Income Taxes - Temporary Differences and Carryforwards and Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets
Net operating loss carryforwards	$ 42,032	$ 35,067
Research and development credits	2,428	2,255
Accruals and reserves	4,222	2,059
Stock compensation	1,512	899
Depreciation and amortization	110	132
Total deferred tax assets	50,304	40,412
Less: Valuation allowance	(50,304)	(40,412)	$ (35,835)
Total deferred tax asset, net of valuation allowance	$ 0	$ 0