Income Taxes - Temporary Differences and Carryforwards and Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Net operating loss carryforwards	$ 52,331	$ 42,032
Research and development credits	3,160	2,428
Accruals and reserves	5,450	4,222
Stock compensation	2,395	1,512
Depreciation and amortization	117	110
Total deferred tax assets	63,453	50,304
Less: Valuation allowance	(63,453)	(50,304)	$ (40,412)
Total deferred tax asset, net of valuation allowance	$ 0	$ 0