Segment Operations (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) business	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segments:
Number of Reportable Segments | business									2
EBITA									$ 1,172.0	$ 1,046.4	$ 959.5
Total (expenses) and other income	$ (63.5)	$ (47.6)	$ (47.7)	$ (48.9)	$ (55.2)	$ (37.6)	$ (37.7)	$ (40.3)	(207.7)	(170.8)	(97.6)
Total revenue	2,901.8	2,438.1	2,520.2	2,361.2	2,856.0	2,297.5	2,391.8	2,169.1	10,221.3	9,714.4	9,047.6
Net revenue	2,433.0	2,061.4	2,125.9	2,004.8	2,413.7	1,895.7	1,948.2	1,774.0	8,625.1	8,031.6	7,473.5
Operating Income (Loss)	491.3	280.3	264.2	50.2	459.1	261.7	249.2	38.8	1,086.0	1,008.8	938.4
Amortization of Intangible Assets									86.0	37.6	21.1
Interest expense									(199.3)	(123.0)	(90.8)
Interest income									34.5	21.8	19.4
Other expense, net	(24.8)	$ (7.4)	$ (3.8)	$ (6.9)	(13.6)	$ (15.3)	$ (16.3)	$ (24.4)	(42.9)	(69.6)	(26.2)
Income before income taxes									878.3	838.0	840.8
Depreciation and amortization									192.5	165.3	136.0
Capital expenditures									198.5	177.1	155.9
Assets	17,751.9				15,620.3				17,751.9	15,620.3
IAN
Segments:
EBITA									1,110.4	1,042.1	891.7
Total revenue									8,026.4	7,556.1	7,009.6
Net revenue									7,348.2	6,767.5	6,266.7
Amortization of Intangible Assets									81.6	32.3	16.6
Depreciation and amortization									166.0	136.8	108.9
Capital expenditures									161.8	143.9	112.0
Assets	15,170.3				13,867.9				15,170.3	13,867.9
CMG
Segments:
EBITA									163.4	180.3	194.4
Total revenue									2,194.9	2,158.3	2,038.0
Net revenue									1,276.9	1,264.1	1,206.8
Amortization of Intangible Assets									4.4	5.3	4.5
Depreciation and amortization									19.2	19.0	19.0
Capital expenditures									13.3	13.0	17.7
Assets	1,710.4				1,516.7				1,710.4	1,516.7
Corporate and Other
Segments:
EBITA									(101.8)	(176.0)	(126.6)
Amortization of Intangible Assets									0.0	0.0	0.0
Depreciation and amortization									7.3	9.5	8.1
Capital expenditures									23.4	20.2	$ 26.2
Assets	$ 871.2				$ 235.7				$ 871.2	$ 235.7