Significant Accounting Policies (Property and Equipment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Retirement of property and equipment	$ 13,400	$ 16,900
Property and equipment, gross	959,767	883,231
Less: accumulated depreciation and amortization	(741,587)	(675,892)
Total property and equipment, net	$ 243,396	252,932
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	$ 189,333	176,140
Software
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Property and equipment, gross	$ 433,033	388,583
Equipment and furniture
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment	7 years
Property and equipment, gross	$ 78,401	73,700
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 182,848	168,656
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	$ 16,769	16,769
Land
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 8,447	$ 28,824