Segment Information (Reconciliation of Revenue from Segments to Consolidated) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 3,232,443	$ 3,145,310	$ 3,129,276	$ 3,008,740	$ 3,069,298	$ 2,900,577	$ 3,039,068	$ 2,512,568	$ 12,515,769	$ 11,521,511	$ 10,418,178
US Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									6,100,171	5,964,968	5,465,026
US Non-Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,150,378	840,446	643,865
Americas [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									7,250,549	6,805,414	6,108,891
Canada Non-Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									48,938	46,938	45,034
Canada Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,103,520	1,118,004	1,023,012
Canada Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,152,458	1,164,942	1,068,046
EMEA Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,362,075	1,195,149	1,190,742
EMEA Non-Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									311,071	340,518	286,666
EMEA [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,673,146	1,535,667	1,477,408
Asia Pacific Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									2,210,686	1,771,150	1,638,357
Asia Pacific Non-Traditional Segment Reporting Information [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									73,775	63,382	56,581
Asia Pacific [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									2,284,461	1,834,532	1,694,938
Corporate Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 155,155	$ 180,956	$ 68,895