Basis of Presentation and Significant Accounting Policies - Adjustments for Adoption (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jan. 01, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Convertible notes payable	$ 190,272		$ 213,645
Additional paid-in-capital	1,109,843		1,222,137
Accumulated deficit	$ (1,352,397)		(1,345,822)
Debt instrument, unamortized discount		$ 4,700	88,100
Debt issuance costs		300	$ 3,200
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Convertible notes payable		300,078
Additional paid-in-capital		1,087,687
Accumulated deficit		(1,297,805)
Accounting Standards Update 2020-06 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Convertible notes payable		86,433
Additional paid-in-capital		(134,450)
Accumulated deficit		$ 48,017