Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT	ACCUMULATED OTHER COMPREHENSIVE LOSS
Beginning balance at Dec. 31, 2014	$ 66,346	$ 17	$ 157,092	$ (90,763)	$ 0
Beginning balance (in shares) at Dec. 31, 2014		16,878,817
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon completion of public offering, net of offering costs	75,359	$ 3	75,356
Issuance of common stock upon completion of public offering, net of offering costs (in shares)		3,047,409
Issuance of common stock under stock-based compensation plans	436	$ 0	436
Issuance of common stock under stock-based compensation plans (in shares)		226,976
Stock-based compensation	6,297		6,297
Net loss	(39,445)			(39,445)	0
Unrealized loss on investments	0
Ending balance at Dec. 31, 2015	108,993	$ 20	239,181	(130,208)	0
Ending balance (in shares) at Dec. 31, 2015		20,153,202
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon completion of public offering, net of offering costs	30,310	$ 5	30,305
Issuance of common stock upon completion of public offering, net of offering costs (in shares)		5,226,369
Issuance of common stock under stock-based compensation plans	144	$ 0	144
Issuance of common stock under stock-based compensation plans (in shares)		33,484
Stock-based compensation	9,283		9,283
Net loss	(53,530)			(53,530)	0
Unrealized loss on investments	(24)				(24)
Ending balance at Dec. 31, 2016	$ 95,176	$ 25	278,913	(183,738)	(24)
Ending balance (in shares) at Dec. 31, 2016	25,413,055	25,413,055
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of accounting standard on stock-based compensation	$ 0		157	(157)
Issuance of common stock upon completion of public offering, net of offering costs	86,576	$ 23	86,553
Issuance of common stock upon completion of public offering, net of offering costs (in shares)		22,425,000
Issuance of common stock under stock-based compensation plans	714	$ 0	714
Issuance of common stock under stock-based compensation plans (in shares)		230,595
Stock-based compensation	8,633		8,633
Net loss	(51,862)			(51,862)	0
Unrealized loss on investments	(25)				(25)
Ending balance at Dec. 31, 2017	$ 139,212	$ 48	$ 374,970	$ (235,757)	$ (49)
Ending balance (in shares) at Dec. 31, 2017	48,068,650	48,068,650