Segment Reporting (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 and Long-Lived Assets and Goodwill by Geographic Region
Revenue	$ 4,176.6	$ 3,719.5	$ 3,790.1	$ 3,587.4	$ 4,241.8	$ 3,791.1	$ 3,884.9	$ 3,499.1	$ 15,273.6	$ 15,416.9	$ 15,134.4
Americas
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									9,180.8	9,597.6	9,359.0
Long-lived assets and goodwill	6,633.8				6,662.7				6,633.8	6,662.7	6,103.4
United States
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									8,196.9	8,627.8	8,526.7
EMEA
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									4,442.5	4,183.1	4,203.5
Long-lived assets and goodwill	2,840.8				2,469.1				2,840.8	2,469.1	2,737.8
Asia Pacific
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									1,650.3	1,636.2	1,571.9
Long-lived assets and goodwill	$ 553.8				$ 519.1				$ 553.8	$ 519.1	$ 527.9