Certain 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 property and equipment	$ 84,205	$ 77,498
Less accumulated depreciation	58,956	53,604
Total	25,249	23,894
Computers and Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	18,570	17,137
Computer Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	29,891	26,686
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	9,094	8,471
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 26,650	$ 25,204