Dropbox (DBX) 10-K 25 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 19, 2019	Jun. 29, 2018
Document Information [Line Items]
Entity Registrant Name	Dropbox, Inc.
Entity Central Index Key	1,467,623
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Emerging Growth Company	false
Entity Small Business	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Public Float			$ 6,438.9
Class A common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		213,030,982
Class B common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		198,242,857
Class C common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		0

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)		Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 519,300,000	$ 430,000,000
Short-term investments		570,000,000	0
Trade and other receivables, net		28,600,000	29,300,000
Prepaid expenses and other current assets		92,300,000	58,800,000
Total current assets		1,210,200,000	518,100,000
Property and equipment, net		310,600,000	341,900,000
Intangible assets, net		14,700,000	17,000,000
Goodwill		96,500,000	98,900,000
Other assets		62,100,000	44,000,000
Total assets		1,694,100,000	1,019,900,000
Current liabilities:
Accounts payable		33,300,000	31,900,000
Accrued and other current liabilities		164,500,000	129,800,000
Accrued compensation and benefits		80,900,000	56,100,000
Capital lease obligation	[1]	73,800,000	102,700,000
Deferred revenue		485,000,000	417,900,000
Total current liabilities		837,500,000	738,400,000
Capital lease obligation, non-current	[1]	89,900,000	71,600,000
Deferred rent, non-current		81,000,000	69,800,000
Other non-current liabilities		8,900,000	37,200,000
Total liabilities		1,017,300,000	917,000,000
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.00001 par value; 240.0 shares authorized and no shares issued and outstanding as of December 31, 2018; no shares authorized, issued and outstanding as of December 31, 2017		0	0
Common stock		0	0
Additional paid-in capital		2,337,500,000	533,100,000
Accumulated deficit		(1,659,500,000)	(1,049,700,000)
Accumulated other comprehensive income (loss)		(1,200,000)	4,200,000
Total stockholders’ equity		676,800,000	102,900,000
Total liabilities and stockholders’ equity		1,694,100,000	1,019,900,000
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017
Stockholders’ equity:
Preferred stock, $0.00001 par value; 240.0 shares authorized and no shares issued and outstanding as of December 31, 2018; no shares authorized, issued and outstanding as of December 31, 2017		$ 0	$ 615,300,000
[1]	Includes amounts attributable to related party transactions. See Note 14, "Related Party Transactions" for further details.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Preferred stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Preferred stock, shares authorized (in shares)	240,000,000	0
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017
Preferred stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Preferred stock, shares authorized (in shares)	0	151,200,000
Preferred stock, shares issued (in shares)	0	147,600,000
Preferred stock, shares outstanding (in shares)	0	147,600,000
Preferred stock, liquidation preference value		$ 624.7
Class A common stock
Common stock, par value (in dollars per share)	$ 0.00001
Common stock, shares authorized (in shares)	2,400,000,000	533,300,000
Common stock, shares issued (in shares)	211,000,000	8,900,000
Common stock, shares outstanding (in shares)	211,000,000	8,900,000
Class B common stock
Common stock, par value (in dollars per share)	$ 0.00001
Common stock, shares authorized (in shares)	475,000,000	466,700,000
Common stock, shares issued (in shares)	198,600,000	187,900,000
Common stock, shares outstanding (in shares)	198,600,000	187,900,000
Class C common stock
Common stock, par value (in dollars per share)	$ 0.00001
Common stock, shares authorized (in shares)	800,000,000	0
Common stock, shares issued (in shares)	0	0
Common stock, shares outstanding (in shares)	0	0

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Income Statement [Abstract]
Revenue		$ 1,391.7		$ 1,106.8		$ 844.8
Cost of revenue	[1]	394.7		368.9		390.6
Gross profit		997		737.9		454.2
Operating expenses
Research and development	[2]	768.2	[3]	380.3	[3]	289.7	[1]
Sales and marketing	[2]	439.6	[3]	314	[3]	250.6	[1]
General and administrative	[2],[3]	283.2		157.3		107.4	[1]
Total operating expenses		1,491	[2],[3]	851.6	[2],[3]	647.7
Loss from operations		(494)		(113.7)		(193.5)
Interest income (expense), net		7.1		(11)		(16.4)
Other income, net		6.8		13.2		4.9
Loss before income taxes		(480.1)		(111.5)		(205)
Provision for income taxes		(4.8)		(0.2)		(5.2)
Net loss		$ (484.9)		$ (111.7)		$ (210.2)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)		$ (1.35)		$ (0.57)		$ (1.11)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)		358.6		195.9		189.1
[1]	Includes stock-based compensation as follows (in millions): for the twelve months ended December 31, 2018, 2017, and 2016. Cost of Revenues of $47.0 million, $12.2 million, and $8.2 million. Research and development of $368.2 million, 93.1 million, and 72.7 million. Sales and marketing of $94.3 million, 33.7 million, and 44.6 million. General and administrative of $140.6 million, 25.6 million, and 22.1 million, respectively.
[2]	During the year ended December 31, 2018, the Company recognized the cumulative unrecognized stock-based compensation of $418.7 million related to the two-tier restricted stock units upon the effectiveness of the Company's registration statement for its initial public offering. See Note 1, "Description of the Business and Summary of Significant Accounting Policies" for further details.
[3]	During the year ended the year ended December 31, 2017, general and administrative expense includes $9.4 million for a non-cash charitable contribution and $1.9 million of cash contributions to the Dropbox Charitable Foundation, a related party. See Note 14, "Related Party Transactions" for further details.

Consolidated Statements of Op_2

Consolidated Statements of Operations (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Donation of common stock to charitable foundation	$ 0	$ 9.4	$ 0
Cash contributions to charitable foundation		1.9
Two-Tier RSUs
Recognized cumulative unrecognized stock-based compensation	418.7
Cost of revenue
Allocated share-based compensation expense	47	12.2	8.2
Research and development
Allocated share-based compensation expense	368.2	93.1	72.7
Sales and marketing
Allocated share-based compensation expense	94.3	33.7	44.6
General and administrative
Allocated share-based compensation expense	$ 140.6	25.6	$ 22.1
Donation of common stock to charitable foundation		$ 9.4

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (484.9)	$ (111.7)	$ (210.2)
Other comprehensive income (loss), net of tax:
Change in foreign currency translation adjustments	(4.9)	5.2	(1.3)
Change in net unrealized losses on short-term investments	(0.5)	0	0
Total other comprehensive income (loss), net of tax	(5.4)	5.2	(1.3)
Comprehensive loss	$ (490.3)	$ (106.5)	$ (211.5)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Class A and Class B common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive income (loss)
Shares outstanding, beginning of the period (in shares) at Dec. 31, 2015		147.6	187.3
Shareholders equity, beginning balance at Dec. 31, 2015	$ 185.6	$ 615.3	$ 0	$ 297.3	$ (727.3)	$ 0.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of early exercised stock options	0.5			0.5
Issuance of common stock, options and awards related to acquisitions (in shares)			0.1
Issuance of common stock, options and awards related to acquisitions	0.7			0.7
Repurchase of unvested common stock (related to early exercised stock options) (in shares)			(0.3)
Repurchase of unvested common stock (related to early exercised stock options)	(0.1)			(0.1)
Stock-based compensation	147.6			147.6
Other comprehensive income (loss)	(1.3)					(1.3)
Net loss	(210.2)				(210.2)
Shareholders equity, ending balance at Dec. 31, 2016	$ 122.8	$ 615.3	$ 0	446	(937.5)	(1)
Shares outstanding, end of the period (in shares) at Dec. 31, 2016		147.6	187.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment from adoption of ASU 2016-09				0.5	(0.5)
Release of restricted stock units (in shares)			14.6
Shares repurchased for tax withholdings on release of restricted stock (in shares)	(5.5)		(5.5)
Shares repurchased for tax withholdings on release of restricted stock	$ (87.9)			(87.9)
Donation of common stock to charitable foundation (in shares)			0.6
Donation of common stock to charitable foundation	$ 9.4			9.4
Exercise of stock options and awards (in shares)	0.1		0.2
Exercise of stock options and awards	$ 0.5			0.5
Repurchase of unvested common stock (related to early exercised stock options) (in shares)			(0.2)
Stock-based compensation	164.6			164.6
Other comprehensive income (loss)	5.2					5.2
Net loss	(111.7)				(111.7)
Shareholders equity, ending balance at Dec. 31, 2017	$ 102.9	$ 615.3	$ 0	533.1	(1,049.7)	4.2
Shares outstanding, end of the period (in shares) at Dec. 31, 2017		147.6	196.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)			40.4
Shares repurchased for tax withholdings on release of restricted stock (in shares)	(15.6)		(15.6)
Shares repurchased for tax withholdings on release of restricted stock	$ (351.8)			(226.9)	(124.9)
Conversion of preferred stock to common stock in connection with initial public offering (in shares)		(147.6)	147.6
Conversion of preferred stock to common stock in connection with initial public offering		$ (615.3)		615.3
Issuance of common stock in connection with initial public offering and private placement, net of underwriters' discounts and commissions and issuance costs (in shares)			37
Issuance of common stock in connection with initial public offering and private placement, net of underwriters' discounts and commissions and issuance costs	$ 739.7			739.7
Exercise of stock options and awards (in shares)	3.4		3.4
Exercise of stock options and awards	$ 26.2			26.2
Stock-based compensation	650.1			650.1
Other comprehensive income (loss)	(5.4)					(5.4)
Net loss	(484.9)
Shareholders equity, ending balance at Dec. 31, 2018	$ 676.8	$ 0	$ 0	$ 2,337.5	$ (1,659.5)	$ (1.2)
Shares outstanding, end of the period (in shares) at Dec. 31, 2018		0	409.6

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Cash flow from operating activities
Net loss	$ (484.9)	$ (111.7)		$ (210.2)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization	166.8	181.8		191.6
Stock-based compensation	650.1	164.6		147.6
Amortization of deferred commissions	12.1	6.6		3.7
Donation of common stock to charitable foundation	0	9.4		0
Other	(1.9)	(1.7)		1.1
Changes in operating assets and liabilities:
Trade and other receivables, net	0.1	(14.4)		1
Prepaid expenses and other current assets	(47.9)	(18.2)		0
Other assets	(11.2)	(10.6)		(7.8)
Accounts payable	(1.7)	16.2		5.5
Accrued and other current liabilities	40.3	34		(12.4)
Accrued compensation and benefits	25	14.4		35.6
Deferred revenue	66.4	64.3		87.6
Non-current liabilities	12.2	(4.4)		9.3
Net cash provided by operating activities	425.4	330.3		252.6
Cash flow from investing activities
Capital expenditures	(63)	(25.3)		(115.2)
Purchase of intangible assets	(3)	(0.8)		(8.5)
Purchases of short-term investments	(850.4)
Proceeds from maturities of short-term investments	212.4	0		0
Proceeds from sales of short-term investments	71.2	0		0
Other	(1)	2.2		5.7
Net cash used in investing activities	(633.8)	(23.9)		(118)
Cash flow from financing activities
Proceeds from initial public offering and private placement, net of underwriters' discounts and commissions	746.6	0		0
Payments of deferred offering costs	(4.5)	(2.5)		0
Shares repurchased for tax withholdings on release of restricted stock	(351.9)	(87.9)		0
Proceeds from issuance of common stock, net of repurchases	26.2	0.5		0
Principal payments on capital lease obligations	(109.1)	(133)	[1]	(137.9)	[1]
Principal payments against note payable	(3.5)	(3.9)		(3.8)
Proceeds from sale-leaseback agreement	0	0		8.8
Fees paid for revolving credit facility	(0.4)	(2.6)		0
Other	(2.6)	(2.3)		(1.6)
Net cash provided by (used in) financing activities	300.8	(231.7)		(134.5)
Effect of exchange rate changes on cash and cash equivalents	(3.1)	2.6		(4.3)
Change in cash and cash equivalents	89.3	77.3		(4.2)
Cash and cash equivalents—beginning of period	430	352.7		356.9
Cash and cash equivalents—end of period	519.3	430		352.7
Supplemental cash flow data:
Interest	8.3	10.8		14.9
Income taxes	1.4	3.4		1.5
Non-cash investing and financing activities:
Property and equipment received and accrued in accounts payable and accrued liabilities	7.3	2.4		7.6
Property and equipment acquired under capital leases	98.5	44.9		92.2
Fair value of shares issued related to acquisitions of businesses and other assets	0	0		0.7
Deferred offering costs accrued in accounts payable and accrued liabilities	$ 0	$ 1.6		$ 0
[1]	Includes amounts attributable to related party transactions. See Note 14, "Related Party Transactions" for further details.

