Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Rate Reconciliation
Tax benefit at federal statutory rate	$ (7,791)	$ (2,924)	$ (2,693)
State taxes	48	127	1,126
Research and development credits	(399)	(161)	(85)
Foreign operations taxed at different rates	(2)	30	31
Stock-based compensation	(216)	327	77
Other nondeductible items	399	660	(43)
Change in valuation allowance	(26,058)	1,901	1,249
Change in statutory tax rate	34,100	0	0
Provision for (benefit from) income taxes	$ 81	$ (40)	$ (338)