Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME	ACCUMULATED DEFICIT
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]
Other comprehensive (loss) income	(158)
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 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)
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
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
Equity component of convertible notes, net of issuance costs	124,666		124,666
Payment for capped call share options	(32,890)		(32,890)
Other comprehensive (loss) income	(158)			(158)
Net loss	(49,402)				(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)
Beginning balance (in shares) at Dec. 31, 2019		46,464,669
Beginning balance at Dec. 31, 2019	166,950	$ 46	1,062,996	(92)	(896,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		5,811
Issuance of common stock upon exercise of stock options and warrants	44		44
Issuance of common stock for restricted stock awards, net (in shares)		5,705
Issuance of common stock for restricted stock awards, net	(1,466)		(1,466)
Stock-based compensation	10,838		10,838
Other comprehensive (loss) income	(28)			(28)
Net loss	(49,129)				(49,129)
Ending balance (in shares) at Mar. 31, 2020		46,476,185
Ending balance at Mar. 31, 2020	127,209	$ 46	1,072,412	(120)	(945,129)
Beginning balance (in shares) at Dec. 31, 2019		46,464,669
Beginning balance at Dec. 31, 2019	$ 166,950	$ 46	1,062,996	(92)	(896,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)	58,676
Other comprehensive (loss) income	$ 80
Net loss	(136,964)
Ending balance (in shares) at Sep. 30, 2020		46,828,333
Ending balance at Sep. 30, 2020	60,097	$ 47	1,093,026	(12)	(1,032,964)
Beginning balance (in shares) at Mar. 31, 2020		46,476,185
Beginning balance at Mar. 31, 2020	127,209	$ 46	1,072,412	(120)	(945,129)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		31,615
Issuance of common stock upon exercise of stock options and warrants	119	$ 1	118
Issuance of common stock for restricted stock awards, net (in shares)		(17,945)
Issuance of common stock for restricted stock awards, net	(150)		(150)
Issuance of common stock upon exercise of stock purchase rights (in shares)		23,494
Issuance of common stock upon exercise of stock purchase rights	295		295
Stock-based compensation	10,289		10,289
Other comprehensive (loss) income	237			237
Net loss	(48,187)				(48,187)
Ending balance (in shares) at Jun. 30, 2020		46,513,349
Ending balance at Jun. 30, 2020	89,812	$ 47	1,082,964	117	(993,316)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and warrants (in shares)		13,907
Issuance of common stock upon exercise of stock options and warrants	9		9
Issuance of common stock for restricted stock awards, net (in shares)		301,077
Issuance of common stock for restricted stock awards, net	(104)		(104)
Stock-based compensation	10,157		10,157
Other comprehensive (loss) income	(129)			(129)
Net loss	(39,648)				(39,648)
Ending balance (in shares) at Sep. 30, 2020		46,828,333
Ending balance at Sep. 30, 2020	$ 60,097	$ 47	$ 1,093,026	$ (12)	$ (1,032,964)