Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 51,240	$ 47,427
Research credit carryforward	6,904	6,296
Stock options and other	2,655	2,296
Total gross deferred tax assets	60,799	56,019
Valuation allowance for deferred tax assets	(60,799)	(56,019)
Net deferred tax assets	0	0
Valuation allowance
Increase (decrease) in the valuation allowance	$ 4,800	$ (14,600)