Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 46,346	$ 43,973
Tax credits	5,262	4,535
California capitalized research and development	80	343
Fixed assets	8	0
Others, net	593	602
Total deferred tax assets	52,289	49,453
Valuation allowance	(52,289)	(49,403)
Deferred tax asset, net of valuation allowance	0	50
Deferred tax liabilities:
Others, net	0	0
Fixed assets	0	(50)
Total deferred tax liabilities	0	(50)
Net deferred tax assets	$ 0	$ 0