Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 43,212	$ 37,070
Research and development credit carryforwards	3,876	3,690
Accruals and other	2,011	2,263
Capitalized start-up costs	122	150
Other	32	38
Total gross deferred tax asset	49,253	43,211
Deferred tax liabilities:
IPR&D	(12,528)	(12,528)
Property and equipment	(60)	(107)
Total gross deferred tax liabilities	(12,588)	(12,635)
Valuation allowance	(49,193)	(43,104)
Net deferred tax liability	$ (12,528)	$ (12,528)