CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In-Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2017	$ 118,942	$ 21	$ 323,482	$ (204,525)	$ (36)
Beginning balance, shares at Dec. 31, 2017		20,973
Issuance of common stock	2,015		2,015
Issuance of common stock, shares		359
Stock-based compensation	2,065		2,065
Net loss	(14,442)			(14,442)
Other comprehensive income (loss)	(20)				(20)
Ending balance at Mar. 31, 2018	108,560	$ 21	327,562	(218,967)	(56)
Ending balance, shares at Mar. 31, 2018		21,332
Beginning balance at Dec. 31, 2017	118,942	$ 21	323,482	(204,525)	(36)
Beginning balance, shares at Dec. 31, 2017		20,973
Net loss	(29,244)
Other comprehensive income (loss)	19
Ending balance at Sep. 30, 2018	100,463	$ 21	334,228	(233,769)	(17)
Ending balance, shares at Sep. 30, 2018		21,482
Beginning balance at Mar. 31, 2018	108,560	$ 21	327,562	(218,967)	(56)
Beginning balance, shares at Mar. 31, 2018		21,332
Issuance of common stock	266		266
Issuance of common stock, shares		63
Stock-based compensation	2,613		2,613
Net loss	(7,732)			(7,732)
Other comprehensive income (loss)	20				20
Ending balance at Jun. 30, 2018	103,727	$ 21	330,441	(226,699)	(36)
Ending balance, shares at Jun. 30, 2018		21,395
Issuance of common stock	452		452
Issuance of common stock, shares		87
Stock-based compensation	3,335		3,335
Net loss	(7,070)			(7,070)
Other comprehensive income (loss)	19				19
Ending balance at Sep. 30, 2018	100,463	$ 21	334,228	(233,769)	(17)
Ending balance, shares at Sep. 30, 2018		21,482
Beginning balance at Dec. 31, 2018	97,883	$ 21	337,627	(239,756)	(9)
Beginning balance, shares at Dec. 31, 2018		21,492
Issuance of common stock	1,863	$ 1	1,862
Issuance of common stock, shares		135
Stock-based compensation	3,477		3,477
Net loss	(7,068)			(7,068)
Other comprehensive income (loss)	16				16
Ending balance at Mar. 31, 2019	96,171	$ 22	342,966	(246,824)	7
Ending balance, shares at Mar. 31, 2019		21,627
Beginning balance at Dec. 31, 2018	97,883	$ 21	337,627	(239,756)	(9)
Beginning balance, shares at Dec. 31, 2018		21,492
Net loss	(15,307)
Other comprehensive income (loss)	45
Ending balance at Sep. 30, 2019	102,536	$ 22	357,541	(255,063)	36
Ending balance, shares at Sep. 30, 2019		22,128
Beginning balance at Mar. 31, 2019	96,171	$ 22	342,966	(246,824)	7
Beginning balance, shares at Mar. 31, 2019		21,627
Issuance of common stock	1,049		1,049
Issuance of common stock, shares		134
Stock-based compensation	3,543		3,543
Net loss	(4,158)			(4,158)
Other comprehensive income (loss)	27				27
Ending balance at Jun. 30, 2019	96,632	$ 22	347,558	(250,982)	34
Ending balance, shares at Jun. 30, 2019		21,761
Issuance of common stock	5,190		5,190
Issuance of common stock, shares		367
Stock-based compensation	4,793		4,793
Net loss	(4,081)			(4,081)
Other comprehensive income (loss)	2				2
Ending balance at Sep. 30, 2019	$ 102,536	$ 22	$ 357,541	$ (255,063)	$ 36
Ending balance, shares at Sep. 30, 2019		22,128