Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current deferred tax assets:
Accruals and allowances	$ 6,489	$ 4,720
Gains on foreign exchange	127	77
Noncurrent deferred tax assets:
Net operating loss carryforwards	64,674	43,553
Depreciation and amortization	7,413	8,329
R&D tax credits	7,969	5,707
Stock-based compensation	9,278	5,235
Valuation allowance	$ (95,950)	$ (67,621)