Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED OTHER COMPREHENSIVE (LOSS) GAIN	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 gain (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]
Net loss	(181,111)
Ending balance (in shares) at Sep. 30, 2018		45,451,227
Ending balance at Sep. 30, 2018	268,582	$ 45	913,499	(1)	(644,961)
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 gain (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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		234,308
Issuance of common stock upon exercise of stock options and warrants	1,146		1,146
Issuance of common stock for restricted stock awards, net (in shares)		8,099
Issuance of common stock for restricted stock awards, net	(107)		(107)
Stock-based compensation	9,945		9,945
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
Other comprehensive gain (loss)	8			8
Net loss	(85,388)				(85,388)
Ending balance (in shares) at Sep. 30, 2018		45,451,227
Ending balance at Sep. 30, 2018	268,582	$ 45	913,499	(1)	(644,961)
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)	139,119
Net loss	$ (144,517)
Ending balance (in shares) at Sep. 30, 2019		45,998,956
Ending balance at Sep. 30, 2019	209,204	$ 46	1,050,252	(158)	(840,936)
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
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		15,000
Issuance of common stock upon exercise of stock options and warrants	159		159
Issuance of common stock for restricted stock awards, net (in shares)		34,538
Issuance of common stock for restricted stock awards, net	267		267
Stock-based compensation	10,804		10,804
Other comprehensive gain (loss)	(158)			(158)
Equity component of Convertible Notes, net of issuance costs of $3,725	124,666		124,666
Payment for capped call share options	(32,890)		(32,890)
Net loss	(49,402)
Ending balance (in shares) at Sep. 30, 2019		45,998,956
Ending balance at Sep. 30, 2019	$ 209,204	$ 46	$ 1,050,252	$ (158)	$ (840,936)