Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended					12 Months Ended
	Apr. 26, 2019	Sep. 30, 2020	Apr. 30, 2020	Oct. 31, 2019	Apr. 30, 2019	Jul. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 23, 2019	Dec. 31, 2017
Policies
Net (loss) income							$ (229,743)	$ (124,827)	$ (128,154)
Accumulated deficit							929,318	699,171
Cash, cash equivalents and restricted cash							48,855	61,981	$ 51,004		$ 13,021
Marketable securities							688,606	379,065
Short-term Credit Line, outstanding balance							$ 50,054	$ 50,123
Common stock, shares issued		4,791,665		5,714,286	5,263,158	4,500,000	86,223,000	78,005,000
Stock issued (in dollars per share)				$ 35	$ 19	$ 20
Payment of offering expenses	$ 600	$ 300		$ 400	$ 600	$ 500
Proceeds from issuance of common stock	$ 108,100	$ 271,000		$ 216,200	$ 108,100	$ 97,300
Net proceeds							$ 278,316
Loss on Debt Extinguishment							5,848
Convertible Notes
Policies
Outstanding principal balance			$ 287,500				287,500
Per annum interest rate (as a percent)			2.25%
Debt principal balance							287,500
2.25% Convertible Senior Notes
Policies
Outstanding principal balance							$ 287,500
Per annum interest rate (as a percent)							2.25%
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
Option for additional shares
Policies
Common stock, shares issued	789,473			857,142		675,000