Stock Options - Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock options
Stock-based compensation expense included in Total operating expenses	$ 7,172	$ 4,872	$ 3,884
Stock options
Stock options
Stock-based compensation expense included in Total operating expenses	5,725	4,230	3,754
Unrecognized stock compensation cost	$ 19,487
Weighted-average amortization period of unrecognized stock compensation cost	2 years 9 months 18 days
Unvested restricted stock units
Stock options
Stock-based compensation expense included in Total operating expenses	$ 1,187	410	0
Unrecognized stock compensation cost	$ 2,699
Weighted-average amortization period of unrecognized stock compensation cost	2 years 3 months 18 days
Employee stock purchase plan
Stock options
Stock-based compensation expense included in Total operating expenses	$ 260	232	130
Research and development
Stock options
Stock-based compensation expense included in Total operating expenses	3,841	2,245	1,788
General and administrative
Stock options
Stock-based compensation expense included in Total operating expenses	$ 3,331	$ 2,627	$ 2,096