Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balances at Dec. 31, 2017	$ 50,337	$ 2	$ 122,950	$ (102)	$ (72,513)
Balances (in shares) at Dec. 31, 2017		16,670,000
Issuance of common stock in connection with employee stock purchase plan	78		78
Issuance of common stock in connection with employee stock purchase plan (in shares)		31,000
Issuance of common stock in connection with exercise of stock options	785		785
Issuance of common stock in connection with exercise of stock options (in shares)		109,000
Stock-based compensation expense	835		835
Unrealized gain on marketable securities	4			4
Net loss	(8,119)				(8,119)
Balances at Mar. 31, 2018	43,920	$ 2	124,648	(98)	(80,632)
Balances (in shares) at Mar. 31, 2018		16,810,000
Balances at Dec. 31, 2018	67,428	$ 2	184,314	(27)	(116,861)
Balances (in shares) at Dec. 31, 2018		24,140,000
Issuance of common stock in connection with employee stock purchase plan	143		143
Issuance of common stock in connection with employee stock purchase plan (in shares)		20,000
Issuance of common stock in connection with exercise of stock options	261		261
Issuance of common stock in connection with exercise of stock options (in shares)		52,000
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs	64,503	$ 1	64,502
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs (in shares)		4,625,000
Stock-based compensation expense	1,081		1,081
Unrealized gain on marketable securities	40			40
Net loss	(17,167)				(17,167)
Balances at Mar. 31, 2019	$ 116,289	$ 3	$ 250,301	$ 13	$ (134,028)
Balances (in shares) at Mar. 31, 2019		28,837,000