Description of the Business and

Description of the Business and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the Business and Summary of Significant Accounting Policies	Description of the Business and Summary of Significant Accounting Policies Business Dropbox, Inc. (the “Company” or “Dropbox”) is a global collaboration platform. Dropbox was incorporated in May 2007 as Evenflow, Inc., a Delaware corporation, and changed its name to Dropbox, Inc. in October 2009. The Company is headquartered in San Francisco, California. Basis of presentation and consolidation The accompanying consolidated financial statements have been prepared in accordance with the United States of America generally accepted accounting principles (“GAAP”). The accompanying consolidated financial statements include the accounts of Dropbox and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Initial public offering and private placement On March 27, 2018, the Company closed its initial public offering ("IPO"), in which the Company issued and sold 26,822,409 shares of Class A common stock at $21.00 per share. The Company received aggregate proceeds of $538.2 million , net of underwriters' discounts and commissions, before deducting offering costs of $6.9 million , net of reimbursements. Immediately prior to the closing of the Company’s IPO, 147,310,563 shares of convertible preferred stock outstanding converted into an equivalent number of shares of Class B common stock. Further, pursuant to transfer agreements with certain of the Company’s stockholders, 258,620 shares of the Company’s convertible preferred stock and 2,609,951 shares of the Company’s Class B common stock automatically converted into an equivalent number of shares of Class A common stock. Immediately subsequent to the closing of the Company's IPO, Salesforce Ventures LLC purchased 4,761,905 shares of Class A common stock from the Company at $21.00 per share. The Company received aggregate proceeds of $100.0 million and did not pay any underwriting discounts or commissions with respect to the shares that were sold in the private placement. On March 28, 2018, the underwriters exercised their option to purchase an additional 5,400,000 shares of the Company's Class A common stock at $21.00 per share. This transaction closed on April 3, 2018, resulting in additional proceeds of $108.4 million , net of underwriters' discounts and commissions. The Company’s net proceeds from the IPO, the concurrent private placement, and underwriters' option totaled $746.6 million , before deducting offering costs of $6.9 million , net of reimbursements. Upon the effectiveness of the registration statement for the Company's IPO, which was March 22, 2018, the liquidity event-related performance vesting condition, referred to as the Performance Vesting Condition, associated with the Company's two-tier restricted stock units ("RSUs") was satisfied. As a result, the Company recognized the cumulative unrecognized stock-based compensation related to its two-tier RSUs using the accelerated attribution method of $418.7 million attributable to service prior to such effective date. As of December 31, 2018 , the remaining unamortized stock-based compensation related to the two-tier RSUs was $0.1 million , which will be recognized if the requisite service is provided over a weighted average period of 0.2 years. During the first quarter of 2018, the Company's Board of Directors approved the acceleration of the Performance Vesting Condition for which the service condition was satisfied, to occur upon the effectiveness of the registration statement for the Company's IPO, rather than six months following an IPO. As a result, the Company released 26.8 million shares of common stock underlying the two-tier RSUs for which the Performance Vesting Condition was satisfied, and recorded $13.9 million in employer related payroll tax expenses associated with these same awards. See “—Stock-based compensation” for further discussion regarding the Company's two-tier RSUs. Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported and disclosed in the Company’s consolidated financial statements and accompanying notes. These estimates are based on information available as of the date of the consolidated financial statements. On a regular basis, management evaluates these estimates and assumptions. Actual results may differ materially from these estimates. The Company’s most significant estimates and judgments involve the measurement of the Company’s stock-based compensation, including the estimation of the underlying deemed fair value of common stock for periods prior to the Company's IPO, and the estimation of the fair value of market-based awards. Financial information about segments and geographic areas The Company manages its operations and allocates resources as a single operating segment. Further, the Company manages, monitors, and reports its financials as a single reporting segment. The Company’s chief operating decision-maker is its Chief Executive Officer, who reviews financial information presented on a consolidated basis for purposes of making operating decisions, assessing financial performance, and allocating resources. See Note 15, "Geographic Areas" for information regarding the Company's long-lived assets and revenue by geography. Stock Split On March 7, 2018, the Company effected a 1-for-1.5 reverse stock split of its capital stock. All of the share and per share information referenced throughout the consolidated financial statements and notes to the consolidated financial statements have been retroactively adjusted to reflect this reverse stock split. Foreign currency transactions The assets and liabilities of the Company’s foreign subsidiaries are translated from their respective functional currencies into U.S. dollars at the rates in effect at the balance sheet date and revenue and expense amounts are translated at the average exchange rate for the period. Foreign currency translation gains and losses are recorded in other comprehensive income (loss). Gains and losses realized from foreign currency transactions (those transactions denominated in currencies other than the foreign subsidiaries’ functional currency) are included in other income, net. Monetary assets and liabilities are remeasured using foreign currency exchange rates at the end of the period, and non-monetary assets are remeasured based on historical exchange rates. The Company recorded $ 1.9 million and $3.6 million in net foreign currency transaction losses in the years ended December 31, 2018 and 2016 , respectively, and recorded $5.0 million in net foreign currency gains in the year ended December 31, 2017 . Revenue recognition The Company derives its revenue from subscription fees from customers for access to its platform. The Company’s policy is to exclude sales and other indirect taxes when measuring the transaction price of its subscription agreements. The Company accounts for revenue contracts with customers 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 The Company’s subscription agreements generally have monthly or annual contractual terms and a small percentage have multi-year contractual terms. Revenue is recognized ratably over the related contractual term beginning on the date that the platform is made available to a customer. Access to the platform represents a series of distinct services as the Company continually provides access to, and fulfills its obligation to the end customer over the subscription term. The series of distinct services represents a single performance obligation that is satisfied over time. The Company recognizes revenue ratably because the customer receives and consumes the benefits of the platform throughout the contract period. The Company’s contracts are generally non-cancelable. The Company bills in advance for monthly contracts and typically bills annually in advance for contracts with terms of one year or longer . The Company also recognizes an immaterial amount of unbilled receivables, primarily relating to consideration for services completed but not billed at the reporting date. Unbilled receivables are classified as receivables when the Company has the right to invoice the customer. The Company records contract liabilities when cash payments are received or due in advance of performance to deferred revenue. Deferred revenue primarily relates to the advance consideration received from the customer. The price of subscriptions is generally fixed at contract inception and therefore, the Company’s contracts do not contain a significant amount of variable consideration. As a result, the amount of revenue recognized in the periods presented from performance obligations satisfied (or partially satisfied) in previous periods was not material. The Company recognized $411.6 million , $353.0 million , and $266.9 million of revenue during the years ended December 31, 2018 , 2017 and 2016 , respectively, that was included in the deferred revenue balances at the beginning of the respective periods. As of December 31, 2018 , future estimated revenue related to performance obligations that are unsatisfied or partially unsatisfied at the end of the reporting period was $529.9 million . The substantial majority of the unsatisfied performance obligations will be satisfied over the next twelve months. Stock-based compensation The Company has granted RSUs to its employees and members of the Board of Directors under the 2008 Equity Incentive Plan (“2008 Plan”), the 2017 Equity Incentive Plan (“2017 Plan”), and the 2018 Equity Incentive Plan ("2018 Plan"). The Company had two types of RSUs outstanding as of December 31, 2018 : • One-tier RSUs, which have a service-based vesting condition over a four -year period. These awards typically have a cliff vesting period of one year and continue to vest quarterly thereafter. The Company began granting one-tier RSUs under its 2008 Plan in August 2015 and it continues to grant one-tier RSUs under its 2018 Plan. The Company recognizes compensation expense associated with one-tier RSUs ratably on a straight-line basis over the requisite service period and accounts for forfeitures in the period in which they occur. • Two-tier RSUs, which have both a service-based vesting condition and a Performance Vesting Condition. The service-based vesting period for these awards is typically four years with a cliff vesting period of one year and continue to vest monthly thereafter. Upon satisfaction of the Performance Vesting Condition, these awards vest quarterly. The Performance Vesting Condition was satisfied on the the effectiveness of the registration statement related to the Company's IPO. Prior to August 2015, the Company granted two-tier RSUs under the 2008 Plan. The last grant date for two-tier RSUs was in May 2015. The Company recognizes compensation expense associated with two-tier RSUs using the accelerated attribution method over the requisite service period. The Performance Vesting Condition for the two-tier RSUs was satisfied upon the effectiveness of the registration statement related to the Company's IPO, which was March 22, 2018. On that date, the Company recognized the cumulative unrecognized expense of the two-tier RSUs, using the accelerated attribution method, which is included in the Company's results for the year ended December 31, 2018. See "—Initial public offering and private placement” for further discussion. As of December 31, 2018, the remaining unamortized stock-based compensation related to the two-tier RSUs was $0.1 million , which will be recognized if the requisite service is provided over a weighted average period of 0.2 years. Since August 2015, the Company has granted RSUs as the only stock-based payment awards to its employees, with the exception of awards granted to its co-founders, and has not granted any stock options since then. The fair values of the common stock underlying the RSUs granted in periods prior to the date of the Company's IPO were determined by the Board of Directors, with input from management and contemporaneous third-party valuations, which were performed at least quarterly. Given the absence of a public trading market for the Company's common stock prior to its IPO, and in accordance with the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation, or AICPA Guide, the Company's Board of Directors exercised reasonable judgment and considered numerous objective and subjective factors to determine the best estimate of the fair value of its common stock including: • The results of contemporaneous valuations of its common stock by unrelated third parties; • The rights, preferences, and privileges of its convertible preferred stock relative to those of its common stock; • Market multiples of comparable public companies in its industry as indicated by their market capitalization and guideline merger and acquisition transactions; • The Company's performance and market position relative to its competitors, who may change from time to time; • The Company's historical financial results and estimated trends and prospects for its future performance; • Valuations published by institutional investors that hold investments in its capital stock; • The economic and competitive environment; • The likelihood and timeline of achieving a liquidity event, such as an initial public offering or sale, given prevailing market conditions; • Any adjustments necessary to recognize a lack of marketability for its common stock; and • Precedent sales of or offers to purchase its capital stock. In valuing the Company's common stock, the Board of Directors determined the fair value of its common stock using both the income and market approach valuation methods. The income approach estimates value based on the expectation of future cash flows that a company will generate. These future cash flows are discounted to their present values using a discount rate based on our weighted average cost of capital, and is adjusted to reflect the risks inherent in the Company's cash flows. The market approach estimates value based on a comparison of the subject company to comparable public companies in a similar line of business. From the comparable companies, a representative market value multiple is determined and then applied to the subject company’s financial forecasts to estimate the value of the subject company. For valuations after the Company's IPO, the Board of Directors determines the fair value of each share of underlying common stock based on the closing price of the Company's Class A common stock as reported on the Nasdaq Global Select Market on the date of the grant. In December 2017, the Board of Directors approved a grant to the Company’s co-founders of restricted stock awards (“RSAs”) with respect to 14.7 million shares of Class A Common Stock in the aggregate (collectively, the “Co-Founder Grants”), of which 10.3 million RSAs were granted to Mr. Houston, the Company’s co-founder and Chief Executive Officer, and 4.4 million RSAs were granted to Mr. Ferdowsi, the Company’s co-founder and Director. These Co-Founder Grants have service-based, market-based, and performance-based vesting conditions. The Co-Founder Grants comprise nine tranches that are eligible to vest based on the achievement of stock price goals, or, each, a Stock Price Target. The Company estimated the grant date fair value of the Co-Founder Grants using a model based on multiple stock price paths developed through the use of a Monte Carlo simulation that incorporates into the valuation the possibility that the Stock Price Targets may not be satisfied. The average grant date fair value of each Co-Founder Grant was estimated to be $10.60 per share, and the Company will recognize aggregate stock-based compensation expense of $156.2 million over the requisite service period of each tranche, which ranged from 2.9 to 6.9 years, using the accelerated attribution method. If the Stock Price Targets are met sooner than the derived service period, the Company will adjust its stock-based compensation to reflect the cumulative expense associated with the vested awards. The Company will recognize expense if the requisite service is provided, regardless of whether the market conditions are achieved. The Co-Founder Grants contain an implied performance-based vesting condition because the Stock Price Targets are based on the trailing 30-day average price of the shares from an established national securities exchange or automated quotation system. Accordingly, no vesting could occur until the completion of the Company’s IPO. The relevant performance-based vesting condition for the Co-Founder Grants was satisfied on the date the Company’s shares of Class A common stock commenced trading on the Nasdaq Global Select Market, in connection with the Company’s IPO, which was March 23, 2018. The Company recognized $37.0 million in stock-based compensation related to the Co-Founder Grants during the year ended December 31, 2018. On January 1, 2017, the Company adopted ASU No. 2016-09: Improvement to Employee Share-based Payment Accounting (Topic 718) issued by the Financial Accounting Standards Board, which among other items, provides an accounting policy election to account for forfeitures as they occur, rather than to account for them based on an estimate of expected forfeitures. The Company elected to account for forfeitures as they occur and therefore, stock-based compensation expense for the years ended December 31, 2018 and 2017, has been calculated based on actual forfeitures in the Company’s consolidated statements of operations, rather than the Company’s previous approach which was net of estimated forfeitures. The net cumulative effect of this change as of January 1, 2017, was not material. Stock-based compensation expense for the year ended December 31, 2016, was recorded net of estimated forfeitures, which were based on historical forfeitures and adjusted to reflect changes in facts and circumstances, if any. Cost of revenue Cost of revenue consists primarily of expenses associated with the storage, delivery, and distribution of the Company’s platform for both paying users and Basic users. These costs, which are referred to as infrastructure costs, include depreciation of servers located in co-location facilities that the Company leases and operates, rent and facilities expense for those datacenters, network and bandwidth costs, support and maintenance costs for infrastructure equipment, and payments to third-party datacenter service providers. Cost of revenue also includes costs, such as salaries, bonuses, benefits, travel-related expenses, and stock-based compensation, which are referred to as employee-related costs, for employees whose primary responsibilities relate to supporting the Company’s infrastructure and delivering user support. Other non-employee costs included in cost of revenue include credit card fees related to processing customer transactions and allocated overhead, such as facilities, including rent, utilities, depreciation on leasehold improvements and other equipment shared by all departments, and shared information technology costs. In addition, cost of revenue includes amortization of developed technologies, professional fees related to user support initiatives, and property taxes related to the datacenters. Advertising and promotional expense Advertising and promotional expenses are included in sales and marketing expenses within the consolidated statements of operations and are expensed when incurred. Advertising and promotional expenses were $ 100.9 million, $80.1 million , and $46.6 million in the years ended December 31, 2018 , 2017 , and 2016 , respectively. Cash and cash equivalents Cash consists primarily of cash on deposit with banks. Cash equivalents include highly liquid investments purchased with an original maturity date of 90 days or less from the date of purchase and primarily consist of money market funds. Cash equivalents also include amounts in transit from payment processors for credit and debit card transactions, which typically settle within five days. Cash and cash equivalents are recorded at cost, which approximates fair value. Short-term investments The Company’s short-term investments are primarily comprised of corporate notes and obligations, U.S. treasury securities, certificates of deposits, U.S. agency obligations, and commercial paper. The Company determines the appropriate classification of its short-term investments at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified and accounted for its short-term investments as available-for-sale securities as the Company may sell these securities at any time for use in its current operations or for other purposes, even prior to maturity. As a result, the Company classifies its short-term investments, including securities with stated maturities beyond twelve months, within current assets in the consolidated balance sheets. The Company's short-term investments are classified as available-for-sale securities and are recorded at fair value each reporting period. Unrealized gains and losses on these short-term investments are reported as a separate component of accumulated other comprehensive income (loss) in the consolidated balance sheets until realized. Interest income is reported within interest income (expense), net in the consolidated statements of operations. The Company periodically evaluates its short-term investments to assess whether those with unrealized loss positions are other-than-temporarily impaired. The Company considers various factors in determining whether to recognize an impairment charge, including the length of time the investment has been in a loss position, the extent to which the fair value is less than the Company’s cost basis, and the financial condition and near-term prospects of the investee. Realized gains and losses are determined based on the specific identification method and are reported in other income, net in the consolidated statements of operations. If the Company determines that the decline in an investment’s fair value is other-than-temporary, the difference is recognized as an impairment loss in the consolidated statements of operations. Trade and other receivables, net Trade and other receivables, net consists primarily of trade receivables that are recorded at the invoice amount, net of an allowance for doubtful accounts. The allowance for doubtful accounts is based on the Company’s assessment of the collectability of accounts. The Company regularly reviews the adequacy of the allowance for doubtful accounts by considering the age of each outstanding invoice, the collection history of each customer, and other relevant factors to determine the appropriate amount of the allowance. Accounts receivable deemed uncollectable are charged against the allowance for doubtful accounts when identified. The Company's allowance for doubtful accounts was $1.2 million and $1.0 million as of December 31, 2018 and 2017, respectively. Concentrations of credit risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, short-term investments, and accounts receivable. The Company places its cash, cash equivalents, and short-term investments with well-established financial institutions. Cash equivalents consist primarily of highly rated money market funds. Trade accounts receivables are typically unsecured and are derived from revenue earned from customers located around the world. Two customers accounted for 14% and 23% of total trade and other receivables, net as of December 31, 2018 . Two customers accounted for 18% and 27% of total trade and other receivables, net as of December 31, 2017 . No customer accounted for more than 10% of the Company’s revenue in the periods presented. Non-trade receivables The Company records non-trade receivables to reflect amounts due for activities outside of its subscription agreements. Historically, the Company’s non-trade receivables have related primarily to receivables resulting from tenant improvement allowances. Non-trade receivables totaled $ 46.2 million and $5.2 million , as of December 31, 2018 and 2017 , respectively, and are classified within prepaid expenses and other current assets in the accompanying consolidated balance sheets. The Company recognized its initial tenant improvement allowance receivable related to its new corporate headquarters once it took initial possession of the first phase in June 2018. As of December 31, 2018 , $40.5 million is included in prepaid expenses and other current assets related to this tenant improvement allowance receivable. See "—Lease obligations” for further discussion on the corresponding recording of the lease incentive obligation. Deferred commissions, net Deferred commissions, net is stated at gross deferred commissions less accumulated amortization. Sales commissions earned by the Company’s sales force and third-party resellers, as well as related payroll taxes, are considered to be incremental and recoverable costs of obtaining a contract with a customer. As a result, these amounts have been capitalized as deferred commissions within prepaid and other current assets and other assets on the consolidated balance sheet. The Company deferred incremental costs of obtaining a contract of $32.0 million and $19.4 million during the years ended December 31, 2018 and 2017 , respectively. Deferred commissions, net included in prepaid and other current assets were $14.5 million and $8.1 million as of December 31, 2018 and 2017 , respectively. Deferred commissions, net included in other assets were $38.3 million and $24.8 million as of December 31, 2018 and 2017 , respectively. Deferred commissions are amortized over a period of benefit of five years. The period of benefit was estimated by considering factors such as historical customer attrition rates, the useful life of the Company’s technology, and the impact of competition in its industry. Amortized costs were $12.1 million , $6.6 million , and $3.7 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Amortized costs are included in sales and marketing expense in the accompanying consolidated statements of operations. There was no impairment loss in relation to the deferred costs for any period presented. Property and equipment, net Equipment is stated at cost less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful life of the related asset, which is generally three to seven years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the related lease. In the first quarter of 2018, the Company determined that the useful lives of certain infrastructure equipment, which are depreciated through cost of revenue, should be increased from three to four years. The Company accounted for this as a change in estimate that was applied prospectively, effective as of January 1, 2018. This change in useful life resulted in a reduction in depreciation expense within cost of revenue of $16.1 million during the year ended December 31, 2018. The following table presents the estimated useful lives of property and equipment: Property and equipment Useful life Buildings 20 to 30 years Datacenter and other computer equipment 3 to 5 years Office equipment and other 3 to 7 years Leasehold improvements Lesser of estimated useful life or remaining lease term Lease obligations The Company leases office space, datacenters, and equipment under non-cancelable capital and operating leases with various expiration dates through 2033. Certain of the Company’s operating lease agreements contain tenant improvement allowances from its landlords. These allowances are accounted for as lease incentive obligations and are amortized as reductions to rent expense over the lease term. In June 2018, the Company took initial possession of the first phase of its new corporate headquarters and recorded a lease incentive obligation related to this phase. As of December 31, 2018, $2.6 million was included in accrued and other current liabilities and $36.4 million was included in deferred rent, non-current in the Company's consolidated balance sheets. In addition, certain of the operating lease agreements contain rent concession, rent escalation, and option to renew provisions. Rent concession and rent escalation provisions are considered in determining the straight-line rent expense to be recorded over the lease term. Lease expense is recognized on a straight-line basis over the lease term commencing on the date the Company has the right to use the leased property. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed to be reasonably assured at lease inception. In 2012, the Company undertook a series of structural improvements to the floor that it occupied in its previous corporate headquarters. As a result of the requirement to fund construction costs and its responsibility for cost overruns during the construction period, the Company was considered the deemed owner of the floor for accounting purposes. Due to the presence of a standby letter of credit as a security deposit, the Company was deemed to have continuing involvement after the construction period. As such, it accounted for this arrangement as owned real estate and recorded a building asset and an imputed financing obligation for its liability to the legal owners. In September 2018, the Company terminated its master lease for its previous corporate headquarters, the impact of which is described in Note 4, "Property and Equipment, Net". The Company leases certain equipment from various third parties, including from a related party, through equipment financing leases under capital leases. See Note 14, “Related Party Transactions” for additional details. These leases either include a bargain purchase option, a full transfer of ownership at the completion of the lease term, or the terms of the leases are at least 75 percent of the useful lives of the assets and are therefore classified as capital leases. These leases are capitalized in property and equipment, net and the related amortization of assets under capital leases is included in depreciation and amortization expense in the Company’s consolidated statements of operations. Initial asset values and lease obligations are based on the present value of future minimum lease payments. Internal use software The Company capitalizes certain costs related to developed or modified software solely for its internal use and cloud based applications used to deliver its platform. The Company capitalizes costs during the application development stage once the preliminary project stage is complete, management authorizes and commits to funding the project, and it is probable that the project will be completed and that the software will be used to perform the function intended. Costs related to preliminary project activities and post implementation activities are expensed as incurred. Capitalized internal use software costs were not material to the Company’s consolidated financial statements during the years ended December 31, 2018 , 2017 , and 2016 . Business combinations The Company uses best estimates and assumptions to assign a fair value to the tangible and intangible assets acquired and liabilities assumed in business combinations as of the acquisition date. These estimates are inherently uncertain and subject to refinement. During the measurement period, which may be up to one year from the acquisition date, adjustments to the fair value of these tangible and intangible assets acquired and liabilities assumed may be recorded, with the corresponding offset to goodwill. Upon the conclusion of the measurement period or final determination of the fair value of assets acquired or liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the Company’s consolidated statements of operations. Long-lived assets, including goodwill and other acquired intangible asset

