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, 2017		36,947,637
Beginning balance 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, net of commissions and expenses	136,375	$ 2	136,373
Issuance of common stock upon exercise of stock options and warrants (in shares)		28,654
Issuance of common stock upon exercise of stock options and warrants	95		95
Issuance of common stock for restricted stock awards, net (in shares)		212,995
Issuance of common stock for restricted stock awards, net	(1,595)	$ 1	(1,596)
Stock-based compensation	8,762		8,762
Other comprehensive loss	(129)			(129)
Net loss	(40,699)				(40,699)
Ending balance (in shares) at Mar. 31, 2018		39,503,110
Ending balance at Mar. 31, 2018	236,271	$ 40	740,952	(157)	(504,564)
Beginning balance (in shares) at Dec. 31, 2017		36,947,637
Beginning balance at Dec. 31, 2017	135,599	$ 37	597,318	(28)	(461,728)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive loss	19
Net loss	(95,723)
Ending balance (in shares) at Jun. 30, 2018		39,839,373
Ending balance at Jun. 30, 2018	194,880	$ 40	754,437	(9)	(559,588)
Beginning balance (in shares) at Mar. 31, 2018		39,503,110
Beginning balance at Mar. 31, 2018	236,271	$ 40	740,952	(157)	(504,564)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock purchase rights (in shares)		16,965
Issuance of common stock upon exercise of stock purchase rights	872		872
Issuance of common stock, net of commissions and expenses	70		70
Issuance of common stock upon exercise of stock options and warrants (in shares)		319,257
Issuance of common stock upon exercise of stock options and warrants	2,732		2,732
Issuance of common stock for restricted stock awards, net (in shares)		41
Issuance of common stock for restricted stock awards, net	(369)		(369)
Stock-based compensation	10,180		10,180
Other comprehensive loss	148			148
Net loss	(55,024)				(55,024)
Ending balance (in shares) at Jun. 30, 2018		39,839,373
Ending balance at Jun. 30, 2018	194,880	$ 40	754,437	(9)	(559,588)
Beginning balance (in shares) at Dec. 31, 2018		45,478,883
Beginning balance at Dec. 31, 2018	227,806	$ 45	924,180	0	(696,419)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		141,245
Issuance of common stock upon exercise of stock options and warrants	1,879		1,879
Issuance of common stock for restricted stock awards, net (in shares)		301,848
Issuance of common stock for restricted stock awards, net	(2,092)	$ 1	(2,093)
Stock-based compensation	12,508		12,508
Net loss	(47,951)				(47,951)
Ending balance (in shares) at Mar. 31, 2019		45,921,976
Ending balance at Mar. 31, 2019	192,150	$ 46	936,474	0	(744,370)
Beginning balance (in shares) at Dec. 31, 2018		45,478,883
Beginning balance at Dec. 31, 2018	$ 227,806	$ 45	924,180	0	(696,419)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)	124,034
Other comprehensive loss	$ 0
Net loss	(95,115)
Ending balance (in shares) at Jun. 30, 2019		45,949,418
Ending balance at Jun. 30, 2019	155,758	$ 46	947,246	0	(791,534)
Beginning balance (in shares) at Mar. 31, 2019		45,921,976
Beginning balance at Mar. 31, 2019	192,150	$ 46	936,474	0	(744,370)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock purchase rights (in shares)		22,648
Issuance of common stock upon exercise of stock purchase rights	569		569
Issuance of common stock upon exercise of stock options and warrants (in shares)		10,480
Issuance of common stock upon exercise of stock options and warrants	71		71
Issuance of common stock for restricted stock awards, net (in shares)		(5,686)
Issuance of common stock for restricted stock awards, net	(891)		(891)
Stock-based compensation	11,023		11,023
Other comprehensive loss	0
Net loss	(47,164)				(47,164)
Ending balance (in shares) at Jun. 30, 2019		45,949,418
Ending balance at Jun. 30, 2019	$ 155,758	$ 46	$ 947,246	$ 0	$ (791,534)