stock Options
Valuation Assumptions
Fair value of common stock (in dollars per share)		$ 31.72
Maximum \| Employee Stock Options
Valuation Assumptions
Fair value of common stock (in dollars per share)		$ 31.96

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock-Based Compensation Expense
Stock-based compensation expense	$ 17,540	$ 9,385
Cost of revenue
Stock-Based Compensation Expense
Stock-based compensation expense	222	138
Research and development
Stock-Based Compensation Expense
Stock-based compensation expense	7,872	4,484
Sales and marketing
Stock-Based Compensation Expense
Stock-based compensation expense	3,859	1,995
General and administrative
Stock-Based Compensation Expense
Stock-based compensation expense	$ 5,587	$ 2,768

Net Loss per Share Attributable to Common Stockholders - General Information (Details)	Mar. 31, 2018 Vote
Common Class A
Net Loss Per Share Attributable to Common Stockholders
Votes per share	1
Common Class B
Net Loss Per Share Attributable to Common Stockholders
Votes per share	10

Net Loss Per Share Attributable to Common Stockholders - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Loss Per Share Attributable to Common Stockholders
Net loss attributable to common stockholders	$ (23,729)	$ (14,227)
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted (in shares)	94,673,557	88,612,804
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.25)	$ (0.16)

Net Loss Per Share Attributable to Common Stockholders - Anti-Dilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Anti-dilutive securities
Total	18,383,913	17,551,471
Employee and Nonemployee Stock Options
Anti-dilutive securities
Total	10,392,199	13,545,334
Restricted Stock Units (RSUs)
Anti-dilutive securities
Total	7,106,203	2,692,636
Class A common stock reserved for Twilio.org
Anti-dilutive securities
Total	635,014	680,397
Employee Stock
Anti-dilutive securities
Total	249,730	594,218
Unvested shares subject to repurchase
Anti-dilutive securities
Total	767	38,886

Transactions with Investors - Vendor Ownership Interest (Details) - Counterparty	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Vendor 1 \| Twilio Inc.
Transactions With Investors
Ownership percentage (as a percent)	1.90%	1.90%
Vendor 2 \| Twilio Inc.
Transactions With Investors
Ownership percentage (as a percent)	1.00%	1.00%
Investor
Transactions With Investors
Vendors, number	2	2	2

Transactions with Investors - Vendor Transactions (Details) - Investor - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Vendor 1
Transactions With Investors
Purchases from vendor	$ 6.3	$ 4.6
Amount due from vendor	2.8
Vendor 2
Transactions With Investors
Purchases from vendor	$ 0.2	$ 0.2

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Taxes
(Provision) benefit for income taxes	$ (137)	$ 30
Tax Cuts and Jobs Act
U.S. corporate income tax rate (as a percent)	21.00%		35.00%