Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended
	Sep. 30, 2020	Apr. 30, 2020	Oct. 31, 2019	Apr. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Policies
Net (loss) income					$ (63,853)	$ (35,372)
Accumulated deficit					986,973		$ 929,318
Cash, cash equivalents and restricted cash					65,289	$ 74,199	48,855	$ 61,981
Marketable securities					588,446		688,606
Short-term Credit Line, outstanding balance					$ 50,052		$ 50,054
Common stock, shares issued					87,430,000		86,223,000
Convertible Notes
Policies
Outstanding principal balance		$ 287,500			$ 287,500
Per annum interest rate (as a percent)		2.25%			2.25%
Net proceeds		$ 278,300
Debt principal balance					$ 287,500
Equity offering
Policies
Common stock, shares issued	4,791,665		6,571,428	6,052,631
Stock issued (in dollars per share)	$ 60.00		$ 35	$ 19
Payment of offering expenses	$ 300		$ 400	$ 600
Proceeds from issuance of common stock	$ 271,000		$ 216,200	$ 108,100