Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2016		40,691,000
Beginning balance at Dec. 31, 2016	$ 869,440	$ 407	$ 1,447,856	$ (1,149)	$ (577,674)
Vesting of restricted stock units (in shares)		88,000
Vesting of restricted stock units	0	$ 1	(1)
Issuance of common stock upon public offering, net of issuance costs (in shares)		7,625,000
Issuance of common stock upon public offering, net of issuance costs of $53,487	1,006,570	$ 76	1,006,494
Exercise of stock options (in shares)		981,000
Exercise of stock options	31,686	$ 10	31,676
Purchase of common stock under ESPP (in shares)		21,000
Purchase of common stock under ESPP	1,153		1,153
Stock-based compensation	53,282		53,282
Other comprehensive income	(3,056)			(3,056)
Net loss	(335,643)				(335,643)
Ending balance (in shares) at Dec. 31, 2017		49,406,000
Ending balance at Dec. 31, 2017	1,623,432	$ 494	2,540,951	(4,205)	(913,808)
Vesting of restricted stock units (in shares)		152,000
Vesting of restricted stock units		$ 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 $53,487	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	54,738,000
Ending balance at Dec. 31, 2018	$ 1,885,070	$ 547	3,386,958	(3,627)	(1,498,808)
Vesting of restricted stock units (in shares)		251,000
Vesting of restricted stock units		$ 3	(3)
Exercise of stock options (in shares)	354,000	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)