Income Taxes - Components of Deferred Tax Assets and Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 21,682	$ 21,501
Depreciation and amortization	339	419
Accruals/other	1,322	908
Research and development credits and foreign credits	2,544	1,143
Total deferred tax assets	25,887	23,971
Valuation allowance	(25,887)	(23,971)
Valuation allowance
Increase in valuation allowance	$ 1,900	$ 13,400	$ 3,000