Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Components of Deferred Tax Assets [Abstract]
Accruals and allowances	$ 8,862	$ 6,489
Gains on foreign exchange	81	127
Net operating loss carryforwards	81,623	64,674
Depreciation and amortization	6,489	7,413
R&D tax credits	10,461	7,969
Stock-based compensation	10,272	9,278
Valuation allowance	(117,788)	(95,950)
Net deferred tax assets	$ 0	$ 0