CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		106,801,409
Beginning balance at Dec. 31, 2019	$ (26,700)	$ 107	$ 266,717	$ (293,524)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	41,564			41,564
Share-based compensation	407		407
Sale of common stock under 2014 ESPP (in shares)		2,785
Sales of common stock under 2014 ESPP	1		1
Issuance of common stock under ATM offering, net of issuance costs (in shares)		3,187,359
Issuance of common stock under ATM offering, net of issuance costs	3,179	$ 3	3,176
Ending balance (in shares) at Mar. 31, 2020		109,991,553
Ending balance at Mar. 31, 2020	18,451	$ 110	270,301	(251,960)
Beginning balance at Dec. 31, 2019	(26,700)	$ 107	266,717	(293,524)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	15,220
Ending balance (in shares) at Jun. 30, 2020		116,627,653
Ending balance at Jun. 30, 2020	(2,628)	$ 116	275,560	(278,304)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		109,991,553
Beginning balance at Mar. 31, 2020	18,451	$ 110	270,301	(251,960)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(26,344)			(26,344)
Share-based compensation	491		491
Issuance of common stock under ATM offering, net of issuance costs (in shares)		6,636,100
Issuance of common stock under ATM offering, net of issuance costs	4,774	$ 6	4,768
Ending balance (in shares) at Jun. 30, 2020		116,627,653
Ending balance at Jun. 30, 2020	(2,628)	$ 116	275,560	(278,304)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		116,627,653
Beginning balance (in shares)		140,449,647
Beginning balance at Dec. 31, 2020	(9,227)	$ 140	306,554	(315,921)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(55,512)			(55,512)
Share-based compensation	958		958
Exercise of stock options (in shares)		30,610
Exercises of stock options	39		39
Exercise of common stock warrants (in shares)		852,840
Exercises of common stock warrants	469	$ 1	468
Issuance of common stock under ATM offering, net of issuance costs (in shares)		30,645,702
Issuance of common stock under ATM offering, net of issuance costs	72,543	$ 31	72,512
Ending balance (in shares) at Mar. 31, 2021		171,978,799
Ending balance at Mar. 31, 2021	9,270	$ 172	380,531	(371,433)
Beginning balance at Dec. 31, 2020	(9,227)	$ 140	306,554	(315,921)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ (80,954)
Exercise of stock options (in shares)	31,000
Exercise of common stock warrants (in shares)	853,000
Ending balance (in shares) at Jun. 30, 2021		188,460,951
Ending balance at Jun. 30, 2021	$ 49,349	$ 188	446,036	(396,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		171,978,799
Beginning balance at Mar. 31, 2021	9,270	$ 172	380,531	(371,433)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(25,442)			(25,442)
Share-based compensation	1,260		1,260
Issuance of common stock under ATM offering, net of issuance costs (in shares)		16,482,152
Issuance of common stock under ATM offering, net of issuance costs	64,261	$ 16	64,245
Ending balance (in shares) at Jun. 30, 2021		188,460,951
Ending balance at Jun. 30, 2021	$ 49,349	$ 188	$ 446,036	$ (396,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		188,460,951