Income Taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 35,623	$ 27,952
Research and development credits	4,678	3,637
Deferred revenue	1,614	1,795
Depreciation	86	629
Amortization	792
Stock compensation	541	185
Other deferred tax assets	1,378	614
Total deferred tax assets	44,712	34,812
Valuation allowance	(44,033)	(34,552)
Subtotal	679	260
Section 481(a) Adjustment - Accrued Bonus	(59)	(118)
Depreciation	(610)
Goodwill	(17)
Stock-based compensation		$ (142)
Net deferred tax assets (liability)	$ (7)