Cash, Cash Equivalents and Shor

Cash, Cash Equivalents and Short-Term Investments	12 Months Ended
	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash, Cash Equivalents and Short-Term Investments	Cash, Cash Equivalents and Short-Term Investments The amortized cost, unrealized gains and losses and estimated fair value of the Company's cash, cash equivalents and short-term investments as of December 31, 2018 consisted of the following: Amortized cost Unrealized gain Unrealized loss Estimated fair value Cash $ 103.0 $ — $ — 103.0 Cash equivalents Money market funds 355.5 — — 355.5 Commercial paper 27.4 — — 27.4 U.S. Treasury securities 33.4 — — 33.4 Total cash and cash equivalents $ 519.3 $ — $ — $ 519.3 Short-term investments Corporate notes and obligations 269.6 0.1 (0.5 ) 269.2 U.S. Treasury securities 176.0 — (0.1 ) 175.9 Certificates of deposit 70.6 — — 70.6 U.S. agency obligations 37.1 — — 37.1 Commercial paper 17.2 — — 17.2 Total short-term investments 570.5 0.1 (0.6 ) 570.0 Total $ 1,089.8 $ 0.1 $ (0.6 ) $ 1,089.3 The Company's cash and cash equivalents at December 31, 2017 consisted of cash of $62.9 million and money market funds of $367.1 million . The Company did not have short-term investments as of December 31, 2017. Included in cash and cash equivalents is cash in transit from payment processors for credit and debit card transactions of $11.9 million and $13.3 million as of December 31, 2018 and December 31, 2017, respectively. All short-term investments were designated as available-for-sale securities as of December 31, 2018 . The following table presents the contractual maturities of the Company’s short-term investments as of December 31, 2018 : Amortized cost Estimated fair value Due within one year $ 327.6 $ 327.5 Due between one to three years 242.9 242.5 Total $ 570.5 $ 570.0 As of December 31, 2018 , the following short-term investments were in unrealized loss positions: Less than 12 months 12 months or greater Total Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses Corporate notes and obligations $ 209.5 $ (0.5 ) $ — $ — $ 209.5 $ (0.5 ) U.S. Treasury securities 127.8 (0.1 ) — — 127.8 (0.1 ) Total $ 337.3 $ (0.6 ) $ — $ — $ 337.3 $ (0.6 ) The Company had 202 short-term investments in unrealized loss positions as of December 31, 2018 . There were no material gross unrealized losses from available-for-sale securities and no material realized gains or losses from available-for-sale securities that were reclassified out of accumulated other comprehensive income for the year ended December 31, 2018 . For investments in available-for-sale debt securities that have unrealized losses, the Company evaluates whether (i) it has the intention to sell any of these investments and (ii) whether 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. Based on this evaluation, the Company determined that there were no other-than-temporary impairments associated with short-term investments as of December 31, 2018 . The Company recorded $16.8 million , $3.0 million , and $0.9 million in interest income from its cash, cash equivalents and short-term investments for the years ended December 31, 2018, 2017 and 2016, respectively.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company measures its financial instruments at fair value each reporting period using a fair value hierarchy that prioritizes the use of observable inputs and minimizes the use of unobservable inputs when measuring fair value. A financial instrument’s classification within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis using the input categories discussed in Note 1: As of December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 355.5 $ — $ — $ 355.5 U.S Treasury securities — 33.4 — 33.4 Commercial paper — 27.4 — 27.4 Total cash equivalents $ 355.5 $ 60.8 $ — $ 416.3 Short-term investments Corporate notes and obligations — 269.2 — 269.2 U.S. Treasury securities — 175.9 — 175.9 Certificates of deposit — 70.6 — 70.6 U.S. agency obligations — 37.1 — 37.1 Commercial paper — 17.2 — 17.2 Total short-term investments — 570.0 — 570.0 Total cash equivalents and short-term investments $ 355.5 $ 630.8 $ — $ 986.3 The total cash equivalents held by the Company as of December 31, 2017 were $367.1 million and were entirely comprised of money market funds classified within Level 1 of the fair value hierarchy. The Company had no transfers between levels of the fair value hierarchy. The carrying amounts of certain financial instruments, including cash held in banks, accounts receivable and accounts payable approximate fair value due to their short-term maturities and are excluded from the fair value table above.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net consisted of the following: As of December 31, 2018 2017 Building $ — $ 36.6 Datacenter and other computer equipment 695.3 663.1 Furniture and fixtures 23.8 21.2 Leasehold improvements 122.6 118.6 Construction in process 32.8 7.2 Total property and equipment 874.5 846.7 Accumulated depreciation and amortization (563.9 ) (504.8 ) Property and equipment, net $ 310.6 $ 341.9 As described in Note 1, "Description of the Business and Summary of Significant Accounting Policies", the Company terminated its master lease for its previous corporate headquarters in September 2018. The termination resulted in de-recognizing the gross building asset of $36.6 million , related accumulated depreciation of $12.2 million , and an imputed financing obligation of $25.5 million , of which $2.8 million was included in accrued and other current liabilities and $22.7 million was included in other non-current liabilities. The Company recorded the difference between the net asset and liability that were de-recognized to other income, net during the year ended December 31, 2018 . The Company leases certain infrastructure from various third parties, including from a former related party, through equipment financing leases under capital leases. See Note 14, “Related Party Transactions” for additional details. Infrastructure assets as of December 31, 2018 and 2017 , respectively, included a total of $ 362.8 million and $417.9 million acquired under capital lease agreements. These leases are capitalized in property and equipment, and the related amortization of assets under capital leases is included in depreciation and amortization expense. The accumulated amortization of the infrastructure assets under capital leases totaled $ 211.7 million and $259.0 million as of December 31, 2018 and 2017 , respectively. Construction in process includes costs primarily related to construction of leasehold improvements for office buildings and datacenters, and as of December 31, 2018, primarily relates to costs incurred related to our new corporate headquarters. Depreciation expense related to property and equipment was $ 158.6 million, $170.7 million , and $173.8 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively.

Business Combinations

Business Combinations	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Business Combinations	Business Combinations The Company did not complete any business combinations during the years ended December 31, 2018, 2017 and 2016.

Intangible Assets, Net

Intangible Assets, Net	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets, Net	Intangible Assets, Net Intangible assets consisted of the following: As of December 31, Weighted- (In years) 2018 2017 Developed technology $ 47.0 $ 50.9 0.0 Patents 13.0 13.0 8.1 Software 19.2 17.8 2.3 Assembled workforce in asset acquisitions 12.6 10.1 1.8 Licenses 4.6 4.6 2.4 Non-compete agreements, trademarks and other 4.0 4.0 6.1 Total intangibles 100.4 100.4 Accumulated amortization (85.7 ) (83.4 ) Intangible assets, net $ 14.7 $ 17.0 Amortization expense was $ 6.1 million, $10.5 million , and $17.3 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Expected future amortization expense for intangible assets as of December 31, 2018 , is as follows: 2019 $ 4.7 2020 3.8 2021 1.5 2022 0.8 2023 0.8 Thereafter 3.1 Total $ 14.7

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill Goodwill represents the excess of the purchase price in a business combination over the fair value of net tangible and intangible assets acquired. Goodwill amounts are not amortized, but tested for impairment on an annual basis. There was no impairment of goodwill as of December 31, 2018 and 2017 . Goodwill consisted of the following: Balance at December 31, 2016 $ 96.0 Effect of foreign currency translation 2.9 Balance at December 31, 2017 98.9 Effect of foreign currency translation (2.4 ) Balance at December 31, 2018 $ 96.5

Accrued and Other Current Liabi

Accrued and Other Current Liabilities	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued and Other Current Liabilities Accrued and other current liabilities consisted of the following: As of December 31, 2018 2017 Non-income taxes payable $ 75.7 $ 69.7 Accrued legal and other external fees 28.1 21.3 Deferred rent 41.0 14.6 Financing obligations, current 2.6 9.7 Income taxes payable 0.3 0.4 Other accrued and current liabilities 16.8 14.1 Total accrued and other current liabilities $ 164.5 $ 129.8 Deferred rent as of December 31, 2018 increased from December 31, 2017 primarily due to the amendment of the lease for the Company's current corporate headquarters and the shortening of its lease term in May 2018, as described further in Note 10, "Commitments and Contingencies". This lease modification resulted in a reclassification of deferred rent from deferred rent, non-current to accrued and other current liabilities, and is included in the deferred rent amount in the table above.

