Business Segments, Geographic Information, and Major Customers - Geographic Summary (Details) - USD ($) $ in Millions	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
Revenue:
Revenue	$ 8,872.0	$ 8,543.0	$ 8,086.2	$ 8,194.3	$ 7,792.1	$ 7,399.8	$ 6,633.0	$ 5,190.8	$ 33,695.5	$ 27,015.8	$ 30,381.4
Property and equipment, net	329.8				311.2				329.8	311.2
United States
Revenue:
Revenue									17,938.0	14,368.8	15,496.3
Property and equipment, net	152.6				137.7				152.6	137.7
EMEA
Revenue:
Revenue									7,553.3	6,018.6	6,382.2
Property and equipment, net	120.2				122.9				120.2	122.9
Asia Pacific
Revenue:
Revenue									4,923.0	4,271.1	5,863.4
Property and equipment, net	10.4				1.4				10.4	1.4
Americas, excluding United States
Revenue:
Revenue									3,281.2	2,357.2	2,639.5
Property and equipment, net	$ 46.7				$ 49.1				46.7	49.1
U.K.
Revenue:
Revenue									5,000.0	4,100.0	4,700.0
Singapore
Revenue:
Revenue									$ 4,800.0	$ 4,200.0	$ 5,800.0