Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Rate Reconciliation
Tax benefit at federal statutory rate	$ (2,924)	$ (2,693)	$ (6,571)
State taxes	127	1,126	249
Research and development credits	(161)	(85)	(57)
Foreign operations taxed at different rates	30	31	447
Stock-based compensation	327	77	(2)
Other nondeductible items	660	(43)	(364)
Change in valuation allowance	1,901	1,249	6,042
Total benefit from income taxes	$ (40)	$ (338)	$ (256)