Revolving Credit Facility

Revolving Credit Facility	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Revolving Credit Facility	Revolving Credit Agreement In April 2017, the Company entered into an amended and restated credit and guaranty agreement which provided for a $600.0 million revolving loan facility (the “revolving credit facility”). In conjunction with the revolving credit facility, the Company paid upfront issuance fees of $2.6 million , which are being amortized over the five -year term of the agreement. In February 2018, the Company amended its revolving credit facility to, among other things, permit the Company to make certain investments, enter into an unsecured standby letter of credit facility and increase its standby letter of credit sublimit to $187.5 million . The Company increased its borrowing capacity under the revolving credit facility from $600.0 million to $725.0 million . The Company may from time to time request increases in its borrowing capacity under the revolving credit facility of up to $275.0 million , provided no event of default has occurred or is continuing or would result from such increase. In conjunction with the amendment, the Company paid upfront issuance fees of $0.4 million , which are being amortized over the remaining term of the agreement. Pursuant to the terms of the revolving credit facility, the Company may issue letters of credit under the revolving credit facility, which reduce the total amount available for borrowing. Pursuant to the terms of the revolving credit facility, the Company is required to pay an annual commitment fee that accrues at a rate of 0.20% per annum on the unused portion of the borrowing commitments under the revolving credit facility. In addition, the Company is required to pay a fee in connection with letters of credit issued under the revolving credit facility, which accrues at a rate of 1.5% per annum on the amount of such letters of credit outstanding. There is an additional fronting fee of 0.125% per annum multiplied by the average aggregate daily maximum amount available under all letters of credit. Borrowings under the revolving credit facility bear interest, at the Company’s option, at an annual rate based on LIBOR plus a spread of 1.50% or at an alternative base rate plus a spread of 0.50% . The revolving credit facility contains customary conditions to borrowing, events of default and covenants, including covenants that restrict the Company’s ability to incur indebtedness, grant liens, make distributions to holders of the Company or its subsidiaries’ equity interests, make investments, or engage in transactions with its affiliates. In addition, the revolving credit facility contains financial covenants, including a consolidated leverage ratio covenant and a minimum liquidity balance of $100.0 million , which includes any available borrowing capacity. The Company was in compliance with the covenants of the revolving credit facility as of December 31, 2018 and December 31, 2017, respectively. The Company had an aggregate of $68.5 million of letters of credit outstanding under the revolving credit facility as of December 31, 2018, and the Company’s total available borrowing capacity under the revolving credit facility was $656.5 million as of December 31, 2018. The Company’s letters of credit expire between April of 2019 and April of 2022.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company has entered into various non-cancelable operating lease agreements for certain offices and datacenters with contractual lease periods expiring at various dates through 2033. The facility lease agreements generally provide for escalating rental payments and for options to renew, which could increase future minimum lease payments if exercised. The Company recognizes rent expense on a straight-line basis over the lease period and accounts for the difference between straight-line rent and actual lease payments as deferred rent. Gross rent expense was $ 85.9 million, $71.0 million , $67.9 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Sublease income, which is recorded as a reduction of rental expense, was $ 11.8 million, $10.6 million , and $4.5 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Sublease income in excess of the Company’s original lease obligation is split with the original lessor per the terms of the sublease agreement, with the Company’s portion recorded to other income, net. In 2018, the Company amended its lease agreement for its current corporate headquarters, modifying the original lease termination date from 2027 to 2019. As a result of the amendment, the Company expects to vacate its current corporate headquarters in August 2019, but is obligated to pay rent through February 2020. Accordingly, the Company's future operating lease commitments were reduced by $192.4 million . As described in Note 1, "Description of the Business and Summary of Significant Accounting Policies”, the Company was considered the deemed owner of a floor in its previous corporate headquarters, for accounting purposes, as part of a build-to-suit lease agreement and initially recorded a building asset and an imputed financing obligation for its obligation to the legal owner in the amount of $36.6 million . As of December 31, 2017, the imputed financing obligation totaled $27.4 million , of which $2.6 million was classified within accrued and other current liabilities, and $24.8 million was classified within other non-current liabilities. In 2018, the Company terminated its master lease for its previous corporate headquarters. As the master lease was nearing expiration, the early termination did not have a material impact on the Company's future operating lease commitments. In 2017, the Company entered into a new lease agreement for office space in San Francisco, California, to serve as its new corporate headquarters. The Company took initial possession of the first phase of its new corporate headquarters in June 2018 and began to recognize rent expense. The Company expects to start making recurring rental payments under the lease in the third quarter of 2019. Included in the operating lease commitments below are total expected minimum obligations under the lease agreement of $831.6 million , which exclude expected tenant improvement reimbursements from the landlord of approximately $75.0 million and variable operating expenses. The Company’s obligations under the lease are supported by a $34.2 million letter of credit, which reduced the borrowing capacity under the revolving credit facility. Other Commitments Other commitments include payments to third-party vendors for services related to the Company’s infrastructure, infrastructure warranty contracts, and asset retirement obligations for office modifications. Future minimum payments under the Company’s non-cancelable leases, financing obligations, and other commitments as of December 31, 2018 , are as follows, and exclude non-cancelable rent payments from the Company’s sub-tenants: Capital lease commitments Operating lease commitments (1) Other commitments Year ended December 31: 2019 $ 82.4 $ 96.4 $ 57.0 2020 45.3 106.2 37.2 2021 30.3 99.0 2.2 2022 17.4 92.9 — 2023 0.6 76.1 0.3 Thereafter — 651.6 — Future minimum payments 176.0 $ 1,122.2 $ 96.7 Less interest and taxes (12.3 ) Less current portion of the present value of minimum lease payments (73.8 ) Capital lease obligations, net of current portion $ 89.9 (1) Consists of future non-cancelable minimum rental payments under operating leases for the Company’s offices and datacenters, excluding rent payments from the Company’s sub-tenants and variable operating expenses. As of December 31, 2018 , the Company is entitled to non-cancelable rent payments from its sub-tenants of $43.5 million , which will be collected over the next five years. Legal matters From time to time, the Company is a party to a variety of claims, lawsuits, and proceedings which arise in the ordinary course of business, including claims of alleged infringement of intellectual property rights. The Company records a liability when it believes that it is probable that a loss will be incurred and the amount of loss or range of loss can be reasonably estimated. The Company discloses losses that are reasonably possible and when the amount of loss or range of loss can be reasonably estimated. In its opinion, resolution of pending matters is not likely to have a material adverse impact on its consolidated results of operations, cash flows, or its financial position. Given the unpredictable nature of legal proceedings, the Company bases its estimate on the information available at the time of the assessment. As additional information becomes available, the Company reassesses the potential liability and may revise the estimate. Indemnification The Company’s arrangements generally include certain provisions for indemnifying customers against liabilities if its products or services infringe a third party’s intellectual property rights. It is not possible to determine the maximum potential amount under these indemnification obligations due to the limited history of prior indemnification claims.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Stockholders' Equity	Stockholders’ Equity Common stock The Company’s amended and restated certificate of incorporation authorizes the issuance of Class A common stock, Class B common stock, and Class C common stock. Holders of Class A common stock, Class B common stock, and Class C common stock are entitled to dividends on a pro rata basis, when, as, and if declared by the Company’s Board of Directors, subject to the rights of the holders of the Company’s preferred stock. Holders of Class A common stock are entitled to one vote per share, holders of Class B common stock are entitled to 10 votes per share, and holders of Class C common stock are entitled to zero votes per share. During the year ended December 31, 2018 , holders of 157.0 million shares of Class B common stock voluntarily converted their shares into an equivalent number of shares of Class A common stock. As of December 31, 2018 , the Company had authorized 2,400.0 million shares of Class A common stock, 475.0 million shares of Class B common stock, and 800.0 million shares of Class C common stock, each at par value of $0.00001 . As of December 31, 2018 , 211.0 million shares of Class A common stock, 198.6 million shares of Class B common stock, and no shares of Class C common stock were issued and outstanding. As of December 31, 2017 , 8.9 million shares of Class A common stock, 187.9 million shares of Class B common stock, and no shares of Class C common stock were issued and outstanding. Class A shares issued and outstanding as of December 31, 2018 and December 31, 2017 exclude 14.7 million unvested restricted stock awards granted to the Company’s co-founders. See "Co-Founder Grants" section below for further details. Convertible preferred stock Immediately prior to the closing of the Company’s IPO, all of the 147.3 million shares of convertible preferred stock converted into an equivalent number of shares of Class B common stock. Further, pursuant to transfer agreements with certain of the Company’s stockholders, 0.3 million shares of the Company’s convertible preferred stock automatically converted into an equivalent number of shares of Class A common stock. The authorized, issued and outstanding shares of convertible preferred stock and liquidation preferences as of December 31, 2017 were as follows: Shares Per share Aggregate preference Dividend amount Authorized Outstanding Series A 63.9 63.9 $ 0.09 $ 6.0 $ 0.02 Series A-1 52.0 51.9 0.03 1.3 0.02 Series B 19.5 19.5 13.58 264.8 1.08 Series C 15.8 12.3 $ 28.65 352.6 $ 2.30 151.2 147.6 $ 624.7 Preferred stock The Company's Board of Directors will have the authority, without further action by the Company's stockholders, to issue up to 240.0 million shares of undesignated preferred stock with rights and preferences, including voting rights, designated from time to time by the Board of Directors. Equity incentive plans Under the 2018 Plan, the Company may grant stock-based awards to purchase or directly issue shares of common stock to employees, directors, and consultants. Options are granted at a price per share equal to the fair market value of the Company's common stock at the date of grant. Options granted are exercisable over a maximum term of 10 years from the date of grant and generally vest over a period of four years. No options have been granted since August of 2015. RSUs and RSAs are also granted under the 2018 Plan. The 2018 Plan will terminate 10 years after the later of (i) its adoption or (ii) the most recent stockholder-approved increase in the number of shares reserved under the 2018 Plan, unless terminated earlier by the Company's Board of Directors. The 2018 Plan was adopted on March 22, 2018 with a reserve of 41.4 million shares of Class A common stock for future issuance. As of December 31, 2018 , there were 26.3 million stock-based awards issued and outstanding and 57.1 million shares available for issuance under the 2008 Equity Incentive Plan, the 2017 Equity Incentive Plan, and the 2018 Plan (collectively, the "Plans"). Stock option and restricted stock activity for the Plans was as follows for the years ended December 31, 2018 , 2017 , and 2016 : Options outstanding Restricted stock outstanding Number of shares available for issuance under the Plans Number of shares outstanding under the Plans Weighted- average exercise price per share Weighted- average remaining contractual term (In years) Number of Plan shares outstanding Weighted- average grant date fair value per share Balance at December 31, 2015 14.6 13.4 $ 17.51 8.1 43.5 $ 17.67 Options and RSUs canceled 14.1 (8.0 ) 22.04 (6.1 ) 20.06 Restricted stock granted (16.6 ) — — 16.6 15.59 Balance at December 31, 2016 12.1 5.4 $ 10.68 6.5 54.0 $ 16.41 Additional shares authorized 6.7 — — — — Options exercised and RSUs released — (0.1 ) 6.00 (14.6 ) 17.12 Options and RSUs canceled 6.2 (0.3 ) 17.44 (6.0 ) 17.96 Shares repurchased for tax withholdings on release of restricted stock 5.5 — — — 17.09 Restricted stock granted (21.5 ) — — 21.5 15.70 Balance at December 31, 2017 9.0 5.0 $ 10.52 5.5 54.9 $ 15.60 Reserved for issuance under the 2018 Plan 41.4 — $ — — $ — Additional shares authorized 1.3 — $ — — $ — Options exercised and RSUs released — (3.4 ) $ 7.75 (40.4 ) $ 15.42 Options and RSUs canceled 6.0 (0.3 ) $ 22.90 (5.7 ) $ 16.78 Shares repurchased for tax withholdings on release of restricted stock 15.6 — $ — — $ 15.45 Restricted stock granted (16.2 ) — $ — 16.2 $ 20.26 Balance at December 31, 2018 57.1 1.3 $ 14.68 5.0 25.0 $ 18.68 Vested at December 31, 2018 1.3 $ 14.68 — $ — Unvested at December 31, 2018 — $ — 25.0 $ 18.68 The following table summarizes information about the pre-tax intrinsic value of options exercised and the weighted-average grant date fair value per share of options granted during the years ended December 31, 2018 , 2017 , and 2016 : Year ended December 31, 2018 2017 2016 Intrinsic value of options exercised $ 59.0 $ 1.2 $ — As of December 31, 2018 , unamortized stock-based compensation expense related to unvested stock options, RSAs (excluding the Co-Founder Grants, which are non-Plan RSAs), and one-tier RSUs was $429.0 million . The weighted-average period over which such compensation expense will be recognized if the requisite service is provided is approximately 2.8 years as of December 31, 2018 . The total fair value of RSUs, as of their respective vesting dates, during the years ended December 31, 2018, 2017 and 2016 were $913.5 million , $232.5 million , and $77.2 million , respectively. Co-Founder Grants In December 2017, the Board of Directors approved a grant to the Company’s co-founders of non-Plan RSAs with respect to 14.7 million shares of Class A Common Stock in the aggregate (collectively, the “Co-Founder Grants”), of which 10.3 million RSAs were granted to Mr. Houston, the Company’s co-founder and Chief Executive Officer, and 4.4 million RSAs were granted to Mr. Ferdowsi, the Company’s co-founder and Director. These Co-Founder Grants have service-based, market-based, and performance-based vesting conditions. The Co-Founder Grants are excluded from Class A common stock issued and outstanding until the satisfaction of these vesting conditions. The Co-Founder Grants also provide the holders with certain stockholder rights, such as the right to vote the shares with the other holders of Class A common stock and a right to cumulative declared dividends. However, the Co-Founder Grants are not considered a participating security for purposes of calculating net loss per share attributable to common stockholders in Note 12, "Net Loss Per Share", as the right to the cumulative declared dividends is forfeitable if the service condition is not met. The Co-Founder Grants are eligible to vest over the ten -year period following the date the Company’s shares of Class A common stock commenced trading on the Nasdaq Global Select Market in connection with the Company’s IPO. The Co-Founder Grants comprise nine tranches that are eligible to vest based on the achievement of stock price goals, each of which are referred to as a Stock Price Target, measured over a consecutive thirty-day trading period during the Performance Period. The Performance Period will begin on January 1, 2019. During the first four years of the Performance Period, no more than 20% of the shares subject to each Co-Founder Grant would be eligible to vest in any calendar year. After the first four years, all shares are eligible to vest based on the achievement of the Stock Price Targets. The Company calculated the grant date fair value of the Co-Founder Grants based on multiple stock price paths developed through the use of a Monte Carlo simulation. A Monte Carlo simulation also calculates a derived service period for each of the nine vesting tranches, which is the measure of the expected time to achieve each Stock Price Target. A Monte Carlo simulation requires the use of various assumptions, including the underlying stock price, volatility, and the risk-free interest rate as of the valuation date, corresponding to the length of time remaining in the performance period, and expected dividend yield. The weighted-average grant date fair value of each Co-Founder Grant was estimated to be $10.60 per share. The weighted-average derived service period of each Co-Founder Grant was estimated to be 5.2 years, and ranged from 2.9 - 6.9 years. The Company will recognize aggregate stock-based compensation expense of $156.2 million over the derived service period of each tranche using the accelerated attribution method as long as the co-founders satisfy their service-based vesting conditions. If the Stock Price Targets are met sooner than the derived service period, the Company will adjust its stock-based compensation to reflect the cumulative expense associated with the vested awards. The Company will recognize expense if the requisite service is provided, regardless of whether the market conditions are achieved. The Performance Vesting Condition for the Co-Founder Grants was satisfied on the date the Company’s shares of Class A common stock commenced trading on the Nasdaq Global Select Market in connection with the Company’s IPO, which was March 23, 2018. The Company recognized $37.0 million in stock-based compensation related to the Co-Founder Grants during the year ended December 31, 2018 . As of December 31, 2018 , unamortized stock-based compensation expense related to the Co-Founder Grants was $119.3 million . The weighted-average period over which such compensation expense will be recognized if the requisite service is provided is approximately 4.0 years as of December 31, 2018 . Award modifications During the year ended December 31, 2017, the Company's Board of Directors voted to approve a modification of vesting schedules for certain unvested one-tier and two-tier RSUs to align the vesting schedules for all RSUs to vest once per quarter. The modification was effective February 15, 2018, which resulted in accelerated vesting of impacted RSUs that had met their service requirement as of that date. As a result, the Company recognized an incremental $10.0 million in stock-based compensation during the first quarter of 2018, which is included in the results for the year ended December 31, 2018, related to these modified one-tier and two-tier RSUs. During the year ended December 31, 2016, the Company’s Board of Directors voted to approve the exchange of stock options previously granted to an executive officer under the Plan for one-tier RSUs. In total, options to purchase 4.3 million shares of common stock were exchanged for 2.2 million RSUs. Total compensation expense for the modified awards is $37.7 million , of which $18.8 million in stock-based compensation expense was recognized on the date of exchange representing the portion that vested immediately. Out of the total $37.7 million in stock-based compensation expense, $8.9 million was incremental to what would have been recognized related to the original stock option award. As of December 31, 2018 , there was no remaining unamortized expense relating to these awards.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company computes net loss per share using the two-class method required for multiple classes of common stock and participating securities. The rights, including the liquidation and dividend rights, of the Class A common stock and Class B common stock are substantially identical, other than voting rights. Accordingly, the Class A common stock and Class B common stock share equally in the Company’s net losses. Before the IPO, the Company’s outstanding securities also included convertible preferred stock. The holders of convertible preferred stock did not have a contractual obligation to share in the Company’s losses, and as a result, net losses were not allocated to these securities. The following table sets forth the calculation of basic and diluted net loss per share attributable to common stockholders during the periods presented. The shares issued in the IPO and the private placement and the shares of Class A and Class B common stock issued upon conversion of the outstanding shares of convertible preferred stock in the IPO are included in the table below weighted for the period outstanding in the year ended December 31, 2018. Additionally, the voluntary conversions of Class B common stock into Class A common stock are included in the table below weighted for the period outstanding in the year ended December 31, 2018: Year ended December 31, 2018 2017 2016 Class A Class B Class A Class B Class A Class B Numerator: Net loss attributable to common stockholders $ (138.7 ) $ (346.2 ) $ (3.6 ) $ (108.1 ) $ (6.1 ) $ (204.1 ) Denominator: Weighted-average number of common shares outstanding used in computing basic and diluted net loss per common share 102.6 256.0 6.3 189.6 5.5 183.6 Net loss per common share, basic and diluted $ (1.35 ) $ (1.35 ) $ (0.57 ) $ (0.57 ) $ (1.11 ) $ (1.11 ) Since the Company was in a loss position for all periods presented, basic net loss per share attributable to common stockholders is the same as diluted net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The weighted-average impact of potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Year ended December 31, 2018 2017 2016 Convertible preferred stock — 147.6 147.6 Restricted stock units 35.0 52.7 47.9 Options to purchase shares of common stock 4.0 5.1 7.9 Co-Founder Grants (1) 14.7 0.8 — Shares subject to repurchase from early-exercised options and unvested restricted stock 0.1 0.2 0.9 Total 53.8 206.4 204.3 (1) In December 2017, the Board of Directors approved a grant to the Company’s co-founders of non-Plan RSAs with respect to 14.7 million shares of Class A Common Stock. The table above reflects the weighted-average impact for the period they were outstanding for the year ended December 31, 2017. See Note 11 "Stockholders' Equity" for further details.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes For the years ended December 31, 2018 , 2017 , and 2016 the Company’s income (loss) from continuing operations before provision for income taxes was as follows: Year ended December 31, 2018 2017 2016 Domestic $ (497.1 ) $ (76.9 ) $ (94.4 ) Foreign 17.0 (34.6 ) (110.6 ) Loss before income taxes $ (480.1 ) $ (111.5 ) $ (205.0 ) The components of the provision for income taxes in the years ended December 31, 2018 , 2017 , and 2016 were as follows: Year ended December 31, 2018 2017 2016 Current: Federal $ (0.1 ) $ 0.1 $ (1.5 ) State (0.2 ) (0.3 ) (0.6 ) Foreign (4.6 ) (2.3 ) (2.9 ) Deferred: — Federal — 1.4 — State — — — Foreign 0.1 0.9 (0.2 ) Provision for income taxes $ (4.8 ) $ (0.2 ) $ (5.2 ) Income tax benefit attributable to loss from continuing operations differed from the amounts computed by applying the statutory U.S. federal income tax rate of 21% for 2018 and 34% for 2017 and 2016 to pretax loss from continuing operations as a result of the following: Year ended December 31, 2018 2017 2016 Tax benefit at federal statutory rate $ 100.8 $ 37.9 $ 69.7 State taxes, net of federal benefit 10.7 1.7 2.2 Foreign rate differential 1.8 (12.3 ) (38.8 ) Research and other credits 86.5 25.4 12.6 Permanent differences (18.4 ) (9.0 ) (3.5 ) Tax Cuts and Jobs Act impact — (61.7 ) — Change in valuation allowance (240.7 ) 38.9 (40.7 ) Stock-based compensation 57.3 (20.1 ) (4.8 ) Other nondeductible items (2.8 ) (1.0 ) (1.9 ) Provision for income taxes $ (4.8 ) $ (0.2 ) $ (5.2 ) The significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 were as follows: As of December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 264.8 $ 119.6 Research credit carryforwards 157.3 67.9 Stock-based compensation 11.1 20.0 Accruals and reserves 42.0 34.0 Fixed assets and intangible assets 0.7 — Other 1.1 — Gross deferred tax assets 477.0 241.5 Valuation allowance (476.0 ) (233.7 ) Total deferred tax assets, net of valuation allowance 1.0 7.8 Deferred tax liabilities: Fixed assets and intangible assets — 6.6 Other — 0.4 Total deferred tax liability — 7.0 Net deferred tax assets $ 1.0 $ 0.8 For the years ended December 31, 2018 and 2017 , based on all available objective evidence, including the existence of cumulative losses, the Company determined that it was not more likely than not that the U.S., Ireland, and Israel net deferred tax assets were fully realizable as of December 31, 2018 and 2017 . Accordingly, the Company established a full valuation allowance against its U.S. and Ireland deferred tax assets and a partial valuation allowance against its Israeli deferred tax assets. As of December 31, 2018 , the Company had $923.6 million of federal, $403.5 million of state, and $288.0 million of foreign net operating loss carryforwards available to reduce future taxable income. Of the federal net operating loss carryforwards, $307.4 million will begin to expire in 2031 and $616.2 million will carryforward indefinitely, while state net operating losses begin to expire in 2029. As of December 31, 2018 , the Company had research credit carryforwards of $145.0 million and $76.8 million for federal and state income tax purposes, respectively, of which $36.1 million and $19.1 million is the unrecognized tax benefit portion related to the research credit carryforwards for federal and state, respectively. The federal credit carryforward will begin to expire in 2027. The state research credits have no expiration date. The Company also had $3.7 million of state enterprise zone credit carryforwards, which will begin to expire in 2023. As of December 31, 2018 , the Company also had $265.1 million of foreign net operating loss carryforwards available to reduce future taxable income, which will carryforward indefinitely. In addition, the Company had $22.9 million of foreign acquired net operating losses, which will carryforward indefinitely. Under Section 382 and 383 of the Internal Revenue Code of 1986, as amended, or the Code, if a corporation undergoes an “ownership change,” the corporation’s ability to use its pre-change net operating loss carryforwards and other pre-change attributes, such as research tax credits, to offset its post-change income may be limited. In general, an “ownership change” will occur if there is a cumulative change in our ownership by “5-percent shareholders” that exceeds 50 percentage points over a rolling three -year period. Similar rules may apply under state tax laws. The Company performed a study for the period through December 31, 2018 , and determined that no ownership changes exceeding 50 percentage points have occurred. The Company’s ability to use net operating loss and tax credit carryforwards to reduce future taxable income and liabilities may be subject to annual limitations as a result of ownership changes from January 1, 2019, and subsequent years. As of December 31, 2018 , the balance of unrecognized tax benefits was $59.8 million of which $4.5 million , if recognized, would affect the effective tax rate and $55.3 million would result in adjustment to deferred tax assets with corresponding adjustments to the valuation allowance. A reconciliation of the beginning and ending amount of unrecognized tax benefit is as follows: Year ended December 31, 2018 2017 2016 Balance of gross unrecognized tax benefits at the beginning of the fiscal year $ 25.6 $ 15.7 $ 7.9 Gross increases related to prior period tax positions 1.1 — 2.2 Gross increases related to current period tax positions 33.1 9.9 5.6 Balance of gross unrecognized tax benefits at the end of the fiscal year $ 59.8 $ 25.6 $ 15.7 The Company recognizes interest and/or penalties related to income tax matters as a component of income tax expense. As of December 31, 2018 , the amount of accrued interest and penalties related to uncertain tax positions was $1.3 million . Interest and penalties recognized for the year ended December 31, 2018 , 2017 , and 2016 was $0.7 million , $0.2 million , and $0.4 million respectively. The Company files income tax returns in the U.S. federal, multiple states, and foreign jurisdictions. All of the Company’s tax years from 2007 remain open for examination by the federal and state authorities, and from 2013 by foreign authorities. The Company generally does not provide deferred income taxes for the undistributed earnings of its foreign subsidiaries as the Company intends to reinvest such earnings indefinitely. Should circumstances change and it becomes apparent that some or all of the undistributed earnings will no longer be indefinitely reinvested, the Company will accrue for income taxes not previously recognized. As of December 31, 2018 , there were no cumulative undistributed earnings in its Irish subsidiary and, as a result, there were no unrecorded deferred tax liabilities. The amount of undistributed earnings in the Company’s other foreign subsidiaries, if any, are immaterial. Impact of the Tax Cuts and Jobs Act The Tax Cuts and Jobs Act (the “Tax Reform Act”) was enacted on December 22, 2017 and provides for significant changes to U.S. tax law. Among other provisions, the Tax Reform Act reduces the U.S. corporate income tax rate to 21% effective in 2018. The Tax Reform Act also contains a number of provisions that may impact the Company in future years. At December 31, 2017, the Company had not completed its accounting for all of the enactment-date income tax effects of the Tax Reform Act for remeasurement of deferred tax assets and liabilities, one-time transition tax, and tax on global intangible low-taxed income. During the twelve months ended December 31, 2018 the Company completed the accounting for all of the enactment-date income tax effects of the Tax Reform Act. Upon completing the analysis, the Company has recognized no material adjustments to its provisional amounts recorded for the year ended December 31, 2017. The Company will continue to monitor ongoing guidance in this area, as the U.S. Treasury Department, the IRS, and other standard-setting bodies could interpret or issue guidance on how provisions of the Tax Reform Act will be applied or otherwise administered that is different from the Company’s interpretation. As a result of the reduction in the corporate rate, the Company has remeasured its U.S. deferred tax assets and liabilities as of December 31, 2017 to reflect the lower rate expected to apply when these temporary differences reverse. As of December 31, 2017, the Company determined the remeasurement resulted in a reduction in deferred tax assets of $63.1 million , which was fully offset by a corresponding change to the Company’s valuation allowance. There were no material changes from provisional amounts recorded for the year ended December 31, 2017. The Tax Reform Act repealed the corporate alternative minimum tax (“AMT”) effective beginning in 2018 and permits AMT credit carryforwards to be refunded to the extent unused through 2021. Since the Company does not anticipate the use of these credits to reduce future federal taxes, it recognized an income tax benefit and established an income tax receivable to reflect anticipated refunds of $1.4 million for its 2016 AMT credit carryforward during the year ended December 31, 2017. There were no material changes from provisional amounts recorded for the year ended December 31, 2017. The Tax Reform Act also provides for a transition to a new territorial system of taxation and generally requires companies to include certain untaxed foreign earnings of non-U.S. subsidiaries into taxable income in 2017 (“Transition Tax”). As a result of the cumulative deficits in the Company’s foreign subsidiaries, the Company has no Transition Tax inclusion. The Tax Reform Act subjects a US shareholder to current tax on global intangible low-taxed income (GILTI) earned by certain foreign subsidiaries. The FASB Staff Q&A, Topic 740 No. 5, Accounting for Global Intangible Low-Taxed Income, states that an entity can make an accounting policy election to either recognize deferred taxes for temporary differences expected to reverse as GILTI in future years or provide for the tax expense related to GILTI in the year the tax is incurred. The Company has elected to recognize the tax on GILTI as a period expense in the period the tax is incurred. As a result of the tested losses in its foreign subsidiaries, under GILTI regulations, the Company has no GILTI inclusion for the year ended 2018.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Dropbox Charitable Foundation During the year ended December 31, 2016, two of the Company’s controlling shareholders formed the Dropbox Charitable Foundation, a Delaware non-stock corporation (the “Foundation”). The primary purpose of the Foundation is to engage in charitable and educational activities within the meaning of Section 501(c)(3) of the Code. The Foundation is governed by a Board of Directors, a majority of which are independent. Both shareholders made contributions to the Foundation during the year ended December 31, 2016, comprised entirely of shares of Dropbox common stock. The Company has not consolidated the Foundation in the accompanying consolidated financial statements, as the Company does not have control of the entity. During the year ended December 31, 2018 , the Company did not make cash contributions to the Foundation. During the year ended December 31, 2017, the Company donated shares of Class B common stock to initially fund the Foundation and recorded $9.4 million of expense to general and administrative expenses based on the Company’s estimate of the then current fair value of the contributed shares. The Company made additional cash contributions to the Foundation of $1.9 million during year ended December 31, 2017. Hewlett Packard Enterprise The Company has engaged in various commercial relationships with Hewlett Packard Enterprise (“HPE”), whose former chief executive officer was appointed to the Dropbox Board of Directors in September 2017. These commercial relationships include infrastructure equipment under capital leases, the purchase of commercial products and other services, and a multi-year subscription agreement for access to the Dropbox platform. The chief executive officer of HPE resigned from her position at HPE effective February 1, 2018. From the beginning of fiscal 2018 through the date of the resignation of the former chief executive officer of HPE, the Company made payments of $5.5 million for infrastructure equipment under capital leases and for commercial products and services provided by HPE. From the date of appointment of HPE’s chief executive officer to the Dropbox Board of Directors through December 31, 2017, the Company made payments of $18.4 million for infrastructure equipment under capital leases and for commercial products and services provided by HPE. As of December 31, 2017, the Company had a remaining obligation of $87.1 million for equipment under capital lease from HPE. Related to the multi-year subscription agreement, the Company recognized an immaterial amount of revenue from the date of appointment of HPE’s former chief executive officer to the Dropbox Board of Directors through December 31, 2017, and had an immaterial balance of deferred revenue and outstanding trade receivables as of December 31, 2017.

