Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryovers	$ 63,494	$ 57,557
R&D tax credits	82,246	79,725
Non-qualified stock options	4,611	5,355
Deferred revenue	1,175	1,242
Charitable contributions	4	4
Accrued expenses	454	407
Fixed assets	92	85
Deferred tax assets	152,076	144,375
Less valuation allowance	(151,939)	(144,375)
Net deferred tax assets	$ 137	$ 0