Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 17, 2019	Apr. 26, 2019	Sep. 30, 2020	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 31, 2019	Apr. 23, 2019	Dec. 31, 2018
Policies
Net (loss) income					$ (58,323)	$ (23,137)	$ (153,331)	$ (89,644)
Accumulated deficit			$ 852,906		852,906		852,906		$ 699,171
Cash, cash equivalents and restricted cash			209,529		209,529	$ 51,664	209,529	$ 51,664	61,981			$ 51,004
Marketable securities			600,205		600,205		600,205		379,065
Short-term Credit Line, outstanding balance			$ 50,052		$ 50,052		$ 50,052		$ 50,123
Common stock, shares issued			85,281,000		85,281,000		85,281,000		78,005,000
Payment of offering expenses	$ 400	$ 600	$ 300
Proceeds from issuance of common stock	$ 216,200	$ 108,100	271,000
Net proceeds							$ 278,316
Debt principal balance			75,000		$ 75,000		75,000
Loss on Debt Extinguishment							5,848
Convertible Notes
Policies
Outstanding principal balance			287,500	$ 287,500	287,500		$ 287,500
Per annum interest rate (as a percent)				2.25%			2.25%
Net proceeds				$ 278,300
Debt principal balance			$ 287,500		$ 287,500		$ 287,500
Equity offering
Policies
Common stock, shares issued			4,791,665		4,791,665		4,791,665			6,571,428	6,052,631
Stock issued (in dollars per share)	$ 35		$ 60.00		$ 60.00		$ 60.00				$ 19