Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balances at Dec. 31, 2018	$ 67,428	$ 2	$ 184,314	$ (27)	$ (116,861)
Balances (in shares) at Dec. 31, 2018		24,140
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
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
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
Stock-based compensation expense	1,081		1,081
Unrealized (loss) 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
Balances at Dec. 31, 2018	67,428	$ 2	184,314	(27)	(116,861)
Balances (in shares) at Dec. 31, 2018		24,140
Unrealized (loss) gain on marketable securities	105
Net loss	(56,759)
Balances at Sep. 30, 2019	79,795	$ 3	253,334	78	(173,620)
Balances (in shares) at Sep. 30, 2019		28,923
Balances at Mar. 31, 2019	116,289	$ 3	250,301	13	(134,028)
Balances (in shares) at Mar. 31, 2019		28,837
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
Stock-based compensation expense	1,245		1,245
Unrealized (loss) 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
Issuance of common stock in connection with employee stock purchase plan	164		164
Issuance of common stock in connection with employee stock purchase plan (in shares)		25
Issuance of common stock in connection with exercise of stock options	212		212
Issuance of common stock in connection with exercise of stock options (in shares)		50
Stock-based compensation expense	1,366		1,366
Unrealized (loss) gain on marketable securities	11			11
Net loss	(21,573)				(21,573)
Balances at Sep. 30, 2019	79,795	$ 3	253,334	78	(173,620)
Balances (in shares) at Sep. 30, 2019		28,923
Balances at Dec. 31, 2019	60,081	$ 3	255,142	51	(195,115)
Balances (in shares) at Dec. 31, 2019		29,084
Issuance of common stock in connection with employee stock purchase plan	174		174
Issuance of common stock in connection with employee stock purchase plan (in shares)		26
Issuance of common stock in connection with exercise of stock options	115		115
Issuance of common stock in connection with exercise of stock options (in shares)		37
Stock-based compensation expense	1,348		1,348
Unrealized (loss) gain on marketable securities	(185)			(185)
Net loss	(18,728)				(18,728)
Balances at Mar. 31, 2020	42,805	$ 3	256,779	(134)	(213,843)
Balances (in shares) at Mar. 31, 2020		29,147
Balances at Dec. 31, 2019	60,081	$ 3	255,142	51	(195,115)
Balances (in shares) at Dec. 31, 2019		29,084
Foreign currency translation adjustment	5
Unrealized (loss) gain on marketable securities	(36)
Net loss	(58,179)
Balances at Sep. 30, 2020	136,274	$ 5	389,543	20	(253,294)
Balances (in shares) at Sep. 30, 2020		44,692
Balances at Mar. 31, 2020	42,805	$ 3	256,779	(134)	(213,843)
Balances (in shares) at Mar. 31, 2020		29,147
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs	129,054	$ 2	129,052
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs (in shares)		15,443
Issuance of common stock in connection with exercise of stock options	44		44
Issuance of common stock in connection with exercise of stock options (in shares)		10
Stock-based compensation expense	1,600		1,600
Unrealized (loss) gain on marketable securities	173			173
Net loss	(21,418)				(21,418)
Balances at Jun. 30, 2020	152,258	$ 5	387,475	39	(235,261)
Balances (in shares) at Jun. 30, 2020		44,600
Offering costs in connection with public offering	(39)		(39)
Issuance of common stock in connection with employee stock purchase plan	192		192
Issuance of common stock in connection with employee stock purchase plan (in shares)		34
Issuance of common stock in connection with exercise of stock options	219		219
Issuance of common stock in connection with exercise of stock options (in shares)		58
Stock-based compensation expense	1,696		1,696
Foreign currency translation adjustment	5			5
Unrealized (loss) gain on marketable securities	(24)			(24)
Net loss	(18,033)				(18,033)
Balances at Sep. 30, 2020	$ 136,274	$ 5	$ 389,543	$ 20	$ (253,294)
Balances (in shares) at Sep. 30, 2020		44,692