Income Taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Federal net operating loss carryforwards	$ 47,845	$ 38,531
State net operating loss carryforwards	7,431	5,553
Stock Options	2,584	1,632
License fees	1,332	1,495
Accrued expenses	277	88
Other	115	149
Total deferred tax assets	63,864	50,641
Valuation allowance	(63,737)	(50,517)
Net deferred tax assets (liability)	127	124
Federal
Deferred Tax Assets
Research tax credits	3,770	2,811
State
Deferred Tax Assets
Research tax credits	$ 510	$ 382