Geographic Areas

Geographic Areas	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Geographic Areas	Geographic Areas Long-lived assets The following table sets forth long-lived assets by geographic area: As of December 31, 2018 2017 United States $ 293.6 $ 323.7 International (1) 17.0 18.2 Total property and equipment, net $ 310.6 $ 341.9 (1) No single country other than the United States had a property and equipment balance greater than 10% of total property and equipment, net, as of December 31, 2018 and 2017 . Revenue Revenue by geography is generally based on the address of the customer as defined in the Company’s subscription agreement. The following table sets forth revenue by geographic area for the years ended December 31, 2018 , 2017 , and 2016 : Year ended December 31, 2018 2017 2016 United States $ 706.5 $ 575.7 $ 455.9 International (1) 685.2 531.1 388.9 Total revenue $ 1,391.7 $ 1,106.8 $ 844.8 (1) No single country outside of the United States accounted for more than 10 percent of total revenue during the years ended December 31, 2018 , 2017 , and 2016

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 8, 2019, the Company acquired all outstanding stock of HelloSign, Inc. ("HelloSign"), which provides an e-signature and document workflow platform. The preliminary purchase consideration is estimated to be approximately $230 million , consisting primarily of cash payments. Of the $230 million of consideration, $48.5 million is subject to on-going employee service. The related expense will be recognized over the required service period of up to three years, while the related payments will begin in the first quarter of 2020 if the requisite service is provided. The purchase price will be allocated to the tangible and intangible assets and liabilities acquired based on their fair values at the acquisition date. The Company is currently performing the procedures necessary to determine the purchase price allocation, and the Company will record the initial fair value estimates during the three months ending March 31, 2019.

Description of the Business a_2

Description of the Business and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The accompanying consolidated financial statements have been prepared in accordance with the United States of America generally accepted accounting principles (“GAAP”). The accompanying consolidated financial statements include the accounts of Dropbox and its wholly owned subsidiaries.
Consolidation	All intercompany balances and transactions have been eliminated in consolidation.
Use of estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported and disclosed in the Company’s consolidated financial statements and accompanying notes. These estimates are based on information available as of the date of the consolidated financial statements. On a regular basis, management evaluates these estimates and assumptions. Actual results may differ materially from these estimates. The Company’s most significant estimates and judgments involve the measurement of the Company’s stock-based compensation, including the estimation of the underlying deemed fair value of common stock for periods prior to the Company's IPO, and the estimation of the fair value of market-based awards.
Financial information about segments and geographic areas	The Company manages its operations and allocates resources as a single operating segment. Further, the Company manages, monitors, and reports its financials as a single reporting segment. The Company’s chief operating decision-maker is its Chief Executive Officer, who reviews financial information presented on a consolidated basis for purposes of making operating decisions, assessing financial performance, and allocating resources.
Foreign currency transactions	The assets and liabilities of the Company’s foreign subsidiaries are translated from their respective functional currencies into U.S. dollars at the rates in effect at the balance sheet date and revenue and expense amounts are translated at the average exchange rate for the period. Foreign currency translation gains and losses are recorded in other comprehensive income (loss). Gains and losses realized from foreign currency transactions (those transactions denominated in currencies other than the foreign subsidiaries’ functional currency) are included in other income, net. Monetary assets and liabilities are remeasured using foreign currency exchange rates at the end of the period, and non-monetary assets are remeasured based on historical exchange rates.
Revenue recognition, Cost of revenue and Deferred commissions, net	The Company derives its revenue from subscription fees from customers for access to its platform. The Company’s policy is to exclude sales and other indirect taxes when measuring the transaction price of its subscription agreements. The Company accounts for revenue contracts with customers 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 The Company’s subscription agreements generally have monthly or annual contractual terms and a small percentage have multi-year contractual terms. Revenue is recognized ratably over the related contractual term beginning on the date that the platform is made available to a customer. Access to the platform represents a series of distinct services as the Company continually provides access to, and fulfills its obligation to the end customer over the subscription term. The series of distinct services represents a single performance obligation that is satisfied over time. The Company recognizes revenue ratably because the customer receives and consumes the benefits of the platform throughout the contract period. The Company’s contracts are generally non-cancelable. The Company bills in advance for monthly contracts and typically bills annually in advance for contracts with terms of one year or longer . The Company also recognizes an immaterial amount of unbilled receivables, primarily relating to consideration for services completed but not billed at the reporting date. Unbilled receivables are classified as receivables when the Company has the right to invoice the customer. The Company records contract liabilities when cash payments are received or due in advance of performance to deferred revenue. Deferred revenue primarily relates to the advance consideration received from the customer. The price of subscriptions is generally fixed at contract inception and therefore, the Company’s contracts do not contain a significant amount of variable consideration. As a result, the amount of revenue recognized in the periods presented from performance obligations satisfied (or partially satisfied) in previous periods was not material. Deferred commissions, net is stated at gross deferred commissions less accumulated amortization. Sales commissions earned by the Company’s sales force and third-party resellers, as well as related payroll taxes, are considered to be incremental and recoverable costs of obtaining a contract with a customer. As a result, these amounts have been capitalized as deferred commissions within prepaid and other current assets and other assets on the consolidated balance sheet. Cost of revenue consists primarily of expenses associated with the storage, delivery, and distribution of the Company’s platform for both paying users and Basic users. These costs, which are referred to as infrastructure costs, include depreciation of servers located in co-location facilities that the Company leases and operates, rent and facilities expense for those datacenters, network and bandwidth costs, support and maintenance costs for infrastructure equipment, and payments to third-party datacenter service providers. Cost of revenue also includes costs, such as salaries, bonuses, benefits, travel-related expenses, and stock-based compensation, which are referred to as employee-related costs, for employees whose primary responsibilities relate to supporting the Company’s infrastructure and delivering user support. Other non-employee costs included in cost of revenue include credit card fees related to processing customer transactions and allocated overhead, such as facilities, including rent, utilities, depreciation on leasehold improvements and other equipment shared by all departments, and shared information technology costs. In addition, cost of revenue includes amortization of developed technologies, professional fees related to user support initiatives, and property taxes related to the datacenters.
Stock-based compensation	The Company has granted RSUs to its employees and members of the Board of Directors under the 2008 Equity Incentive Plan (“2008 Plan”), the 2017 Equity Incentive Plan (“2017 Plan”), and the 2018 Equity Incentive Plan ("2018 Plan"). The Company had two types of RSUs outstanding as of December 31, 2018 : • One-tier RSUs, which have a service-based vesting condition over a four -year period. These awards typically have a cliff vesting period of one year and continue to vest quarterly thereafter. The Company began granting one-tier RSUs under its 2008 Plan in August 2015 and it continues to grant one-tier RSUs under its 2018 Plan. The Company recognizes compensation expense associated with one-tier RSUs ratably on a straight-line basis over the requisite service period and accounts for forfeitures in the period in which they occur. • Two-tier RSUs, which have both a service-based vesting condition and a Performance Vesting Condition. The service-based vesting period for these awards is typically four years with a cliff vesting period of one year and continue to vest monthly thereafter. Upon satisfaction of the Performance Vesting Condition, these awards vest quarterly. The Performance Vesting Condition was satisfied on the the effectiveness of the registration statement related to the Company's IPO. Prior to August 2015, the Company granted two-tier RSUs under the 2008 Plan. The last grant date for two-tier RSUs was in May 2015. The Company recognizes compensation expense associated with two-tier RSUs using the accelerated attribution method over the requisite service period. The Performance Vesting Condition for the two-tier RSUs was satisfied upon the effectiveness of the registration statement related to the Company's IPO, which was March 22, 2018. On that date, the Company recognized the cumulative unrecognized expense of the two-tier RSUs, using the accelerated attribution method, which is included in the Company's results for the year ended December 31, 2018. See "—Initial public offering and private placement” for further discussion. As of December 31, 2018, the remaining unamortized stock-based compensation related to the two-tier RSUs was $0.1 million , which will be recognized if the requisite service is provided over a weighted average period of 0.2 years. Since August 2015, the Company has granted RSUs as the only stock-based payment awards to its employees, with the exception of awards granted to its co-founders, and has not granted any stock options since then. The fair values of the common stock underlying the RSUs granted in periods prior to the date of the Company's IPO were determined by the Board of Directors, with input from management and contemporaneous third-party valuations, which were performed at least quarterly. Given the absence of a public trading market for the Company's common stock prior to its IPO, and in accordance with the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation, or AICPA Guide, the Company's Board of Directors exercised reasonable judgment and considered numerous objective and subjective factors to determine the best estimate of the fair value of its common stock including: • The results of contemporaneous valuations of its common stock by unrelated third parties; • The rights, preferences, and privileges of its convertible preferred stock relative to those of its common stock; • Market multiples of comparable public companies in its industry as indicated by their market capitalization and guideline merger and acquisition transactions; • The Company's performance and market position relative to its competitors, who may change from time to time; • The Company's historical financial results and estimated trends and prospects for its future performance; • Valuations published by institutional investors that hold investments in its capital stock; • The economic and competitive environment; • The likelihood and timeline of achieving a liquidity event, such as an initial public offering or sale, given prevailing market conditions; • Any adjustments necessary to recognize a lack of marketability for its common stock; and • Precedent sales of or offers to purchase its capital stock. In valuing the Company's common stock, the Board of Directors determined the fair value of its common stock using both the income and market approach valuation methods. The income approach estimates value based on the expectation of future cash flows that a company will generate. These future cash flows are discounted to their present values using a discount rate based on our weighted average cost of capital, and is adjusted to reflect the risks inherent in the Company's cash flows. The market approach estimates value based on a comparison of the subject company to comparable public companies in a similar line of business. From the comparable companies, a representative market value multiple is determined and then applied to the subject company’s financial forecasts to estimate the value of the subject company. For valuations after the Company's IPO, the Board of Directors determines the fair value of each share of underlying common stock based on the closing price of the Company's Class A common stock as reported on the Nasdaq Global Select Market on the date of the grant. In December 2017, the Board of Directors approved a grant to the Company’s co-founders of restricted stock awards (“RSAs”) with respect to 14.7 million shares of Class A Common Stock in the aggregate (collectively, the “Co-Founder Grants”), of which 10.3 million RSAs were granted to Mr. Houston, the Company’s co-founder and Chief Executive Officer, and 4.4 million RSAs were granted to Mr. Ferdowsi, the Company’s co-founder and Director. These Co-Founder Grants have service-based, market-based, and performance-based vesting conditions. The Co-Founder Grants comprise nine tranches that are eligible to vest based on the achievement of stock price goals, or, each, a Stock Price Target. The Company estimated the grant date fair value of the Co-Founder Grants using a model based on multiple stock price paths developed through the use of a Monte Carlo simulation that incorporates into the valuation the possibility that the Stock Price Targets may not be satisfied. The average grant date fair value of each Co-Founder Grant was estimated to be $10.60 per share, and the Company will recognize aggregate stock-based compensation expense of $156.2 million over the requisite service period of each tranche, which ranged from 2.9 to 6.9 years, using the accelerated attribution method. If the Stock Price Targets are met sooner than the derived service period, the Company will adjust its stock-based compensation to reflect the cumulative expense associated with the vested awards. The Company will recognize expense if the requisite service is provided, regardless of whether the market conditions are achieved. The Co-Founder Grants contain an implied performance-based vesting condition because the Stock Price Targets are based on the trailing 30-day average price of the shares from an established national securities exchange or automated quotation system. Accordingly, no vesting could occur until the completion of the Company’s IPO. The relevant performance-based vesting condition for the Co-Founder Grants was satisfied on the date the Company’s shares of Class A common stock commenced trading on the Nasdaq Global Select Market, in connection with the Company’s IPO, which was March 23, 2018. The Company recognized $37.0 million in stock-based compensation related to the Co-Founder Grants during the year ended December 31, 2018.
Advertising and promotional expense	Advertising and promotional expenses are included in sales and marketing expenses within the consolidated statements of operations and are expensed when incurred.
Cash and cash equivalents	Cash consists primarily of cash on deposit with banks. Cash equivalents include highly liquid investments purchased with an original maturity date of 90 days or less from the date of purchase and primarily consist of money market funds. Cash equivalents also include amounts in transit from payment processors for credit and debit card transactions, which typically settle within five days. Cash and cash equivalents are recorded at cost, which approximates fair value.
Short-term investments	The Company’s short-term investments are primarily comprised of corporate notes and obligations, U.S. treasury securities, certificates of deposits, U.S. agency obligations, and commercial paper. The Company determines the appropriate classification of its short-term investments at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified and accounted for its short-term investments as available-for-sale securities as the Company may sell these securities at any time for use in its current operations or for other purposes, even prior to maturity. As a result, the Company classifies its short-term investments, including securities with stated maturities beyond twelve months, within current assets in the consolidated balance sheets. The Company's short-term investments are classified as available-for-sale securities and are recorded at fair value each reporting period. Unrealized gains and losses on these short-term investments are reported as a separate component of accumulated other comprehensive income (loss) in the consolidated balance sheets until realized. Interest income is reported within interest income (expense), net in the consolidated statements of operations. The Company periodically evaluates its short-term investments to assess whether those with unrealized loss positions are other-than-temporarily impaired. The Company considers various factors in determining whether to recognize an impairment charge, including the length of time the investment has been in a loss position, the extent to which the fair value is less than the Company’s cost basis, and the financial condition and near-term prospects of the investee. Realized gains and losses are determined based on the specific identification method and are reported in other income, net in the consolidated statements of operations. If the Company determines that the decline in an investment’s fair value is other-than-temporary, the difference is recognized as an impairment loss in the consolidated statements of operations.
Trade and other receivables, net/Non-trade receivables	Trade and other receivables, net consists primarily of trade receivables that are recorded at the invoice amount, net of an allowance for doubtful accounts. The Company records non-trade receivables to reflect amounts due for activities outside of its subscription agreements. Historically, the Company’s non-trade receivables have related primarily to receivables resulting from tenant improvement allowances.
Trade and other receivables, net	The allowance for doubtful accounts is based on the Company’s assessment of the collectability of accounts. The Company regularly reviews the adequacy of the allowance for doubtful accounts by considering the age of each outstanding invoice, the collection history of each customer, and other relevant factors to determine the appropriate amount of the allowance. Accounts receivable deemed uncollectable are charged against the allowance for doubtful accounts when identified.
Concentrations of credit risk	Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, short-term investments, and accounts receivable. The Company places its cash, cash equivalents, and short-term investments with well-established financial institutions. Cash equivalents consist primarily of highly rated money market funds. Trade accounts receivables are typically unsecured and are derived from revenue earned from customers located around the world.
Property and equipment, net	Equipment is stated at cost less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful life of the related asset, which is generally three to seven years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the related lease. In the first quarter of 2018, the Company determined that the useful lives of certain infrastructure equipment, which are depreciated through cost of revenue, should be increased from three to four years. The Company accounted for this as a change in estimate that was applied prospectively, effective as of January 1, 2018. This change in useful life resulted in a reduction in depreciation expense within cost of revenue of $16.1 million during the year ended December 31, 2018. The following table presents the estimated useful lives of property and equipment: Property and equipment Useful life Buildings 20 to 30 years Datacenter and other computer equipment 3 to 5 years Office equipment and other 3 to 7 years Leasehold improvements Lesser of estimated useful life or remaining lease term
Lease obligations	The Company leases office space, datacenters, and equipment under non-cancelable capital and operating leases with various expiration dates through 2033. Certain of the Company’s operating lease agreements contain tenant improvement allowances from its landlords. These allowances are accounted for as lease incentive obligations and are amortized as reductions to rent expense over the lease term. In June 2018, the Company took initial possession of the first phase of its new corporate headquarters and recorded a lease incentive obligation related to this phase. As of December 31, 2018, $2.6 million was included in accrued and other current liabilities and $36.4 million was included in deferred rent, non-current in the Company's consolidated balance sheets. In addition, certain of the operating lease agreements contain rent concession, rent escalation, and option to renew provisions. Rent concession and rent escalation provisions are considered in determining the straight-line rent expense to be recorded over the lease term. Lease expense is recognized on a straight-line basis over the lease term commencing on the date the Company has the right to use the leased property. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed to be reasonably assured at lease inception. In 2012, the Company undertook a series of structural improvements to the floor that it occupied in its previous corporate headquarters. As a result of the requirement to fund construction costs and its responsibility for cost overruns during the construction period, the Company was considered the deemed owner of the floor for accounting purposes. Due to the presence of a standby letter of credit as a security deposit, the Company was deemed to have continuing involvement after the construction period. As such, it accounted for this arrangement as owned real estate and recorded a building asset and an imputed financing obligation for its liability to the legal owners. In September 2018, the Company terminated its master lease for its previous corporate headquarters, the impact of which is described in Note 4, "Property and Equipment, Net". The Company leases certain equipment from various third parties, including from a related party, through equipment financing leases under capital leases. See Note 14, “Related Party Transactions” for additional details. These leases either include a bargain purchase option, a full transfer of ownership at the completion of the lease term, or the terms of the leases are at least 75 percent of the useful lives of the assets and are therefore classified as capital leases. These leases are capitalized in property and equipment, net and the related amortization of assets under capital leases is included in depreciation and amortization expense in the Company’s consolidated statements of operations. Initial asset values and lease obligations are based on the present value of future minimum lease payments.
Internal use software	The Company capitalizes certain costs related to developed or modified software solely for its internal use and cloud based applications used to deliver its platform. The Company capitalizes costs during the application development stage once the preliminary project stage is complete, management authorizes and commits to funding the project, and it is probable that the project will be completed and that the software will be used to perform the function intended. Costs related to preliminary project activities and post implementation activities are expensed as incurred.
Business combinations	The Company uses best estimates and assumptions to assign a fair value to the tangible and intangible assets acquired and liabilities assumed in business combinations as of the acquisition date. These estimates are inherently uncertain and subject to refinement. During the measurement period, which may be up to one year from the acquisition date, adjustments to the fair value of these tangible and intangible assets acquired and liabilities assumed may be recorded, with the corresponding offset to goodwill. Upon the conclusion of the measurement period or final determination of the fair value of assets acquired or liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the Company’s consolidated statements of operations.
Long-lived assets, including goodwill and other acquired intangible assets, net	The Company evaluates the recoverability of property and equipment and finite-lived intangible assets for possible impairment whenever events or circumstances indicate that the carrying amount of such assets may not be recoverable. The evaluation is performed at the lowest level for which identifiable cash flows are largely independent of the cash flows of other assets and liabilities. Recoverability of these assets is measured by a comparison of the carrying amounts to the future undiscounted cash flows the assets are expected to generate. If such review determines that the carrying amount of specific property and equipment or intangible assets is not recoverable, the carrying amount of such assets is reduced to its fair value. The Company reviews goodwill for impairment at least annually in the fourth quarter, or more frequently if events or changes in circumstances would more likely than not reduce the fair value of its single reporting unit below its carrying value. The Company has not recorded impairment charges on property and equipment, goodwill, or intangible assets for the periods presented in these consolidated financial statements. Acquired property and equipment and finite-lived intangible assets are amortized over their useful lives. The Company evaluates the estimated remaining useful life of these assets when events or changes in circumstances warrant a revision to the remaining period of amortization. If the Company reduces the estimated useful life assumption for any asset, the remaining unamortized balance is amortized or depreciated over the revised estimated useful life on a prospective basis.
Deferred offering costs	Deferred offering costs, which consist of direct incremental legal, accounting, and consulting fees related to the Company's IPO, were capitalized.
Income taxes	Deferred income tax balances reflect the effects of temporary differences between the financial reporting and tax bases of the Company’s assets and liabilities using enacted tax rates expected to apply when taxes are actually paid or recovered. In addition, deferred tax assets are recorded for net operating loss and credit carryforwards. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized based on all available positive and negative evidence. Such evidence includes, but is not limited to, recent cumulative earnings or losses, expectations of future taxable income by taxing jurisdiction, and the carry-forward periods available for the utilization of deferred tax assets. The Company uses a two-step approach to recognizing and measuring uncertain income tax positions. The first step is to evaluate the tax position for recognition by determining if the weight of available evidence indicates it is more likely than not that the position will be sustained on audit. The second step is to measure the tax benefit as the largest amount, which is more than 50% likely of being realized upon ultimate settlement. The Company recognizes interest and penalties related to unrecognized tax benefits as income tax expense. Significant judgment is required to evaluate uncertain tax positions. Although the Company believes that it has adequately reserved for its uncertain tax positions, it can provide no assurance that the final tax outcome of these matters will not be materially different. The Company evaluates its uncertain tax positions on a regular basis and evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, such as the Tax Cuts and Jobs Act, correspondence with tax authorities during the course of an audit, and effective settlement of audit issues. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences will affect the provision for income taxes in the period in which such determination is made and could have a material impact on the Company’s financial condition and results of operations.
Fair value measurement	The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. The Company defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining fair value measurements for assets and liabilities, the Company considers the principal or most advantageous market in which it would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions, and credit risk. Fair value is estimated by applying the following hierarchy, which prioritizes the inputs used to measure fair value into three levels and bases the categorization within the hierarchy upon the lowest level of input that is available and significant to the fair value measurement: Level 1 —Quoted prices in active markets for identical assets or liabilities. Level 2 —Observable inputs other than quoted prices in active markets for identical assets and liabilities, quoted prices for identical or similar assets or liabilities in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 —Inputs that are generally unobservable and typically reflect management’s estimate of assumptions that market participants would use in pricing the asset or liability.
Recently issued accounting pronouncements not yet adopted and Recently adopted accounting pronouncements	Recently issued accounting pronouncements not yet adopted In February 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2016-02, Leases (Topic 842) . Most prominent among the changes in the standard is the recognition of right of use assets and lease liabilities by lessees for those leases classified as operating leases under current GAAP. Under the standard, disclosures are required to meet the objective of enabling users of financial statements to assess the amount, timing, and uncertainty of cash flows arising from leases. The Company is required to adopt the standard using the modified retrospective approach and has elected to use the optional transition method related to comparative reporting at adoption. The new standard is effective for fiscal years beginning after December 15, 2018. The Company will recognize a material operating lease liability and related right-of-use asset on the consolidated balance sheet on January 1, 2019. The lease liability and corresponding right-of-use asset primarily relate to its office space and data center operating leases. The prospective impact on the consolidated statement of operations under the new standard is substantially similar compared to the current lease accounting model. The Company's accounting for capital leases related to infrastructure equipment, now referred to as financing leases under the new standard, is substantially unchanged under the new standard. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes will result in more timely recognition of credit losses. The guidance is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact and timing of adopting ASU No. 2016-13. In February 2018, the FASB issued ASU No. 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income . Under existing U.S. GAAP, the effects of changes in tax rates and laws on deferred tax balances are recorded as a component of income tax expense in the period in which the law was enacted. When deferred tax balances related to items originally recorded in accumulated other comprehensive income are adjusted, certain tax effects become stranded in accumulated other comprehensive income. The amendments in ASU No. 2018-02 allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the 2017 Tax Reform Act. The amendments in ASU No. 2018-02 also require certain disclosures about stranded tax effects. The guidance is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company does not expect the impact of adoption to have a material impact on its consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting. Under existing U.S. GAAP, the measurement date for equity awards granted to nonemployees is the earlier of the performance commitment date or the date the performance is complete. The amendments in ASU No. 2018-07 allow for measurement of these awards on the grant date, consistent with equity awards granted to employees. The guidance is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company does not expect the impact of adoption to have a material impact on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820), which amends disclosure requirements for fair value measurements by requiring new disclosures, modifying existing requirements, and eliminating others. The amendments are the result of a broader disclosure project, which aims to improve the effectiveness of disclosures. ASU No. 2018-13 is effective for fiscal years beginning after December 15, 2019 and interim periods within those fiscal years. Early adoption is permitted. The Company does not expect the adoption to have a significant impact on its disclosures and is currently evaluating the timing of adopting ASU No. 2018-13. In August 2018, the FASB issued ASU No. 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40), Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. Under existing U.S. GAAP, there is diversity in practice in accounting for the costs of implementing cloud computing arrangements that are service contracts. The amendments in ASU No. 2018-15 amend the definition of a hosting arrangement and requires a customer in a hosting arrangement that is a service contract to capitalize certain costs as if the arrangement were an internal-use software project. The guidance is effective for fiscal years beginning after December 15, 2019 and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact and timing of adopting ASU No. 2018-15. Recently adopted accounting pronouncements In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (Subtopic 825) , which primarily affects the accounting for equity investments, financial liabilities under the fair value option and the presentation and disclosure requirements for financial instruments. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. The accounting for other financial instruments, such as loans, investments in debt securities and financial liabilities is largely unchanged. The Company adopted ASU No. 2016-01 on January 1, 2018. The adoption of the standard did not have a material impact on the Company's consolidated financial statements. In October 2016, the FASB issued ASU No. 2016-16, Income Taxes: Intra-Entity Transfers Other than Inventory (Topic 740) , 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 No. 2016-16 on January 1, 2018. The adoption of the guidance did not have a material impact on the Company's consolidated financial statements.
Goodwill	Goodwill represents the excess of the purchase price in a business combination over the fair value of net tangible and intangible assets acquired. Goodwill amounts are not amortized, but tested for impairment on an annual basis.

