Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segments:
Total revenue	$ 2,438.1	$ 2,297.5	$ 7,319.5	$ 6,858.4
Net revenue	2,061.4	1,895.7	6,192.1	5,617.9
Segment EBITA	302.0	266.8	659.3	565.3
Amortization of acquired intangibles	(21.7)	(5.1)	(64.6)	(15.6)
Depreciation	47.3	38.9	148.5	118.4
Capital expenditures	53.7	44.2	133.8	105.7
Total assets	15,997.6		15,997.6		$ 15,620.3
Operating income	280.3	261.7	594.7	549.7
Total (expenses) and other income	(47.6)	(37.6)	(144.2)	(115.6)
Income before income taxes	232.7	224.1	450.5	434.1
IAN
Segments:
Total revenue	1,903.9	1,774.7	5,744.7	5,272.7
Net revenue	1,743.9	1,580.5	5,260.9	4,690.9
Segment EBITA	261.2	240.2	637.4	543.4
Amortization of acquired intangibles	(20.6)	(3.9)	(61.3)	(11.9)
Depreciation	41.3	33.0	128.8	98.2
Capital expenditures	44.5	35.2	106.9	82.9
Total assets	13,998.5		13,998.5		13,867.9
CMG
Segments:
Total revenue	534.2	522.8	1,574.8	1,585.7
Net revenue	317.5	315.2	931.2	927.0
Segment EBITA	51.8	49.2	97.3	113.1
Amortization of acquired intangibles	(1.1)	(1.2)	(3.3)	(3.7)
Depreciation	4.8	4.9	14.3	14.5
Capital expenditures	4.0	4.2	7.9	7.2
Total assets	1,656.1		1,656.1		1,516.7
Corporate and Other
Segments:
Segment EBITA	(11.0)	(22.6)	(75.4)	(91.2)
Amortization of acquired intangibles	(0.0)	(0.0)	(0.0)	(0.0)
Depreciation	1.2	1.0	5.4	5.7
Capital expenditures	5.2	$ 4.8	19.0	$ 15.6
Total assets	$ 343.0		$ 343.0		$ 235.7