Segments and Related Information (Foreign Operations) (Narrative) (Details) - USD ($) $ in Thousands				3 Months Ended								12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment and Related Information [Line Items]
Revenue				$ 1,636,800	$ 1,698,300	$ 1,569,300	$ 1,416,600	$ 1,709,300	$ 2,016,600	$ 1,939,000	$ 1,518,300	$ 6,320,975	$ 7,183,188	$ 6,909,417
Property and equipment, net	$ 982,328	$ 905,835		982,328				905,835				$ 982,328	$ 905,835
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Government transactions [Member]
Segment and Related Information [Line Items]
Concentration risk, percentage of total												2.00%	1.00%	1.00%
United States [Member]
Segment and Related Information [Line Items]
Revenue												$ 6,200,000	$ 6,900,000	$ 6,700,000
Property and equipment, net	959,500	874,700	$ 707,400	959,500				874,700				959,500	874,700	707,400
Intangible assets and goodwill, net	1,400,000	1,400,000	1,200,000	1,400,000				1,400,000				1,400,000	1,400,000	1,200,000
Foreign Operations [Member]
Segment and Related Information [Line Items]
Revenue												133,100	233,500	164,300
Property and equipment, net	22,800	31,100	40,400	22,800				31,100				22,800	31,100	40,400
Intangible assets and goodwill, net	$ 50,500	$ 56,400	$ 61,500	$ 50,500				$ 56,400				$ 50,500	$ 56,400	$ 61,500
Foreign Operations [Member] | Accounts Receivable, Net, Less Contract Liabilities [Member] | Geographic Concentration Risk [Member]
Segment and Related Information [Line Items]
Concentration risk, percentage of total	5.00%	5.00%	5.00%