Income Taxes - Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 58,457	$ 52,145
Deferred revenue	372	80
Tax credits	2,829	2,065
Accrued payroll	935	1,071
Accrued expenses not currently deductible	2,121	2,171
Non-qualified stock options	3,164	1,716
Depreciation and amortization	128	0
Other	46	128
Gross deferred assets	68,052	59,376
Valuation allowance	(68,052)	(58,542)	$ (45,011)	$ (34,357)
Total deferred tax assets, net of valuation allowance	0	834
Deferred tax liabilities:
Depreciation and amortization	0	(399)
Unrealized foreign currency gain/loss	0	(435)
Total deferred tax liabilities	0	(834)
Net deferred assets	$ 0	$ 0