Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Balance, Beginning at Dec. 31, 2017		$ 222,188	$ (6)	$ (101,550)	$ 120,632
Balance, Beginning (in shares) at Dec. 31, 2017	21,088,306
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		4,847			4,847
Issuance of common stock upon private placement, net of issuance costs		21,679			21,679
Issuance of common stock upon private placement net of issuance costs(in shares)	2,750,000
Common stock issued pursuant to at-the-market offering, net of issuance costs		1,025			1,025
Common stock issued pursuant to at-the-market offering, net of issuance costs (in shares)	103,500
Common stock issued under equity incentive and employee stock purchase plans		931			931
Common stock issued under equity incentive and employee stock purchase plans (in shares)	196,296
Other comprehensive gain (loss)			(97)		(97)
Net loss				(25,059)	(25,059)
Balance, Ending at Sep. 30, 2018		250,670	(103)	(126,609)	123,958
Balance, Ending (in shares) at Sep. 30, 2018	24,138,102
Balance, Beginning at Jun. 30, 2018		225,622	(86)	(117,874)	107,662
Balance, Beginning (in shares) at Jun. 30, 2018	21,217,494
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		2,017			2,017
Issuance of common stock upon private placement, net of issuance costs		21,679			21,679
Issuance of common stock upon private placement net of issuance costs(in shares)	2,750,000
Common stock issued pursuant to at-the-market offering, net of issuance costs		1,025			1,025
Common stock issued pursuant to at-the-market offering, net of issuance costs (in shares)	103,500
Common stock issued under equity incentive and employee stock purchase plans		327			327
Common stock issued under equity incentive and employee stock purchase plans (in shares)	67,108
Other comprehensive gain (loss)			(17)		(17)
Net loss				(8,735)	(8,735)
Balance, Ending at Sep. 30, 2018		250,670	(103)	(126,609)	123,958
Balance, Ending (in shares) at Sep. 30, 2018	24,138,102
Balance, Beginning at Dec. 31, 2018		253,222	(233)	(140,474)	$ 112,515
Balance, Beginning (in shares) at Dec. 31, 2018	23,187,219				23,187,219
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		6,193			$ 6,193
Common stock issued pursuant to at-the-market offering, net of issuance costs		34,492			34,492
Common stock issued pursuant to at-the-market offering, net of issuance costs (in shares)	2,846,641
Common stock issued pursuant to exercise of Exchange Warrants (in shares)	599,997
Common stock issued under equity incentive and employee stock purchase plans		1,765			1,765
Common stock issued under equity incentive and employee stock purchase plans (in shares)	572,590
Other comprehensive gain (loss)			79		79
Net loss				(59,686)	(59,686)
Balance, Ending at Sep. 30, 2019		295,672	(154)	(200,160)	$ 95,358
Balance, Ending (in shares) at Sep. 30, 2019	27,206,447				27,206,447
Balance, Beginning at Jun. 30, 2019		268,234	(167)	(183,751)	$ 84,316
Balance, Beginning (in shares) at Jun. 30, 2019	24,967,603
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		2,201			2,201
Common stock issued pursuant to at-the-market offering, net of issuance costs		23,949			23,949
Common stock issued pursuant to at-the-market offering, net of issuance costs (in shares)	1,924,957
Common stock issued under equity incentive and employee stock purchase plans		1,288			1,288
Common stock issued under equity incentive and employee stock purchase plans (in shares)	313,887
Other comprehensive gain (loss)			13		13
Net loss				(16,409)	(16,409)
Balance, Ending at Sep. 30, 2019		$ 295,672	$ (154)	$ (200,160)	$ 95,358
Balance, Ending (in shares) at Sep. 30, 2019	27,206,447				27,206,447