Income Taxes - Temporary Differences and Carryforwards and Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Net operating loss carryforwards	$ 35,067	$ 32,210
Research and development credits	2,255	2,070
Depreciation and amortization	132	179
Accruals and reserves	2,958	1,376
Deferred tax asset, gross	40,412	35,835
Less: Valuation allowance	(40,412)	(35,835)	$ (43,348)
Total deferred tax asset	$ 0	$ 0