Nature of Business and Summary of Significant Accounting Policies - Net 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	$ 182,762	$ 162,106
Less: accumulated depreciation and amortization	(102,534)	(83,156)
Property and equipment, net	80,228	78,950
Computer and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 123,804	105,692
Computer and office equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	3 years
Computer and office equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	Lesser of useful life of improvement or remaining life of lease
Property and equipment, gross	$ 32,364	33,093
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	7 years
Property and equipment, gross	$ 12,158	11,145
Building and improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 12,651	10,391
Building and improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	7 years
Building and improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	30 years
Land
Property, Plant and Equipment [Line Items]
Property, plant & equipment useful lives	Non depreciable
Property and equipment, gross	$ 1,785	$ 1,785