Income Taxes - Reconciliation of Federal Statutory Income Tax to Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Expected provision at statutory federal rate	$ (5,400)	$ (7,134)	$ (7,741)
State tax—net of federal benefit	1	1	1
U.S. federal research credit	(843)	(924)	(1,125)
Non deductible expenses	66	119	104
Others	(9)	8	8
Change in valuation allowance	6,192	7,938	8,760
Provision for income taxes	$ 7	$ 8	$ 7