Description of the Business a_3

Description of the Business and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Estimated Useful Lives of Property and Equipment	The following table presents the estimated useful lives of property and equipment: Property and equipment Useful life Buildings 20 to 30 years Datacenter and other computer equipment 3 to 5 years Office equipment and other 3 to 7 years Leasehold improvements Lesser of estimated useful life or remaining lease term Property and equipment, net consisted of the following: As of December 31, 2018 2017 Building $ — $ 36.6 Datacenter and other computer equipment 695.3 663.1 Furniture and fixtures 23.8 21.2 Leasehold improvements 122.6 118.6 Construction in process 32.8 7.2 Total property and equipment 874.5 846.7 Accumulated depreciation and amortization (563.9 ) (504.8 ) Property and equipment, net $ 310.6 $ 341.9

Cash, Cash Equivalents and Sh_2

Cash, Cash Equivalents and Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Schedule of Cash, Cash Equivalents and Short-Term Investments	The amortized cost, unrealized gains and losses and estimated fair value of the Company's cash, cash equivalents and short-term investments as of December 31, 2018 consisted of the following: Amortized cost Unrealized gain Unrealized loss Estimated fair value Cash $ 103.0 $ — $ — 103.0 Cash equivalents Money market funds 355.5 — — 355.5 Commercial paper 27.4 — — 27.4 U.S. Treasury securities 33.4 — — 33.4 Total cash and cash equivalents $ 519.3 $ — $ — $ 519.3 Short-term investments Corporate notes and obligations 269.6 0.1 (0.5 ) 269.2 U.S. Treasury securities 176.0 — (0.1 ) 175.9 Certificates of deposit 70.6 — — 70.6 U.S. agency obligations 37.1 — — 37.1 Commercial paper 17.2 — — 17.2 Total short-term investments 570.5 0.1 (0.6 ) 570.0 Total $ 1,089.8 $ 0.1 $ (0.6 ) $ 1,089.3
Contractual Maturities of Short Term Investments	The following table presents the contractual maturities of the Company’s short-term investments as of December 31, 2018 : Amortized cost Estimated fair value Due within one year $ 327.6 $ 327.5 Due between one to three years 242.9 242.5 Total $ 570.5 $ 570.0
Short-Term Investments In Unrealized Loss Positions	As of December 31, 2018 , the following short-term investments were in unrealized loss positions: Less than 12 months 12 months or greater Total Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses Corporate notes and obligations $ 209.5 $ (0.5 ) $ — $ — $ 209.5 $ (0.5 ) U.S. Treasury securities 127.8 (0.1 ) — — 127.8 (0.1 ) Total $ 337.3 $ (0.6 ) $ — $ — $ 337.3 $ (0.6 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value Assets Measured On Recurring Basis	The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis using the input categories discussed in Note 1: As of December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 355.5 $ — $ — $ 355.5 U.S Treasury securities — 33.4 — 33.4 Commercial paper — 27.4 — 27.4 Total cash equivalents $ 355.5 $ 60.8 $ — $ 416.3 Short-term investments Corporate notes and obligations — 269.2 — 269.2 U.S. Treasury securities — 175.9 — 175.9 Certificates of deposit — 70.6 — 70.6 U.S. agency obligations — 37.1 — 37.1 Commercial paper — 17.2 — 17.2 Total short-term investments — 570.0 — 570.0 Total cash equivalents and short-term investments $ 355.5 $ 630.8 $ — $ 986.3

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	The following table presents the estimated useful lives of property and equipment: Property and equipment Useful life Buildings 20 to 30 years Datacenter and other computer equipment 3 to 5 years Office equipment and other 3 to 7 years Leasehold improvements Lesser of estimated useful life or remaining lease term Property and equipment, net consisted of the following: As of December 31, 2018 2017 Building $ — $ 36.6 Datacenter and other computer equipment 695.3 663.1 Furniture and fixtures 23.8 21.2 Leasehold improvements 122.6 118.6 Construction in process 32.8 7.2 Total property and equipment 874.5 846.7 Accumulated depreciation and amortization (563.9 ) (504.8 ) Property and equipment, net $ 310.6 $ 341.9

Intangible Assets, Net (Tables)

Intangible Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets	Intangible assets consisted of the following: As of December 31, Weighted- (In years) 2018 2017 Developed technology $ 47.0 $ 50.9 0.0 Patents 13.0 13.0 8.1 Software 19.2 17.8 2.3 Assembled workforce in asset acquisitions 12.6 10.1 1.8 Licenses 4.6 4.6 2.4 Non-compete agreements, trademarks and other 4.0 4.0 6.1 Total intangibles 100.4 100.4 Accumulated amortization (85.7 ) (83.4 ) Intangible assets, net $ 14.7 $ 17.0
Schedule of Future Amortization Expense	Expected future amortization expense for intangible assets as of December 31, 2018 , is as follows: 2019 $ 4.7 2020 3.8 2021 1.5 2022 0.8 2023 0.8 Thereafter 3.1 Total $ 14.7

Goodwill (Tables)

Goodwill (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	Goodwill consisted of the following: Balance at December 31, 2016 $ 96.0 Effect of foreign currency translation 2.9 Balance at December 31, 2017 98.9 Effect of foreign currency translation (2.4 ) Balance at December 31, 2018 $ 96.5

Accrued and Other Current Lia_2

Accrued and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule Of Accrued And Other Current Liabilities	Accrued and other current liabilities consisted of the following: As of December 31, 2018 2017 Non-income taxes payable $ 75.7 $ 69.7 Accrued legal and other external fees 28.1 21.3 Deferred rent 41.0 14.6 Financing obligations, current 2.6 9.7 Income taxes payable 0.3 0.4 Other accrued and current liabilities 16.8 14.1 Total accrued and other current liabilities $ 164.5 $ 129.8

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Payments	Future minimum payments under the Company’s non-cancelable leases, financing obligations, and other commitments as of December 31, 2018 , are as follows, and exclude non-cancelable rent payments from the Company’s sub-tenants: Capital lease commitments Operating lease commitments (1) Other commitments Year ended December 31: 2019 $ 82.4 $ 96.4 $ 57.0 2020 45.3 106.2 37.2 2021 30.3 99.0 2.2 2022 17.4 92.9 — 2023 0.6 76.1 0.3 Thereafter — 651.6 — Future minimum payments 176.0 $ 1,122.2 $ 96.7 Less interest and taxes (12.3 ) Less current portion of the present value of minimum lease payments (73.8 ) Capital lease obligations, net of current portion $ 89.9 (1) Consists of future non-cancelable minimum rental payments under operating leases for the Company’s offices and datacenters, excluding rent payments from the Company’s sub-tenants and variable operating expenses. As of December 31, 2018 , the Company is entitled to non-cancelable rent payments from its sub-tenants of $43.5 million , which will be collected over the next five years.

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Schedule of Convertible Preferred Stock	The authorized, issued and outstanding shares of convertible preferred stock and liquidation preferences as of December 31, 2017 were as follows: Shares Per share Aggregate preference Dividend amount Authorized Outstanding Series A 63.9 63.9 $ 0.09 $ 6.0 $ 0.02 Series A-1 52.0 51.9 0.03 1.3 0.02 Series B 19.5 19.5 13.58 264.8 1.08 Series C 15.8 12.3 $ 28.65 352.6 $ 2.30 151.2 147.6 $ 624.7
Schedule of Stock Option and Restricted Stock Activity	Stock option and restricted stock activity for the Plans was as follows for the years ended December 31, 2018 , 2017 , and 2016 : Options outstanding Restricted stock outstanding Number of shares available for issuance under the Plans Number of shares outstanding under the Plans Weighted- average exercise price per share Weighted- average remaining contractual term (In years) Number of Plan shares outstanding Weighted- average grant date fair value per share Balance at December 31, 2015 14.6 13.4 $ 17.51 8.1 43.5 $ 17.67 Options and RSUs canceled 14.1 (8.0 ) 22.04 (6.1 ) 20.06 Restricted stock granted (16.6 ) — — 16.6 15.59 Balance at December 31, 2016 12.1 5.4 $ 10.68 6.5 54.0 $ 16.41 Additional shares authorized 6.7 — — — — Options exercised and RSUs released — (0.1 ) 6.00 (14.6 ) 17.12 Options and RSUs canceled 6.2 (0.3 ) 17.44 (6.0 ) 17.96 Shares repurchased for tax withholdings on release of restricted stock 5.5 — — — 17.09 Restricted stock granted (21.5 ) — — 21.5 15.70 Balance at December 31, 2017 9.0 5.0 $ 10.52 5.5 54.9 $ 15.60 Reserved for issuance under the 2018 Plan 41.4 — $ — — $ — Additional shares authorized 1.3 — $ — — $ — Options exercised and RSUs released — (3.4 ) $ 7.75 (40.4 ) $ 15.42 Options and RSUs canceled 6.0 (0.3 ) $ 22.90 (5.7 ) $ 16.78 Shares repurchased for tax withholdings on release of restricted stock 15.6 — $ — — $ 15.45 Restricted stock granted (16.2 ) — $ — 16.2 $ 20.26 Balance at December 31, 2018 57.1 1.3 $ 14.68 5.0 25.0 $ 18.68 Vested at December 31, 2018 1.3 $ 14.68 — $ — Unvested at December 31, 2018 — $ — 25.0 $ 18.68
Schedule of Pre-Tax Intrinsic Value	The following table summarizes information about the pre-tax intrinsic value of options exercised and the weighted-average grant date fair value per share of options granted during the years ended December 31, 2018 , 2017 , and 2016 : Year ended December 31, 2018 2017 2016 Intrinsic value of options exercised $ 59.0 $ 1.2 $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	Additionally, the voluntary conversions of Class B common stock into Class A common stock are included in the table below weighted for the period outstanding in the year ended December 31, 2018: Year ended December 31, 2018 2017 2016 Class A Class B Class A Class B Class A Class B Numerator: Net loss attributable to common stockholders $ (138.7 ) $ (346.2 ) $ (3.6 ) $ (108.1 ) $ (6.1 ) $ (204.1 ) Denominator: Weighted-average number of common shares outstanding used in computing basic and diluted net loss per common share 102.6 256.0 6.3 189.6 5.5 183.6 Net loss per common share, basic and diluted $ (1.35 ) $ (1.35 ) $ (0.57 ) $ (0.57 ) $ (1.11 ) $ (1.11 )
Schedule of Potentially Dilutive Securities Excluded from Computation of Earnings Per Share	The weighted-average impact of potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Year ended December 31, 2018 2017 2016 Convertible preferred stock — 147.6 147.6 Restricted stock units 35.0 52.7 47.9 Options to purchase shares of common stock 4.0 5.1 7.9 Co-Founder Grants (1) 14.7 0.8 — Shares subject to repurchase from early-exercised options and unvested restricted stock 0.1 0.2 0.9 Total 53.8 206.4 204.3 (1) In December 2017, the Board of Directors approved a grant to the Company’s co-founders of non-Plan RSAs with respect to 14.7 million shares of Class A Common Stock. The table above reflects the weighted-average impact for the period they were outstanding for the year ended December 31, 2017. See Note 11 "Stockholders' Equity" for further details.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Income before Income Tax, Domestic and Foreign	For the years ended December 31, 2018 , 2017 , and 2016 the Company’s income (loss) from continuing operations before provision for income taxes was as follows: Year ended December 31, 2018 2017 2016 Domestic $ (497.1 ) $ (76.9 ) $ (94.4 ) Foreign 17.0 (34.6 ) (110.6 ) Loss before income taxes $ (480.1 ) $ (111.5 ) $ (205.0 )
Schedule of Components of Income Tax Expense (Benefit)	The components of the provision for income taxes in the years ended December 31, 2018 , 2017 , and 2016 were as follows: Year ended December 31, 2018 2017 2016 Current: Federal $ (0.1 ) $ 0.1 $ (1.5 ) State (0.2 ) (0.3 ) (0.6 ) Foreign (4.6 ) (2.3 ) (2.9 ) Deferred: — Federal — 1.4 — State — — — Foreign 0.1 0.9 (0.2 ) Provision for income taxes $ (4.8 ) $ (0.2 ) $ (5.2 )
Schedule of Effective Income Tax Rate Reconciliation	Income tax benefit attributable to loss from continuing operations differed from the amounts computed by applying the statutory U.S. federal income tax rate of 21% for 2018 and 34% for 2017 and 2016 to pretax loss from continuing operations as a result of the following: Year ended December 31, 2018 2017 2016 Tax benefit at federal statutory rate $ 100.8 $ 37.9 $ 69.7 State taxes, net of federal benefit 10.7 1.7 2.2 Foreign rate differential 1.8 (12.3 ) (38.8 ) Research and other credits 86.5 25.4 12.6 Permanent differences (18.4 ) (9.0 ) (3.5 ) Tax Cuts and Jobs Act impact — (61.7 ) — Change in valuation allowance (240.7 ) 38.9 (40.7 ) Stock-based compensation 57.3 (20.1 ) (4.8 ) Other nondeductible items (2.8 ) (1.0 ) (1.9 ) Provision for income taxes $ (4.8 ) $ (0.2 ) $ (5.2 )
Schedule of Deferred Tax Assets and Liabilities	The significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 were as follows: As of December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 264.8 $ 119.6 Research credit carryforwards 157.3 67.9 Stock-based compensation 11.1 20.0 Accruals and reserves 42.0 34.0 Fixed assets and intangible assets 0.7 — Other 1.1 — Gross deferred tax assets 477.0 241.5 Valuation allowance (476.0 ) (233.7 ) Total deferred tax assets, net of valuation allowance 1.0 7.8 Deferred tax liabilities: Fixed assets and intangible assets — 6.6 Other — 0.4 Total deferred tax liability — 7.0 Net deferred tax assets $ 1.0 $ 0.8
Schedule of Unrecognized Tax Benefits Roll Forward	A reconciliation of the beginning and ending amount of unrecognized tax benefit is as follows: Year ended December 31, 2018 2017 2016 Balance of gross unrecognized tax benefits at the beginning of the fiscal year $ 25.6 $ 15.7 $ 7.9 Gross increases related to prior period tax positions 1.1 — 2.2 Gross increases related to current period tax positions 33.1 9.9 5.6 Balance of gross unrecognized tax benefits at the end of the fiscal year $ 59.8 $ 25.6 $ 15.7

