Income Taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Net operating loss carryforwards	$ 57,772	$ 55,722
Research and development credits	17,614	14,244
Equity compensation	5,329	4,067
Other, net	769	799
Total deferred tax assets	81,484	74,832
Deferred Tax Liabilities:
Fixed assets	(150)	(349)
Total deferred tax liabilities	(150)	(349)
Net deferred tax assets	81,334	74,483
Less: valuation allowance	(81,334)	(74,483)
Deferred tax assets, net of valuation allowance	$ 0	$ 0