Summary of Significant Accounting Policies - Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (1,254,272)	$ (1,254,272)		$ (929,318)
Convertible Notes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest expense	1,925	5,769
Outstanding principle	287,500	287,500	$ 287,500
Net carrying amount	280,085	280,085
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest expense	$ 2,400	$ 7,000
Previously reported | Convertible Notes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Outstanding principle				287,500
Unamortized debt discount and issuance cost				(85,007)
Net carrying amount				202,493
Additional paid-in-capital				(1,411,286)
Accumulated deficit				(929,318)
ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment					$ 400
Accounting Standards Update 2020-06 | Convertible Notes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Outstanding principle				287,500
Unamortized debt discount and issuance cost				(8,333)
Net carrying amount				279,167
Additional paid-in-capital				(1,328,414)
Accumulated deficit				(923,120)
Accounting Standards Update 2020-06 | Adoption Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reduction in net loss from continuing operations per share	$ 0.03	$ 0.08
Accounting Standards Update 2020-06 | Adoption Adjustment | Convertible Notes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt discount and issuance cost				76,674
Net carrying amount				76,674
Additional paid-in-capital				82,872
Accumulated deficit				$ 6,198