Significant Accounting Policies (Property and Equipment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Retirement of property and equipment	$ 10,900	$ 13,400
Property and equipment, gross	1,018,311	959,767
Less: accumulated depreciation and amortization	(806,099)	(741,587)
Total property and equipment, net	$ 231,894	243,396
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	40 years
Property and equipment, gross	$ 76,152	76,152
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Property and equipment, gross	$ 205,063	189,333
Software
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Property and equipment, gross	$ 451,927	433,033
Equipment and furniture
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	7 years
Property and equipment, gross	$ 85,356	78,401
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 199,813	182,848
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	10 years
Assets under construction
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,913	8,447
Land
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 16,769	$ 16,769