Business and Significant Accounting Policies - Property, Equipment and Leasehold Improvements, Less Accumulated Depreciation and Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 635,935	$ 514,362
Less — accumulated depreciation and amortization	(291,356)	(246,697)
Property, equipment and leasehold improvements, net	344,579	267,665
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 256,451	210,955
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	$ 104,370	85,002
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	$ 275,114	$ 218,405
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