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, 2015	$ 173,695	$ 3		$ 335,032		$ (161,340)
Balances, Shares at Dec. 31, 2015		28,823,549
Issuance of common stock from exercise of stock options, Amount	1,011			1,011
Issuance of common stock from exercise of stock options, Shares		124,557
Vesting of restricted stock units, net of employee tax obligations	11			11
Vesting of restricted stock units, net of employee tax obligations, Shares		42,781
Issuance of common stock under employee stock purchase plan	510			510
Issuance of common stock under employee stock purchase plan, Shares		10,499
Purchase of treasury stock, Amount	$ (17)		$ (17)
Purchase of treasury stock, Shares	(346)		346
Stock-based compensation expense	$ 22,855			22,855
Public offering of common stock, net of offering costs, Amount	329,541	$ 1		329,540
Public offering of common stock, net of offering costs, Shares		8,220,786
Unrealized gain (loss) on available-for-sale securities	(102)				$ (102)
Net loss	(158,987)					(158,987)
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)		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
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
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)		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)
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