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, 2015		26,458,495
Beginning balance, value at Dec. 31, 2015	$ 18,775	$ 26	$ 236,492	$ (179)	$ (217,564)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of commissions and expenses (in shares)		6,721,529
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
Issuance of common stock upon exercise (in shares)		149,186
Issuance of common stock upon exercise	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	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]
Issuance of common stock, net of commissions and expenses (in shares)		2,506,387
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
Issuance of common stock upon exercise (in shares)		201,592
Issuance of common stock upon exercise	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
Stock-based compensation	$ 26,078		26,078
Shares issued for asset acquisition	14,302		14,302
Other comprehensive income	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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of commissions and expenses (in shares)		2,313,824
Issuance of common stock	136,445	$ 2	136,443
Issuance of common stock upon exercise of stock purchase rights (in shares)		34,193
Issuance of common stock upon exercise of stock purchase rights	$ 1,401		1,401
Issuance of common stock upon exercise (in shares)	616,248	597,777
Issuance of common stock upon exercise	$ 4,251	$ 1	4,250
Issuance of common stock for restricted stock awards, net (in shares)		216,005
Issuance of common stock for restricted stock awards, net	(2,172)		(2,172)
Issuance of shares upon conversion of 2014 Convertible Notes (in shares)		5,369,447
Issuance of shares upon conversion of 2014 Convertible Notes	148,083	$ 5	148,078
Stock-based compensation	38,862		38,862
Other comprehensive income	28			28
Net loss	(232,569)				(232,569)
Ending balance (in shares) at Dec. 31, 2018		45,478,883
Ending balance, value at Dec. 31, 2018	$ 227,806	$ 45	$ 924,180	$ 0	$ (696,419)