Background and Basis of Presentation (Policies)	3 Months Ended
Background and Basis of Presentation (Policies)	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated balance sheet as of March 31, 2021, the condensed consolidated statements of operations, the condensed consolidated statements of comprehensive loss, the condensed consolidated statements of stockholder's equity, and the condensed consolidated statements of cash flows for the three months ended March 31, 2021 and 2020, and the related footnote disclosures are unaudited. In the opinion of management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, including normal recurring adjustments, necessary to present fairly our financial position as of March 31, 2021, our results of operations, results of comprehensive loss, stockholder's equity, and cash flows for the three months ended March 31, 2021 and 2020. Our results of operations, results of comprehensive loss, stockholder's equity, and cash flows for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the full year. We operate in a single segment. Our fiscal year ends on December 31 and in this report we refer to the year ended December 31, 2020 as 2020. The condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto that are included in our Annual Report on Form 10-K for the year ended December 31, 2020 (the Annual Report on Form 10-K) filed with the U.S. Securities and Exchange Commission (SEC). Except for our policies on convertible senior notes, net, share-based compensation expense, and net loss per share, there have been no material changes to our significant accounting policies as compared to the significant accounting policies described in our Annual Report on Form 10-K. Our policies on convertible senior notes, net and net loss per share were updated as a result of our adoption of Accounting Standards Update (ASU) 2020-06, Accounting for Convertible Instruments and Contracts in an Entity's Own Equity on January 1, 2021 under the modified retrospective method. For further information on ASU 2020-06, see the section below titled “Recent Accounting Pronouncements” of this Note 1, “Background and Basis of Presentation.”
Convertible Senior Notes, net	Convertible Senior Notes, net In August 2020, we issued $1.0 billion in aggregate principal amount of 0% convertible senior notes due in 2026 (2026 notes). In March 2019, we issued $700 million in aggregate principal amount of 0.125% convertible senior notes due in 2025 (2025 notes) and in April 2019, the initial purchasers fully exercised their option to purchase $100 million of additional 2025 notes for aggregate total gross proceeds of $800 million. In April 2018, we issued $345 million in aggregate principal amount of 0.25% convertible senior notes due in 2023 (2023 notes). Collectively, the 2026 notes, 2025 notes, and the 2023 notes are referred to as the “notes.” The notes, including the embedded conversion features, are accounted for under the traditional convertible debt accounting model entirely as a liability net of unamortized issuance costs. The carrying amount of the liability is classified as a current liability if we have committed to settle with current assets; otherwise, we classify it as a long-term liability as we retain the election to settle conversion requests in shares of our common stock. The embedded conversion features are not remeasured as long as they do not meet the separation requirement of a derivative; otherwise, they are classified as derivative instruments and recorded at fair value with changes in fair value recorded in other (expense) income, net on our condensed consolidated statements of operations. The fair value of any derivative instruments related to the notes are determined utilizing Level 2 inputs. Issuance costs are amortized on a straight-line basis, which approximates the effective interest rate method, to interest expense over the term of the notes. In accounting for conversions of the notes, the carrying amount of the converted notes is reduced by the total consideration paid or issued for the respective converted notes and the difference is recorded to additional paid-in capital on our condensed consolidated balance sheets. In accounting for extinguishments of the notes, the reacquisition price of the extinguished notes is compared to the carrying amount of the respective extinguished notes and a gain or loss is recorded in other (expense) income, net on our condensed consolidated statements of operations.
Share-based Compensation Expense	Share-based Compensation Expense Share-based compensation expense for restricted stock units (RSUs), performance-based restricted stock units (PSUs) with either a market-based condition or financial performance targets, and the employee stock purchase plan (ESPP) is accounted for under the fair value method based on the grant-date fair value of the award. Share-based compensation expense for RSUs and PSUs with financial performance targets is measured based on the closing fair market value of the Company’s common stock, PSUs with a market-based condition is estimated using a Monte Carlo simulation model, and the ESPP is estimated using the Black-Scholes-Merton option pricing model. We recognize share-based compensation expense, subject to continuing service over the requisite service period, which is generally the vesting period, on a straight-line basis for RSUs and the ESPP and on a graded basis for PSUs. Share-based compensation expense for PSUs with a market-based condition is recognized regardless of whether the market condition is satisfied whereas share-based compensation expense for PSUs with financial performance targets is recognized upon estimated or actual achievement of such targets. These amounts are reduced by estimated forfeitures, which are estimated at the time of the grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing net loss by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by giving effect to all potential shares of common stock, including stock options, PSUs, RSUs, and shares related to convertible senior notes, to the extent dilutive. Diluted net loss per share assumes that all stock options and dilutive convertible shares were exercised or converted and is computed by applying the treasury stock method for outstanding stock options, PSUs, and RSUs, and the if-converted method for outstanding convertible senior notes. Under the treasury stock method, options, PSUs, and RSUs are assumed to be exercised or vested at the beginning of the period (or at the time of issuance, if later) and as if funds obtained thereby were used to purchase common stock at the average market price during the period. Under the if-converted method, outstanding convertible senior notes are assumed to be converted into common stock at the beginning of the period (or at the time of issuance, if later).
Use of Estimates	Use of EstimatesThe preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates, judgments, and assumptions that affect the reported amounts of assets and liabilities; the disclosure of contingent liabilities at the date of the financial statements; and the reported amounts of revenues and expenses during the reporting periods. We base our estimates on historical experience, knowledge of current business conditions, and various other factors we believe to be reasonable under the circumstances. These estimates are based on management’s knowledge about current events and expectations about actions we may undertake in the future. Actual results could differ from these estimates, and such differences could be material to our financial position and results of operations.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Issued Accounting Pronouncements Not Yet Adopted There were no accounting pronouncements issued during the three months ended March 31, 2021 that would have an impact on our financial statements. Recently Adopted Accounting Pronouncements In August 2020, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2020-06, Accounting for Convertible Instruments and Contracts in an Entity's Own Equity. ASU 2020-06 simplifies the guidance in Accounting Standards Codification (ASC) 470-20, Debt - Debt with Conversion and Other Options. Under ASU 2020-06, convertible instruments with embedded conversion features, that are not required to be accounted for as a derivative or that do not result in a substantial premium, are no longer required to be separated from the host contract thereby eliminating the cash conversion feature model. Instead, these convertible debt instruments will be accounted for as a single liability measured at amortized cost under the traditional convertible debt accounting model. ASU 2020-06 also requires the if-converted method to be applied for all convertible instruments when calculating diluted earnings per share. We adopted ASU 2020-06 on January 1, 2021 under the modified retrospective method applied to convertible senior notes outstanding as of January 1, 2021 and have not changed previously disclosed amounts or provided additional disclosures for comparative periods. Adoption of ASU 2020-06 resulted in an increase to convertible senior notes of $378.1 million and a decrease to additional paid-in capital of $465.0 million due to the application of the traditional convertible debt model and no longer separating the embedded conversion feature. Accumulated deficit also decreased by $86.9 million due to the reduction in non-cash interest expense related to the debt discount and we expect interest expense to decrease in future periods. Refer to Note 7, “Convertible Senior Notes” for more information.