Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance (in shares) at Dec. 31, 2017			49,406,000
Beginning balance at Dec. 31, 2017	$ 1,623,432	$ (29,375)	$ 494	$ 2,540,951	$ (4,205)	$ (913,808)	$ (29,375)
Vesting of restricted stock units (in shares)			152,000
Vesting of restricted stock units	0		$ 2	(2)
Issuance of common stock upon public offering, net of issuance costs (in shares)			4,169,000
Issuance of common stock upon public offering, net of issuance costs of $34,588	649,368		$ 42	649,326
Issuance of common stock to Regeneron (in shares)			420,000
Issuance of common stock to Regeneron	54,484		$ 4	54,480
Exercise of stock options (in shares)			575,000
Exercise of stock options	29,768		$ 5	29,763
Purchase of common stock under ESPP (in shares)			16,000
Purchase of common stock under ESPP	1,604			1,604
Stock-based compensation	110,836			110,836
Other comprehensive income	578				578
Net loss	(555,625)					(555,625)
Ending balance (in shares) at Dec. 31, 2018			54,738,000
Ending balance at Dec. 31, 2018	$ 1,885,070	$ 6,564	$ 547	3,386,958	(3,627)	(1,498,808)	$ 6,564
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Vesting of restricted stock units (in shares)			251,000
Vesting of restricted stock units	$ 0		$ 3	(3)
Exercise of stock options (in shares)			354,000
Exercise of stock options	17,838		$ 4	17,834
Purchase of common stock under ESPP (in shares)			25,000
Purchase of common stock under ESPP	2,766			2,766
Stock-based compensation	160,629			160,629
Other comprehensive income	1,734				1,734
Net loss	$ (789,608)					(789,608)
Ending balance (in shares) at Dec. 31, 2019	55,368,000		55,368,000
Ending balance at Dec. 31, 2019	$ 1,284,993		$ 554	3,568,184	(1,893)	(2,281,852)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Vesting of restricted stock units (in shares)			434,000
Vesting of restricted stock units	$ 0		$ 4	(4)
Issuance of common stock upon public offering, net of issuance costs (in shares)			10,455,000
Issuance of common stock upon public offering, net of issuance costs of $34,588	$ 541,536		$ 105	541,431
Exercise of stock options (in shares)	95,000		95,000
Exercise of stock options	$ 1,847		$ 1	1,846
Purchase of common stock under ESPP (in shares)			80,000
Purchase of common stock under ESPP	3,775		$ 1	3,774
Stock-based compensation	145,212			145,212
Other comprehensive income	(3,612)				(3,612)
Net loss	$ (618,695)					(618,695)
Ending balance (in shares) at Dec. 31, 2020	66,432,000		66,432,000
Ending balance at Dec. 31, 2020	$ 1,355,056		$ 665	$ 4,260,443	$ (5,505)	$ (2,900,547)