Business and Significant Accounting Policies - Property, Equipment and Leasehold Improvements, Less Accumulated Depreciation and Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 514,362	$ 432,019
Less — accumulated depreciation and amortization	(246,697)	(210,512)
Property, equipment and leasehold improvements, net	267,665	221,507
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 210,955	189,015
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	$ 85,002	67,288
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	$ 218,405	$ 175,716
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