Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 55,216	$ 51,240
Research credit carryforward	7,609	6,904
Stock options and other	3,225	2,655
Total gross deferred tax assets	66,050	60,799
Valuation allowance for deferred tax assets	(66,050)	(60,799)
Net deferred tax assets	0	0
Valuation allowance
Increase (decrease) in the valuation allowance	$ 5,300	$ 4,800