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, 2014	$ 121,885	$ 3		$ 188,727		$ (66,845)
Balances, Shares at Dec. 31, 2014		25,621,791
Issuance of common stock from exercise of stock options, Amount	603			603
Issuance of common stock from exercise of stock options, Shares		417,475
Vesting of restricted stock, Amount	17			17
Vesting of restricted stock, Shares		128,051
Issuance of common stock under employee stock purchase plan	127			127
Issuance of common stock under employee stock purchase plan, Shares		3,852
Issuance of common stock in payment of consultant fees, Amount	1,211			1,211
Issuance of common stock in payment of consultant fees, Shares		23,809
Stock-based compensation expense	15,176			15,176
Public offering of common stock, net of offering costs, Amount	129,171			129,171
Public offering of common stock, net of offering costs, Shares		2,628,571
Net loss	(94,495)					(94,495)
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, Amount	11			11
Vesting of restricted stock, 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