Business and Significant Accounting Policies - Property, Equipment and Leasehold Improvements, Less Accumulated Depreciation and Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 676,368	$ 635,935
Less — accumulated depreciation and amortization	(339,603)	(291,356)
Property, equipment and leasehold improvements, net	336,765	344,579
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, equipment and leasehold improvements, gross	$ 277,973	256,451
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	$ 114,622	104,370
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	$ 283,773	$ 275,114
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