Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, cost	$ 809,371	$ 789,242
Accumulated depreciation	(664,484)	(627,733)
Property and equipment, net	144,887	161,509	$ 142,906
Computer hardware
Property, Plant and Equipment [Line Items]
Property and equipment, cost	171,515	170,746
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment, cost	376,761	374,490
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, cost	102,424	100,472
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, cost	148,764	133,541
Other
Property, Plant and Equipment [Line Items]
Property and equipment, cost	$ 9,907	$ 9,993