Business and Significant Accounting Policies (Details) - Property, Equipment and Leasehold Improvements, Less Accumulated Depreciation and Amortization - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 432,019	$ 306,125
Less — accumulated depreciation and amortization	(210,512)	(184,519)
Property, equipment, and leasehold improvements, net	221,507	121,606
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 189,015	166,385
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	2 years
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	7 years
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 67,288	43,137
Furniture and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	3 years
Furniture and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	8 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 175,716	$ 96,603
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	2 years
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	15 years