Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryovers	$ 79,675	$ 74,965
Research and development and other tax credits	33,429	33,121
Deferred revenue	1	16
Intangibles and property and equipment basis difference	782	0
Stock compensation expense	1,389	5,033
Lease liability	164
Other	461	2,027
Total deferred tax assets	115,901	115,162
Total deferred tax liabilities	(325)	(176)
Net deferred tax asset	115,576	114,986
Valuation allowance	(115,564)	(114,960)
Net deferred tax asset	$ 12	$ 26