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, 2014		24,018,577
Beginning balance, value at Dec. 31, 2014	$ 28,042	$ 24	$ 171,326	$ (107)	$ (143,201)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock, net of commissions and expenses (in shares)		1,754,556
Proceeds from issuance of common stock	50,373	$ 2	50,371
Issuance of common stock upon exercise of stock purchase rights (in shares)		5,029
Issuance of common stock upon exercise of stock purchase rights	96		96
Exercise of stock options (in shares)		296,716
Exercise of stock options	1,282		1,282
Issuance of common stock upon net exercise of warrants (in shares)		314,368
Issuance of common stock upon net exercise of warrants	9		9
Issuance of common stock for restricted stock awards, net (in shares)		69,249
Issuance of common stock for restricted stock awards, net	0
Stock-based compensation	12,945		12,945
Excess tax benefit	463		463
Other comprehensive income (loss)	(72)			(72)
Net loss	(74,363)				(74,363)
Ending balance (in shares) at Dec. 31, 2015		26,458,495
Ending balance, value at Dec. 31, 2015	18,775	$ 26	236,492	(179)	(217,564)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock, net of commissions and expenses (in shares)		6,721,529
Proceeds from issuance of common stock	167,165	$ 7	167,158
Issuance of common stock upon exercise of stock purchase rights (in shares)		75,205
Issuance of common stock upon exercise of stock purchase rights	1,120		1,120
Exercise of stock options (in shares)		149,186
Exercise of stock options	591		591
Issuance of common stock for restricted stock awards, net (in shares)		54,192
Issuance of common stock for restricted stock awards, net	(153)		(153)
Stock-based compensation	16,794		16,794
Other comprehensive income (loss)	111			111
Net loss	(99,059)				(99,059)
Ending balance (in shares) at Dec. 31, 2016		33,458,607
Ending balance, value at Dec. 31, 2016	105,344	$ 33	422,002	(68)	(316,623)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock, net of commissions and expenses (in shares)		2,506,387
Proceeds from issuance of common stock	133,812	$ 2	133,810
Issuance of common stock upon exercise of stock purchase rights (in shares)		27,953
Issuance of common stock upon exercise of stock purchase rights	$ 1,050		1,050
Exercise of stock options (in shares)	241,050	201,592
Exercise of stock options	$ 828	$ 1	827
Issuance of common stock for restricted stock awards, net (in shares)		489,952
Issuance of common stock for restricted stock awards, net	$ (750)	$ 1	(751)
Shares issued for asset acquisition (in shares)	263,146
Shares issued for asset acquisition	$ 14,302		14,302
Stock-based compensation	26,078		26,078
Other comprehensive income (loss)	40			40
Net loss	(145,105)				(145,105)
Ending balance (in shares) at Dec. 31, 2017		36,947,637
Ending balance, value at Dec. 31, 2017	$ 135,599	$ 37	$ 597,318	$ (28)	$ (461,728)