Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended						3 Months Ended		6 Months Ended
	Jul. 31, 2021	May 31, 2021	Sep. 30, 2020	Apr. 30, 2020	Oct. 31, 2019	Apr. 30, 2019	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Policies
Net (loss) income							$ (116,026)	$ (59,637)	$ (179,879)	$ (95,009)
Accumulated deficit							1,102,999		1,102,999		$ 929,318
Cash, cash equivalents and restricted cash							63,039	77,377	63,039	77,377	48,855	$ 61,981
Marketable securities							517,432		517,432		688,606
Short-term Credit Line, outstanding balance							$ 50,049		$ 50,049		$ 50,054
Contractual commitment for development and option agreement		$ 10,000
Payment for third party assets if purchase option is exercised		$ 290,000
Common stock, shares issued							88,498,000		88,498,000		86,223,000
Net proceeds										278,317
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
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
Equity offering | Subsequent Event
Policies
Common stock, shares issued	5,175,000
Stock issued (in dollars per share)	$ 113
Payment of offering expenses	$ 400
Proceeds from issuance of common stock	$ 551,200