CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series C Preferred Stock [Member]	Series A, B, C and D Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member] Series C Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Series C Preferred Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2019	$ 74,574					$ 19	$ 273,966		$ (199,427)	$ 16
Balances, Shares at Dec. 31, 2019				2,720		19,190,695
Issuance of common stock upon the exercise of stock options	808						808
Issuance of common stock upon the exercise of stock options, shares						131,933
Issuance of common stock, net of financing/offering costs and net of issuance costs	16,226	$ 20,542				$ 2	16,224	$ 20,542
Issuance of common stock and preferred stock, net of financing/offering costs, shares					2,287	1,691,467
Accretion of Series C preferred stock					$ 549			(549)
Beneficial conversion feature of Series C preferred stock					$ (549)			$ 549
Share-based compensation expense	2,324						2,324
Unrealized gain (loss) on available-for-sale securities	(1)									(1)
Net loss	(40,759)								(40,759)
Balances at Jun. 30, 2020	73,714					$ 21	313,864		(240,186)	15
Balances, Shares at Jun. 30, 2020				5,007		21,014,095
Balances at Mar. 31, 2020	82,427					$ 20	305,111		(222,685)	(19)
Balances, Shares at Mar. 31, 2020				5,007		20,328,340
Issuance of common stock upon the exercise of stock options	269						269
Issuance of common stock upon the exercise of stock options, shares						40,537
Issuance of common stock, net of financing/offering costs and net of issuance costs	7,271					$ 1	7,270
Issuance of common stock and preferred stock, net of financing/offering costs, shares						645,218
Share-based compensation expense	1,214						1,214
Unrealized gain (loss) on available-for-sale securities	34									34
Net loss	(17,501)								(17,501)
Balances at Jun. 30, 2020	73,714					$ 21	313,864		(240,186)	15
Balances, Shares at Jun. 30, 2020				5,007		21,014,095
Balances at Dec. 31, 2020	132,040		$ 3			$ 29	409,722		(277,707)	(7)
Balances, Shares at Dec. 31, 2020			3,218,287			29,260,247
Issuance of common stock upon the exercise of stock options	$ 370						370
Issuance of common stock upon the exercise of stock options, shares	46,715					46,715
Issuance of common stock, net of financing/offering costs and net of issuance costs	$ 4,123					$ 1	4,122
Issuance of common stock and preferred stock, net of financing/offering costs, shares						257,185
Conversion of convertible preferred stock to common stock, shares			(135)			135,000
Premium for commitment to sell common stock							12,463
Share-based compensation expense	4,172						4,172
Unrealized gain (loss) on available-for-sale securities	5									5
Net loss	(37,995)								(37,995)
Balances at Jun. 30, 2021	90,252		$ 3			$ 30	405,923		(315,702)	(2)
Balances, Shares at Jun. 30, 2021			3,218,152			29,699,147
Balances at Mar. 31, 2021	118,846		$ 3			$ 30	415,946		(297,130)	(3)
Balances, Shares at Mar. 31, 2021			3,218,152			29,663,946
Issuance of common stock upon the exercise of stock options	277						277
Issuance of common stock upon the exercise of stock options, shares						35,201
Premium for commitment to sell common stock	12,463						12,463
Share-based compensation expense	2,163						2,163
Unrealized gain (loss) on available-for-sale securities	1									1
Net loss	(18,572)								(18,572)
Balances at Jun. 30, 2021	$ 90,252		$ 3			$ 30	$ 405,923		$ (315,702)	$ (2)
Balances, Shares at Jun. 30, 2021			3,218,152			29,699,147