Balance Sheet Components - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 60,071	$ 57,125
Less: Accumulated depreciation and amortization	(44,512)	(36,544)
Property and equipment, net	$ 15,559	20,581
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Property and equipment, gross	$ 29,938	28,905
Software, Including Internally Developed Software [Member]
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Property and equipment, gross	$ 23,389	21,323
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	Shorter of useful life or lease term
Property and equipment, gross	$ 4,617	4,613
Office Equipment, Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,127	$ 2,284
Office Equipment, Furniture and Fixtures [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Office Equipment, Furniture and Fixtures [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years