Stock-Based Compensation - Accounting for Stock-based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	$ 475	$ 769	$ 1,990	$ 2,273
Research and development
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	58	158	491	569
Research and development | Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	7	17	22	43
Research and development | Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	51	141	469	526
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	417	611	1,499	1,704
General and administrative | Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation		2	3	4
General and administrative | Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	$ 417	$ 609	$ 1,496	$ 1,700