Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment	$ 107,500	$ 71,891	$ (134,553)		$ (98,578)
Accounting Standards Update 2016-09 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment					0
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment	$ (238,891)	$ (191,526)	$ (139,111)		(101,556)
Accumulated Deficit | Accounting Standards Update 2016-09 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment				$ 13	$ (13)