Income Taxes - Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 49,996	$ 58,457
Deferred revenue	604	372
Tax credits	3,662	2,829
Accrued payroll	646	935
Accrued expenses not currently deductible	1,239	2,121
Non-qualified stock options	2,689	3,164
Depreciation and amortization	350	128
Other	84	46
Gross deferred assets	59,270	68,052
Valuation allowance	(59,270)	(68,052)	$ (58,542)	$ (45,011)
Total deferred tax assets, net of valuation allowance	0	0
Deferred tax liabilities:
Depreciation and amortization	0	0
Unrealized foreign currency gain/loss	0	0
Total deferred tax liabilities	0	0
Net deferred assets	$ 0	$ 0