Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Rate Reconciliation
Tax benefit at federal statutory rate	$ (2,292)	$ (7,791)	$ (2,924)
State taxes	222	48	127
Research and development credits	(499)	(399)	(161)
Foreign operations taxed at different rates	(17)	(2)	30
Stock-based compensation	(2,587)	(216)	327
Other nondeductible items	(3)	326	405
Executive compensation	838	73	255
Change in valuation allowance	4,301	(26,058)	1,901
Change in statutory tax rate	0	34,100	0
Provision for (benefit from) income taxes	$ (37)	$ 81	$ (40)