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, 2017	50,337	$ 2	122,950	(102)	(72,513)
Balances (in shares) at Dec. 31, 2017		16,670,000
Unrealized gain on marketable securities	46
Net loss	(17,533)
Balances at Jun. 30, 2018	73,447	$ 2	163,547	(56)	(90,046)
Balances (in shares) at Jun. 30, 2018		21,908,000
Balances at Mar. 31, 2018	43,920	$ 2	124,648	(98)	(80,632)
Balances (in shares) at Mar. 31, 2018		16,810,000
Issuance of common stock in connection with exercise of stock options	248		248
Issuance of common stock in connection with exercise of stock options (in shares)		51,000
Issuance of common stock in connection with public offering, net of offering costs	37,655		37,655
Issuance of common stock in connection with public offering,net of offering costs (in shares)		5,047,000
Stock-based compensation expense	996		996
Unrealized gain on marketable securities	42			42
Net loss	(9,414)				(9,414)
Balances at Jun. 30, 2018	73,447	$ 2	163,547	(56)	(90,046)
Balances (in shares) at Jun. 30, 2018		21,908,000
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
Balances at Dec. 31, 2018	67,428	$ 2	184,314	(27)	(116,861)
Balances (in shares) at Dec. 31, 2018		24,140,000
Unrealized gain on marketable securities	94
Net loss	(35,186)
Balances at Jun. 30, 2019	99,615	$ 3	251,592	67	(152,047)
Balances (in shares) at Jun. 30, 2019		28,848,000
Balances at Mar. 31, 2019	116,289	$ 3	250,301	13	(134,028)
Balances (in shares) at Mar. 31, 2019		28,837,000
Issuance of common stock in connection with exercise of stock options	46		46
Issuance of common stock in connection with exercise of stock options (in shares)		11,000
Stock-based compensation expense	1,245		1,245
Unrealized gain on marketable securities	54			54
Net loss	(18,019)				(18,019)
Balances at Jun. 30, 2019	$ 99,615	$ 3	$ 251,592	$ 67	$ (152,047)
Balances (in shares) at Jun. 30, 2019		28,848,000