Geographic Areas (Tables)

Geographic Areas (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Long-lived Assets by Geographic Areas	The following table sets forth long-lived assets by geographic area: As of December 31, 2018 2017 United States $ 293.6 $ 323.7 International (1) 17.0 18.2 Total property and equipment, net $ 310.6 $ 341.9 (1) No single country other than the United States had a property and equipment balance greater than 10% of total property and equipment, net, as of December 31, 2018 and 2017 .
Revenue by Geographic Areas	The following table sets forth revenue by geographic area for the years ended December 31, 2018 , 2017 , and 2016 : Year ended December 31, 2018 2017 2016 United States $ 706.5 $ 575.7 $ 455.9 International (1) 685.2 531.1 388.9 Total revenue $ 1,391.7 $ 1,106.8 $ 844.8 (1) No single country outside of the United States accounted for more than 10 percent of total revenue during the years ended December 31, 2018 , 2017 , and 2016

Description of the Business a_4

Description of the Business and Summary of Significant Accounting Policies - Initial Public Offering and Private Placement (Details) $ / shares in Units, $ in Millions	Apr. 03, 2018USD ($)	Mar. 28, 2018USD ($)$ / sharesshares	Mar. 27, 2018USD ($)$ / sharesshares	Mar. 26, 2018shares	Mar. 22, 2018USD ($)	Mar. 07, 2018	Mar. 31, 2018USD ($)shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Conversion of Stock [Line Items]
Reverse stock split ratio						0.6667
Stock issued in IPO (in shares)			26,822,409
Share price of stock issued in IPO (in dollars per share) | $ / shares			$ 21
Proceeds from initial public offering and private placement | $			$ 538.2					$ 746.6	$ 0	$ 0
Offering costs | $			$ 6.9					$ 4.5	$ 2.5	$ 0
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017 | Conversion of Convertible Preferred Stock to Class B Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				147,310,563
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017 | Conversion of Convertible Preferred Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				258,620
Class B common stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)								157,000,000
Class B common stock | Conversion of Convertible Preferred Stock to Class B Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				147,310,563
Class B common stock | Conversion of Class B Common Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				2,609,951
Class A common stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)								0
Class A common stock | Conversion of Convertible Preferred Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				258,620
Class A common stock | Conversion of Class B Common Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				2,609,951
Private Placement
Conversion of Stock [Line Items]
Proceeds from initial public offering and private placement | $		$ 100
Private Placement | Class A common stock
Conversion of Stock [Line Items]
Sale of stock (in shares)		4,761,905
Share price of stock sold in shares (in dollars per share) | $ / shares		$ 21
Underwriters Over-allotment Option | Class A common stock
Conversion of Stock [Line Items]
Sale of stock (in shares)		5,400,000
Share price of stock sold in shares (in dollars per share) | $ / shares		$ 21
Proceeds received in sale of stock | $	$ 108.4
Two-Tier RSUs
Conversion of Stock [Line Items]
Recognized cumulative unrecognized stock-based compensation | $					$ 418.7			$ 418.7
Remaining unamortized stock-based compensation | $								$ 0.1
Unamortized stock-based compensation, requisite service period								2 months 2 days
Shares released (in shares)							26,800,000
Employer related payroll expense | $							$ 13.9

Description of the Business a_5

Description of the Business and Summary of Significant Accounting Policies - Foreign Currency Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net foreign currency transaction gain (loss)	$ (1.9)	$ 5	$ (3.6)

Description of the Business a_6

Description of the Business and Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue, performance obligation, description of timing	one year or longer
Revenue recognized	$ 411.6	$ 353	$ 266.9
Remaining performance obligation	$ 529.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Remaining performance obligation, expected timing of satisfaction	12 months

Description of the Business a_7

Description of the Business and Summary of Significant Accounting Policies - Stock-based Compensation (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017USD ($)tranche$ / sharesshares	Dec. 31, 2018USD ($)award$ / sharesshares	Dec. 31, 2017USD ($)tranche$ / sharesshares	Dec. 31, 2016$ / sharesshares
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of award types | award		2
One-Tier RSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Two-Tier RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Remaining unamortized stock-based compensation | $		$ 0.1
Unamortized stock-based compensation, requisite service period		2 months 2 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)		16.2	21.5	16.6
Shares granted (in dollars per share) | $ / shares		$ 20.26	$ 15.70	$ 15.59
Tranche One | One-Tier RSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		1 year
Tranche One | Two-Tier RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		1 year
Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining unamortized stock-based compensation | $	$ 156.2	$ 119.3	$ 156.2
Unamortized stock-based compensation, requisite service period		4 years	5 years 2 months 12 days
Shares granted (in shares)	14.7	14.7	14.7
Number of tranches | tranche	9		9
Shares granted (in dollars per share) | $ / shares	$ 10.60
Allocated share-based compensation expense | $		$ 37
Co-Founder Grants | Tranche One | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Chief Executive Officer | Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	10.3
Director | Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	4.4
Minimum | Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock-based compensation, requisite service period			2 years 10 months 24 days
Award requisite period	2 years 10 months 24 days
Maximum | Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock-based compensation, requisite service period			6 years 10 months 24 days
Award requisite period	6 years 10 months 24 days

Description of the Business a_8

Description of the Business and Summary of Significant Accounting Policies - Advertising and Promotional Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Advertising and promotional expense	$ 100.9	$ 80.1	$ 46.6

Description of the Business a_9

Description of the Business and Summary of Significant Accounting Policies - Trade and Other Receivables, Net (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Allowance for doubtful accounts	$ 1.2	$ 1

Description of the Business _10

Description of the Business and Summary of Significant Accounting Policies - Concentrations of Credit Risk (Details) - Customer Concentration Risk - Trade and Other Receivables	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	18.00%
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	23.00%	27.00%

Description of the Business _11

Description of the Business and Summary of Significant Accounting Policies - Non-trade Receivables (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Non-trade receivables	$ 46.2	$ 5.2
Tenant improvement allowance receivable related to the new corporate headquarters included in prepaid expenses and other current assets and other assets	$ 40.5

Description of the Business _12

Description of the Business and Summary of Significant Accounting Policies - Deferred Commissions, Net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Contract Cost [Line Items]
Additional contract costs deferred	$ 32,000,000	$ 19,400,000
Deferred contract costs, amortization period	5 years
Amortization of deferred commissions	$ 12,100,000	6,600,000	$ 3,700,000
Impairment loss related to deferred costs	0	0	$ 0
Prepaid Expenses and Other Current Assets | Deferred Commissions
Capitalized Contract Cost [Line Items]
Deferred contract costs	14,500,000	8,100,000
Other Assets | Deferred Commissions
Capitalized Contract Cost [Line Items]
Deferred contract costs	$ 38,300,000	$ 24,800,000

Description of the Business _13

Description of the Business and Summary of Significant Accounting Policies - Property and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation	$ 158.6	$ 170.7	$ 173.8
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Minimum | Infrastructure Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Minimum | Buildings
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	20 years
Minimum | Datacenter and other computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Minimum | Office equipment and other
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	7 years
Maximum | Infrastructure Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	4 years
Maximum | Buildings
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	30 years
Maximum | Datacenter and other computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Maximum | Office equipment and other
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	7 years
Change in Accounting Method Accounted for as Change in Estimate
Property, Plant and Equipment [Line Items]
Depreciation	$ (16.1)

Description of the Business _14

Description of the Business and Summary of Significant Accounting Policies - Lease Obligations (Details) $ in Millions	Dec. 31, 2018USD ($)
Accrued and other current liabilities
Lessor, Lease, Description [Line Items]
Lease incentive obligation	$ 2.6
Deferred rent, non-current
Lessor, Lease, Description [Line Items]
Lease incentive obligation	$ 36.4

Description of the Business _15

Description of the Business and Summary of Significant Accounting Policies - Internal Use Software (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Capitalized internal use software costs	$ 0	$ 0	$ 0

Description of the Business _16

Description of the Business and Summary of Significant Accounting Policies - Deferred Offering Costs (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred offering costs	$ 0	$ 4,100,000

Cash, Cash Equivalents and Sh_3

Cash, Cash Equivalents and Short-Term Investments - Schedule of Cash, Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 519.3	$ 430	$ 352.7	$ 356.9
Total cash equivalents	519.3
Short-term investments, amortized cost	570.5
Short-term investments, unrealized gain	0.1
Short-term investments, unrealized loss	(0.6)
Short-term investments, estimated fair value	570
Total cash, cash equivalents, and short term investments, before unrealized gains (losses) on investments	1,089.8
Total cash, cash equivalents, and short term investments	1,089.3
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	269.6
Short-term investments, unrealized gain	0.1
Short-term investments, unrealized loss	(0.5)
Short-term investments, estimated fair value	269.2
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	176
Short-term investments, unrealized gain	0
Short-term investments, unrealized loss	(0.1)
Short-term investments, estimated fair value	175.9
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	70.6
Short-term investments, unrealized gain	0
Short-term investments, unrealized loss	0
Short-term investments, estimated fair value	70.6
U.S. agency obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	37.1
Short-term investments, unrealized gain	0
Short-term investments, unrealized loss	0
Short-term investments, estimated fair value	37.1
Commercial paper
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	17.2
Short-term investments, unrealized gain	0
Short-term investments, unrealized loss	0
Short-term investments, estimated fair value	17.2
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	103	62.9
Total cash equivalents	103
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	355.5	$ 367.1
Total cash equivalents	355.5
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	27.4
Total cash equivalents	27.4
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	33.4
Total cash equivalents	$ 33.4

Cash, Cash Equivalents and Sh_4

Cash, Cash Equivalents and Short-Term Investments - Narrative (Details)	12 Months Ended
	Dec. 31, 2018USD ($)investment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 519,300,000	$ 430,000,000	$ 352,700,000	$ 356,900,000
Short-term investments	570,000,000	0
Cash in transit for credit and debit card transactions	$ 11,900,000	13,300,000
Number of investments in unrealized loss positions | investment	202
Other-than-temporary impairment loss	$ 0
Investment income	16,800,000	3,000,000	$ 900,000
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	103,000,000	62,900,000
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 355,500,000	$ 367,100,000

Cash, Cash Equivalents and Sh_5

Cash, Cash Equivalents and Short-Term Investments - Contractual Maturities of Short Term Investments (Details) $ in Millions	Dec. 31, 2018USD ($)
Amortized cost
Due within one year	$ 327.6
Due between one to three years	242.9
Total	570.5
Estimated fair value
Due within one year	327.5
Due between one to three years	242.5
Total	$ 570

Cash, Cash Equivalents and Sh_6

Cash, Cash Equivalents and Short-Term Investments - Short-term Investments In Unrealized Loss Positions (Details) $ in Millions	Dec. 31, 2018USD ($)
Fair value
Less than 12 months	$ 337.3
12 months or greater	0
Total	337.3
Unrealized losses
Less than 12 months	(0.6)
12 months or greater	0
Total	(0.6)
Corporate notes and obligations
Fair value
Less than 12 months	209.5
12 months or greater	0
Total	209.5
Unrealized losses
Less than 12 months	(0.5)
12 months or greater	0
Total	(0.5)
U.S. Treasury securities
Fair value
Less than 12 months	127.8
12 months or greater	0
Total	127.8
Unrealized losses
Less than 12 months	(0.1)
12 months or greater	0
Total	$ (0.1)

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 519.3
Total short-term investments	570
Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	269.2
U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	175.9
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	70.6
U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	17.2
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	355.5
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	27.4
U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	33.4
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	570
Total cash equivalents and short-term investments	986.3
Fair Value, Measurements, Recurring | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	269.2
Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	175.9
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	70.6
Fair Value, Measurements, Recurring | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	17.2
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	355.5
Fair Value, Measurements, Recurring | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	27.4
Fair Value, Measurements, Recurring | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	416.3
Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	33.4
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Total cash equivalents and short-term investments	355.5
Fair Value, Measurements, Recurring | Level 1 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 1 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	355.5	$ 367.1
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	355.5
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	570
Total cash equivalents and short-term investments	630.8
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	269.2
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	175.9
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	70.6
Fair Value, Measurements, Recurring | Level 2 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	37.1
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	17.2
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	27.4
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	60.8
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	33.4
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Total cash equivalents and short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes and obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total short-term investments	0
Fair Value, Measurements, Recurring | Level 3 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 874.5	$ 846.7
Accumulated depreciation and amortization	(563.9)	(504.8)
Property and equipment, net	310.6	341.9
Building
Property, Plant and Equipment [Line Items]
Total property and equipment	0	36.6
Datacenter and other computer equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	695.3	663.1
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	23.8	21.2
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	122.6	118.6
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 32.8	$ 7.2

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Imputed financing lease obligation	$ 25.5
Property and equipment, gross	874.5	$ 846.7
Accumulated depreciation	563.9	504.8
Depreciation	158.6	170.7	$ 173.8
Building
Property, Plant and Equipment [Line Items]
Gross building asset, derecognized	36.6
Accumulated depreciation, derecognized	12.2
Property and equipment, gross	0	36.6
Infrastructure Assets Held under Capital Leases
Property, Plant and Equipment [Line Items]
Property and equipment, gross	362.8	417.9
Accumulated depreciation	211.7	$ 259
Accrued and other current liabilities
Property, Plant and Equipment [Line Items]
Imputed financing lease obligation	2.8
Other Noncurrent Liabilities
Property, Plant and Equipment [Line Items]
Imputed financing lease obligation	$ 22.7

Intangible Assets, Net - Schedu

Intangible Assets, Net - Schedule of Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 100.4	$ 100.4
Accumulated amortization	(85.7)	(83.4)
Intangible assets, net	14.7	17
Developed technology
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 47	50.9
Developed technology | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	0 years
Patents
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 13	13
Patents | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	8 years 1 month 6 days
Software
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 19.2	17.8
Software | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	2 years 3 months 18 days
Assembled workforce in asset acquisitions
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 12.6	10.1
Assembled workforce in asset acquisitions | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	1 year 9 months 18 days
Licenses
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 4.6	4.6
Licenses | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	2 years 4 months 24 days
Non-compete agreements, trademarks and other
Finite-Lived Intangible Assets [Line Items]
Total intangibles	$ 4	$ 4
Non-compete agreements, trademarks and other | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted- average remaining useful life (In years)	6 years 1 month 6 days

Intangible Assets, Net - Sche_2

Intangible Assets, Net - Schedule of Future Amortization Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$ 6.1	$ 10.5	$ 17.3
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
2,019	4.7
2,020	3.8
2,021	1.5
2,022	0.8
2,023	0.8
Thereafter	3.1
Total	$ 14.7

Goodwill (Details)

Goodwill (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment		$ 0	$ 0
Goodwill [Roll Forward]
Goodwill, beginning of period		98,900,000	96,000,000
Effect of foreign currency translation	$ (2,400,000)		2,900,000
Goodwill, end of period	$ 96,500,000	$ 96,500,000	$ 98,900,000

Accrued and Other Current Lia_3

Accrued and Other Current Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Non-income taxes payable	$ 75.7	$ 69.7
Accrued legal and other external fees	28.1	21.3
Deferred rent	41	14.6
Financing obligations, current	2.6	9.7
Income taxes payable	0.3	0.4
Other accrued and current liabilities	16.8	14.1
Total accrued and other current liabilities	$ 164.5	$ 129.8

Revolving Credit Facility (Deta

Revolving Credit Facility (Details) - Credit And Guarantee Agreement - Line of Credit - USD ($)	Apr. 30, 2017	Feb. 28, 2018	Dec. 31, 2018
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 600,000,000	$ 725,000,000
Debt issuance fees	$ 2,600,000	400,000
Debt instrument, term	5 years
Line of credit facility, accordion feature, increase limit		$ 275,000,000
Unused capacity, commitment fee percentage		0.20%
Covenant terms, minimum liquidity balance		$ 100,000,000
Aggregate letters of credit outstanding amount			$ 68,500,000
Remaining borrowing capacity			$ 656,500,000
Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate		1.50%
Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.50%
Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 187,500,000
Commitment fee percentage		1.50%
Fronting fee		0.125%

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Gross rent expense	$ 85.9	$ 71	$ 67.9
Sublease income	11.8	10.6	$ 4.5
Operating leases, future minimum payments no longer obligated to pay	192.4
Property and equipment, gross	874.5	846.7
Imputed financing obligation		27.4
Operating lease, minimum obligation		831.6
Tenant improvement reimbursements		75
Letter of credit
Loss Contingencies [Line Items]
Letter of credit supporting obligations		34.2
Accrued and other current liabilities
Loss Contingencies [Line Items]
Imputed financing obligation		2.6
Other Noncurrent Liabilities
Loss Contingencies [Line Items]
Imputed financing obligation		24.8
Building
Loss Contingencies [Line Items]
Property and equipment, gross	$ 0	$ 36.6

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Capital lease commitments
2,019		$ 82.4
2,020		45.3
2,021		30.3
2,022		17.4
2,023		0.6
Thereafter		0
Future minimum payments		176
Less interest and taxes		(12.3)
Less current portion of the present value of minimum lease payments	[1]	(73.8)	$ (102.7)
Capital lease obligations, net of current portion	[1]	89.9	$ 71.6
Operating lease commitments
2,019		96.4
2,020		106.2
2,021		99
2,022		92.9
2,023		76.1
Thereafter		651.6
Future minimum payments		1,122.2
Other commitments
2,019		57
2,020		37.2
2,021		2.2
2,022		0
2,023		0.3
Thereafter		0
Future minimum payments		96.7
Rent payments from sub-tenants		$ 43.5
[1]	Includes amounts attributable to related party transactions. See Note 14, "Related Party Transactions" for further details.

