Income Taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 76,335	$ 68,915
Research and development credits	15,845	14,198
Stock-based compensation	946	761
Capitalized research expenses	716	1,028
Capital loss carryforwards	39,087	39,087
Depreciable and amortizable assets	11,089	10,859
Business interest expense limitation	11,609	0
Other temporary differences	12,584	11,761
Total gross deferred tax assets	168,211	146,609
Valuation allowance	(168,211)	(145,753)
Net deferred tax assets	$ 0	$ 856