Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue	$ 4,086.7	$ 3,714.3	$ 3,859.6	$ 3,629.6	$ 4,176.6	$ 3,719.5	$ 3,790.1	$ 3,587.4	$ 15,290.2	$ 15,273.6	$ 15,416.9
Americas
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									8,900.0	9,180.8	9,597.6
Long-lived assets and goodwill	6,946.1				6,633.8				6,946.1	6,633.8	6,662.7
United States
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									7,999.8	8,196.9	8,627.8
EMEA
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									4,679.8	4,442.5	4,183.1
Long-lived assets and goodwill	2,578.9				2,840.8				2,578.9	2,840.8	2,469.1
Asia-Pacific
Revenue and Long-Lived Assets and Goodwill by Geographic Region
Revenue									1,710.4	1,650.3	1,636.2
Long-lived assets and goodwill	$ 553.7				$ 553.8				$ 553.7	$ 553.8	$ 519.1