Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating losses	$ 60,455	$ 53,901
Credits	7,174	6,221
Deferred revenues	1,879	3,334
Stock-based compensation	2,967	2,872
Reserves and accruals	1,876	2,028
Depreciation	1,376	1,573
Intangible assets	2,557	3,172
Capital losses	576	576
Unrealized gain/loss	297	295
Other assets	83	78
Total deferred tax assets:	79,240	74,050
Deferred tax liabilities:
Other	(64)	(115)
Total deferred tax liabilities:	(64)	(115)
Valuation allowance	(79,222)	(74,010)
Net deferred tax liabilities	$ (46)	$ (75)