Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended		6 Months Ended
	Oct. 17, 2019	Apr. 26, 2019	Apr. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Oct. 31, 2019	Apr. 23, 2019	Dec. 31, 2018
Policies
Net (loss) income				$ (59,637)	$ (32,416)	$ (95,009)	$ (66,507)
Accumulated deficit				794,583		794,583		$ 699,171
Cash, cash equivalents and restricted cash				77,377	$ 29,983	77,377	$ 29,983	61,981			$ 51,004
Marketable securities				493,852		493,852		379,065
Short-term Credit Line, outstanding balance				$ 50,053		$ 50,053		$ 50,123
Common stock, shares issued				79,717,000		79,717,000		78,005,000
Payment of offering expenses	$ 400	$ 600
Proceeds from issuance of common stock	$ 216,200	$ 108,100
Net proceeds						$ 278,317
Debt principal balance				$ 75,000		75,000
Loss on debt extinguishment				5,848		5,848
Convertible Notes
Policies
Outstanding principal balance			$ 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
Equity offering
Policies
Common stock, shares issued									6,571,428	6,052,631
Stock issued (in dollars per share)	$ 35									$ 19