Stockholders' Equity - Common S

Stockholders' Equity - Common Stock (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2017$ / sharesshares	Dec. 31, 2018vote$ / sharesshares	Dec. 31, 2017$ / sharesshares	Dec. 31, 2016shares
Class of Stock [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.00001	$ 0.00001	$ 0.00001
Restricted Stock
Class of Stock [Line Items]
Shares granted (in shares)		16,200,000	21,500,000	16,600,000
Co-Founder Grants | Restricted Stock
Class of Stock [Line Items]
Shares granted (in shares)	14,700,000	14,700,000	14,700,000
Class A common stock
Class of Stock [Line Items]
Votes per share | vote		1
Common stock, shares authorized (in shares)	533,300,000	2,400,000,000	533,300,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.00001
Common stock, shares issued (in shares)	8,900,000	211,000,000	8,900,000
Common stock, shares outstanding (in shares)	8,900,000	211,000,000	8,900,000
Class B common stock
Class of Stock [Line Items]
Votes per share | vote		10
Shares converted in conversion (in shares)		157,000,000
Common stock, shares authorized (in shares)	466,700,000	475,000,000	466,700,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.00001
Common stock, shares issued (in shares)	187,900,000	198,600,000	187,900,000
Common stock, shares outstanding (in shares)	187,900,000	198,600,000	187,900,000
Class C common stock
Class of Stock [Line Items]
Votes per share | vote		0
Common stock, shares authorized (in shares)	0	800,000,000	0
Common stock, par value (in dollars per share) | $ / shares		$ 0.00001
Common stock, shares issued (in shares)	0	0	0
Common stock, shares outstanding (in shares)	0	0	0

Stockholders' Equity - Converti

Stockholders' Equity - Convertible Preferred Stock and Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	Mar. 26, 2018	Dec. 31, 2018	Dec. 31, 2017	Mar. 27, 2018
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		240,000,000	0
Preferred stock, shares outstanding (in shares)		0	0
Preferred stock, per share price at issuance (in dollars per share)				$ 21
Class A common stock
Class of Stock [Line Items]
Shares issued in conversion (in shares)		0
Class B common stock
Class of Stock [Line Items]
Shares converted in conversion (in shares)		157,000,000
Convertible preferred stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		0	151,200,000
Preferred stock, shares outstanding (in shares)		0	147,600,000
Aggregate liquidation preference			$ 624.7
Series A Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			63,900,000
Preferred stock, shares outstanding (in shares)			63,900,000
Preferred stock, per share price at issuance (in dollars per share)			$ 0.09
Aggregate liquidation preference			$ 6
Preferred stock, dividend per share amount (in dollars per share)			$ 0.02
Series A-1 Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			52,000,000
Preferred stock, shares outstanding (in shares)			51,900,000
Preferred stock, per share price at issuance (in dollars per share)			$ 0.03
Aggregate liquidation preference			$ 1.3
Preferred stock, dividend per share amount (in dollars per share)			$ 0.02
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			19,500,000
Preferred stock, shares outstanding (in shares)			19,500,000
Preferred stock, per share price at issuance (in dollars per share)			$ 13.58
Aggregate liquidation preference			$ 264.8
Preferred stock, dividend per share amount (in dollars per share)			$ 1.08
Series C Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			15,800,000
Preferred stock, shares outstanding (in shares)			12,300,000
Preferred stock, per share price at issuance (in dollars per share)			$ 28.65
Aggregate liquidation preference			$ 352.6
Preferred stock, dividend per share amount (in dollars per share)			$ 2.30
Conversion of Convertible Preferred Stock to Class B Common Stock | Class B common stock
Class of Stock [Line Items]
Shares issued in conversion (in shares)	147,310,563
Conversion of Convertible Preferred Stock to Class B Common Stock | Convertible preferred stock
Class of Stock [Line Items]
Shares converted in conversion (in shares)	147,310,563
Conversion of Convertible Preferred Stock to Class A Common Stock | Class A common stock
Class of Stock [Line Items]
Shares issued in conversion (in shares)	258,620
Conversion of Convertible Preferred Stock to Class A Common Stock | Convertible preferred stock
Class of Stock [Line Items]
Shares converted in conversion (in shares)	258,620

Stockholders' Equity - Equity I

Stockholders' Equity - Equity Incentive Plans (Details) - USD ($) $ in Millions	12 Months Ended	40 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Mar. 22, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for future issuance (in shares)	41,400,000	41,400,000
Shares issued and outstanding (in shares)	1,300,000	1,300,000		5,000,000	5,400,000	13,400,000
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
Options granted (in shares)		0
Remaining unamortized stock-based compensation	$ 429	$ 429
Unamortized stock-based compensation, requisite service period	2 years 9 months 18 days
2018 Plan | Class A common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for future issuance (in shares)			41,400,000
2018 Plan | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
Vesting period	4 years
2008 Equity Incentive Plan, 2017 Equity Incentive Plan, and 2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued and outstanding (in shares)	26,300,000	26,300,000
Shares available for grant (in shares)	57,100,000	57,100,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Stock Option and Restricted Stock Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares available for issuance under the Plans
Beginning balance (in shares)	9	12.1	14.6
Reserved for issuance under the 2018 Plan (in shares)	41.4
Additional shares authorized (in shares)	1.3	6.7
Options and RSUs canceled (in shares)	6	6.2	14.1
Shares repurchased for tax withholdings on release of restricted stock (unaudited) (in shares)	15.6	5.5
Restricted stock granted (in shares)	(16.2)	(21.5)	(16.6)
Ending balance (in shares)	57.1	9	12.1	14.6
Options Outstanding, Number of shares outstanding under the Plans
Beginning balance (in shares)	5	5.4	13.4
Options canceled (in shares)	(0.3)	(0.3)	(8)
Options exercised (in shares)	(3.4)	(0.1)
Ending balance (in shares)	1.3	5	5.4	13.4
Vested during period (in shares)	1.3
Unvested at end of period (in shares)	0
Options Outstanding, Weighted- average exercise price per share
Beginning balance (in dollars per share)	$ 10.52	$ 10.68	$ 17.51
Options canceled (in dollars per share)	22.90	17.44	22.04
Options exercised (in dollars per share)	7.75	6
Ending balance (in dollars per share)	14.68	$ 10.52	$ 10.68	$ 17.51
Vested during period (in dollars per share)	14.68
Unvested at end of period (in dollars per shares)	$ 0
Options Outstanding, Weighted- average remaining contractual term (In years)
Weighted-average contractual term	5 years	5 years 6 months	6 years 6 months	8 years 1 month 6 days
Restricted Stock
Restricted Stock Outstanding, Number of Plan shares outstanding
Beginning balance (in shares)	54.9	54	43.5
RSUs canceled (in shares)	(5.7)	(6)	(6.1)
Restricted stock granted (in shares)	16.2	21.5	16.6
RSUs released (in shares)	(40.4)	(14.6)
Ending balance (in shares)	25	54.9	54	43.5
Vested (in shares)	0
Restricted Stock Outstanding, Weighted- average grant date fair value per share
Beginning balance (in dollars per share)	$ 15.60	$ 16.41	$ 17.67
RSUs released (in dollars per share)	15.42	17.12
RSUs canceled (in dollars per share)	16.78	17.96	20.06
Shares repurchased for tax withholdings on release of restricted stock (in dollars per share)	15.45	17.09
Restricted stock granted (in dollars per share)	20.26	15.70	15.59
Ending balance (in dollars per share)	18.68	$ 15.60	$ 16.41	$ 17.67
RSUs vested (in dollars per share)	$ 0
Restricted Stock Units (RSUs)
Restricted Stock Outstanding, Weighted- average grant date fair value per share
Fair value of RSUs	$ 913.5	$ 232.5	$ 77.2

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Pre-Tax Intrinsic Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Intrinsic value of options exercised	$ 59	$ 1.2	$ 0

Stockholders' Equity - Co-Found

Stockholders' Equity - Co-Founder Grants (Details) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017USD ($)tranche$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)tranche$ / sharesshares	Dec. 31, 2016shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share) | $ / shares		$ 0
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)		16.2	21.5	16.6
Co-Founder Grants | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	14.7	14.7	14.7
Expiration period	10 years
Number of tranches | tranche	9		9
Weighted average grant date fair value (in dollars per share) | $ / shares	$ 10.60		$ 10.60
Award requisite period		4 years	5 years 2 months 12 days
Remaining unamortized stock-based compensation | $	$ 156.2	$ 119.3	$ 156.2
Stock-based compensation related to the Co-Founder Grants | $		$ 37
Co-Founder Grants | Restricted Stock | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite period			2 years 10 months 24 days
Co-Founder Grants | Restricted Stock | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite period			6 years 10 months 24 days
Co-Founder Grants | Restricted Stock | Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Co-Founder Grants | Restricted Stock | Tranche One | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage vested maximum	20.00%
Co-Founder Grants | Chief Executive Officer | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	10.3
Co-Founder Grants | Director | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	4.4

Stockholders' Equity - Award Mo

Stockholders' Equity - Award Modifications (Details) - USD ($) shares in Millions	Mar. 30, 2016	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2018
2016 Plan Modification
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Plan modification, stock-based compensation	$ 18,800,000		$ 8,900,000
Allocated share-based compensation expense			$ 37,700,000
Plan modification, remaining unamortized expense				$ 0
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Plan modification, stock-based compensation		$ 10,000,000
Restricted Stock Units (RSUs) | 2016 Plan Modification
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs received on exchange (in shares)			2.2
Employee Stock Option | 2016 Plan Modification
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exchanged (in shares)			4.3

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Net loss	$ (484.9)	$ (111.7)	$ (210.2)
Weighted-average number of common shares outstanding used in computing basic and diluted net loss per common share (in shares)	358.6	195.9	189.1
Net loss per common share, basic and diluted (in dollars per share)	$ (1.35)	$ (0.57)	$ (1.11)
Class A
Class of Stock [Line Items]
Net loss	$ (138.7)	$ (3.6)	$ (6.1)
Weighted-average number of common shares outstanding used in computing basic and diluted net loss per common share (in shares)	102.6	6.3	5.5
Net loss per common share, basic and diluted (in dollars per share)	$ (1.35)	$ (0.57)	$ (1.11)
Class B
Class of Stock [Line Items]
Net loss	$ (346.2)	$ (108.1)	$ (204.1)
Weighted-average number of common shares outstanding used in computing basic and diluted net loss per common share (in shares)	256	189.6	183.6
Net loss per common share, basic and diluted (in dollars per share)	$ (1.35)	$ (0.57)	$ (1.11)

Net Loss Per Share - Schedule_2

Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares shares in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		53.8	206.4	204.3
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares granted (in shares)		16.2	21.5	16.6
Co-Founder Grants | Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares granted (in shares)	14.7	14.7	14.7
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		0	147.6	147.6
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		35	52.7	47.9
Options to purchase shares of common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		4	5.1	7.9
Co-Founder Grants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		14.7	0.8	0
Shares subject to repurchase from early-exercised options and unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		0.1	0.2	0.9

Income Taxes - Schedule of Loss

Income Taxes - Schedule of Loss from Continuing Operations Before Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Domestic	$ (497.1)	$ (76.9)	$ (94.4)
Foreign	17	(34.6)	(110.6)
Loss before income taxes	$ (480.1)	$ (111.5)	$ (205)

Income Taxes - Components of In

Income Taxes - Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ (0.1)	$ 0.1	$ (1.5)
State	(0.2)	(0.3)	(0.6)
Foreign	(4.6)	(2.3)	(2.9)
Deferred:
Federal	0	1.4	0
State	0	0	0
Foreign	0.1	0.9	(0.2)
Provision for income taxes	$ (4.8)	$ (0.2)	$ (5.2)

Income Taxes - Income Tax Rate

Income Taxes - Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ 100.8	$ 37.9	$ 69.7
State taxes, net of federal benefit	10.7	1.7	2.2
Foreign rate differential	1.8	(12.3)	(38.8)
Research and other credits	86.5	25.4	12.6
Permanent differences	(18.4)	(9)	(3.5)
Tax Cuts and Jobs Act impact	0	(61.7)	0
Change in valuation allowance	(240.7)	38.9	(40.7)
Stock-based compensation	57.3	(20.1)	(4.8)
Other nondeductible items	(2.8)	(1)	(1.9)
Provision for income taxes	$ (4.8)	$ (0.2)	$ (5.2)

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 264.8	$ 119.6
Research credit carryforwards	157.3	67.9
Stock-based compensation	11.1	20
Accruals and reserves	42	34
Fixed assets and intangible assets	0.7	0
Other	1.1	0
Gross deferred tax assets	477	241.5
Valuation allowance	(476)	(233.7)
Total deferred tax assets, net of valuation allowance	1	7.8
Deferred tax liabilities:
Fixed assets and intangible assets	0	6.6
Other	0	0.4
Total deferred tax liability	0	7
Net deferred tax assets	$ 1	$ 0.8

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Federal income tax rates	21.00%	34.00%	34.00%
Operating loss carryforwards, expiring in future	$ 307,400,000
Operating loss carryforwards, carryforward indefinitely	616,200,000
Unrecognized tax benefits	59,800,000	$ 25,600,000	$ 15,700,000	$ 7,900,000
Unrecognized tax benefits that would impact effective tax rate	4,500,000
Unrecognized tax benefits that would impact effective tax rate, adjustment to deferred tax assets and valuation allowance	55,300,000
Penalties and interest accrued	1,300,000
Penalties and interest expense	700,000	200,000	$ 400,000
Undistributed earnings of foreign subsidiaries	0
Estimated reduction in deferred tax assets due to remeasurement		63,100,000
Estimated tax benefit from repealed corporate AMT		1,400,000
Estimated transition tax inclusion		$ 0
Domestic
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	923,600,000
Domestic | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, amount	145,000,000
Tax credit carryforward, valuation allowance	36,100,000
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	403,500,000
State | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, amount	76,800,000
Tax credit carryforward, valuation allowance	19,100,000
State | State Enterprise Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, amount	3,700,000
Foreign
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	288,000,000
Operating loss carryforwards, carryforward indefinitely	265,100,000
Operating loss carryforwards, acquired	$ 22,900,000

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance of gross unrecognized tax benefits at the beginning of the fiscal year	$ 25.6	$ 15.7	$ 7.9
Gross increases related to prior period tax positions	1.1	0	2.2
Gross increases related to current period tax positions	33.1	9.9	5.6
Balance of gross unrecognized tax benefits at the end of the fiscal year	$ 59.8	$ 25.6	$ 15.7

Related Party Transactions (Det

Related Party Transactions (Details)	1 Months Ended	4 Months Ended	12 Months Ended
	Jan. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016shareholder
Controlling Shareholders | Dropbox Charitable Foundation
Related Party Transaction [Line Items]
Number of controlling shareholders in related party transaction | shareholder					2
Payments to related party			$ 0	$ 1,900,000
General and administrative expense from donated shares				9,400,000
Affiliated Entity | Infrastructure Equipment Under Capital Leases and Commercial Products and Services
Related Party Transaction [Line Items]
Payments to related party	$ 5,500,000	$ 18,400,000		$ 87,100,000

Geographic Areas (Details)

Geographic Areas (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Property and equipment, net	$ 310.6	$ 341.9
Revenue	1,391.7	1,106.8	$ 844.8
United States
Segment Reporting Information [Line Items]
Property and equipment, net	293.6	323.7
Revenue	706.5	575.7	455.9
International
Segment Reporting Information [Line Items]
Property and equipment, net	17	18.2
Revenue	$ 685.2	$ 531.1	$ 388.9

Subsequent Events (Details)

Subsequent Events (Details) - HelloSign Inc. - Subsequent Event $ in Millions	Feb. 08, 2019USD ($)
Subsequent Event [Line Items]
Consideration transferred	$ 230
Consideration transferred, subject to on-going employee service	$ 48.5
Employee service period	3 years