Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2016	$ 368,517	$ 4	$ (17)	$ 688,959	$ (102)	$ (320,327)
Balances, Shares at Dec. 31, 2016		37,222,172	346
Issuance of common stock from exercise of stock options, Amount	15,437	$ 1		15,436
Issuance of common stock from exercise of stock options, Shares		706,222
Issuance of common stock under employee stock purchase plan	771			771
Issuance of common stock under employee stock purchase plan, Shares		15,718
Purchase of treasury stock, Amount	(96)		$ (96)
Purchase of treasury stock, Shares			614
Stock-based compensation expense	35,142			35,142
Public offering of common stock, net of offering costs, Amount	325,751			325,751
Public offering of common stock, net of offering costs, Shares		4,058,822
Unrealized gain (loss) on available-for-sale securities	73				73
Net loss	(270,120)					(270,120)
Balances at Dec. 31, 2017	475,475	$ 5	$ (113)	1,066,059	(29)	(590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960
Issuance of common stock from exercise of stock options, Amount	27,014			27,014
Issuance of common stock from exercise of stock options, Shares		824,188
Issuance of common stock under employee stock purchase plan	2,705			2,705
Issuance of common stock under employee stock purchase plan, Shares		19,687
Purchase of treasury stock, Amount	(98)		$ (98)
Purchase of treasury stock, Shares			2,073
Stock-based compensation expense	100,993			100,993
Public offering of common stock, net of offering costs, Amount	631,154			631,154
Public offering of common stock, net of offering costs, Shares		4,032,012
Unrealized gain (loss) on available-for-sale securities	(486)				(486)
Vesting of restricted stock units, net of employee tax obligations	(904)			(904)
Vesting of restricted stock units, net of employee tax obligations, Shares		9,442
Net loss	(372,882)					(372,882)
Balances at Dec. 31, 2018	862,971	$ 5	$ (211)	1,827,021	(515)	(963,329)
Balances, Shares at Dec. 31, 2018		46,888,263	3,033
Issuance of common stock from exercise of stock options, Amount	44,276			44,276
Issuance of common stock from exercise of stock options, Shares		1,031,989
Issuance of common stock under employee stock purchase plan	5,744			5,744
Issuance of common stock under employee stock purchase plan, Shares		55,404
Purchase of treasury stock, Amount	$ (189)		$ (189)
Purchase of treasury stock, Shares	(3,033)
Stock-based compensation expense	$ 151,508			151,508
Public offering of common stock, net of offering costs, Amount	560,948			560,948
Public offering of common stock, net of offering costs, Shares		3,833,334
Unrealized gain (loss) on available-for-sale securities	1,810				1,810
Vesting of restricted stock units, net of employee tax obligations	(2,175)			(2,175)
Vesting of restricted stock units, net of employee tax obligations, Shares		68,204
Net loss	(680,238)					(680,238)
Balances at Dec. 31, 2019	$ 944,655	$ 5	$ (400)	$ 2,587,322	$ 1,295	$ (1,643,567)
Balances, Shares at Dec. 31, 2019		51,877,194	3,033