Other Balance Sheet Accounts - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total of property and equipment	$ 25,052	$ 25,187
Less accumulated depreciation and amortization	(24,446)	(24,014)
Property and equipment, net	606	1,173
Equipment [Member]
Property, Plant and Equipment [Line Items]
Total of property and equipment	16,908	17,043
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total of property and equipment	$ 8,144	$ 8,144