Income Taxes - Schedule of Reconciliation of Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes
Expected provision at statutory federal rate	$ (7,134)	$ (7,741)	$ (3,080)
State tax-net of federal benefit	1	1	1
U.S. federal research credit	(924)	(1,125)	(2,272)
Non deductible expenses	119	104	28
Others	8	8	(9)
Change in valuation allowance	7,938	8,760	5,349
Provision for income taxes	8	$ 7	$ 17
Foreign subsidiaries
Undistributed earnings of foreign subsidiaries	$ 300