Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2018	$ 21	$ 142,032	$ (1)	$ (75,718)	$ 66,334
Balance, shares at Dec. 31, 2018	20,593,000
Issuance of shares upon exercise of stock options		272			272
Issuance of shares upon exercise of stock options, shares	99,000
Issuance of shares under employee stock purchase plan		423			423
Issuance of shares under employee stock purchase plan, shares	38,000
Issuance of shares upon vesting of restriced stock units, shares	58,000
Stock-based compensation expense		7,739			7,739
Tax payments related to shares withheld for vested restricted stock units		(613)			(613)
Unrealized gain on available-for-sale securities			9		9
Net loss				(33,183)	(33,183)
Balance at Sep. 30, 2019	$ 21	149,853	8	(108,901)	40,981
Balance, shares at Sep. 30, 2019	20,788,000
Balance at Jun. 30, 2019	$ 21	146,973	22	(97,155)	49,861
Balance, shares at Jun. 30, 2019	20,768,000
Issuance of shares under employee stock purchase plan		201			201
Issuance of shares under employee stock purchase plan, shares	20,000
Stock-based compensation expense		2,679			2,679
Unrealized gain on available-for-sale securities			(14)		(14)
Net loss				(11,746)	(11,746)
Balance at Sep. 30, 2019	$ 21	149,853	8	(108,901)	40,981
Balance, shares at Sep. 30, 2019	20,788,000
Balance at Dec. 31, 2019	$ 21	153,401	4	(122,689)	$ 30,737
Balance, shares at Dec. 31, 2019	20,825,000				20,825,000
Issuance of shares upon exercise of stock options		349			$ 349
Issuance of shares upon exercise of stock options, shares	86,000				85,750
Issuance of shares under employee stock purchase plan		490			$ 490
Issuance of shares under employee stock purchase plan, shares	83,000
Issuance of shares upon exercise of warrants		112			112
Issuance of shares upon exercise of warrants, shares	16,000
Issuance of common stock and warrants in connection with rights offering, net of issuance cost of $565	$ 4	29,430			29,434
Issuance of common stock and warrants in connection with rights offering, net of issuance cost of $565, shares	4,280,000
Stock-based compensation expense		7,686			7,686
Unrealized gain on available-for-sale securities			(3)		(3)
Net loss				(36,101)	(36,101)
Balance at Sep. 30, 2020	$ 25	191,468	1	(158,790)	$ 32,704
Balance, shares at Sep. 30, 2020	25,290,000				25,290,000
Balance at Jun. 30, 2020	$ 25	188,197		(145,894)	$ 42,328
Balance, shares at Jun. 30, 2020	25,149,000
Issuance of shares upon exercise of stock options		340			340
Issuance of shares upon exercise of stock options, shares	85,000
Issuance of shares under employee stock purchase plan		235			235
Issuance of shares under employee stock purchase plan, shares	40,000
Issuance of shares upon exercise of warrants		112			112
Issuance of shares upon exercise of warrants, shares	16,000
Stock-based compensation expense		2,648			2,648
Rights offering, additional issuance costs		(64)			(64)
Unrealized gain on available-for-sale securities			1		1
Net loss				(12,896)	(12,896)
Balance at Sep. 30, 2020	$ 25	$ 191,468	$ 1	$ (158,790)	$ 32,704
Balance, shares at Sep. 30, 2020	25,290,000				25,290,000