Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segments:
Net revenue	$ 2,125.9	$ 1,948.2	$ 4,130.7	$ 3,722.2
Document Period End Date			Jun. 30, 2019
Total revenue	2,520.2	2,391.8	$ 4,881.4	4,560.9
EBITA	285.5	254.4	357.3	298.5
Amortization	(21.3)	(5.2)	(42.9)	(10.5)
Depreciation	51.7	38.8	101.2	79.5
Capital expenditures	47.3	38.7	80.1	61.5
Total assets	16,526.9		16,526.9		$ 15,620.3
Operating income	264.2	249.2	314.4	288.0
Total (expenses) and other income	(47.7)	(37.7)	(96.6)	(78.0)
Income (loss) before income taxes	216.5	211.5	217.8	210.0
IAN
Segments:
Net revenue	1,805.8	1,629.1	3,517.0	3,110.4
Total revenue	1,968.6	1,812.5	3,840.8	3,498.0
EBITA	261.7	242.2	376.2	303.2
Amortization	(20.2)	(3.9)	(40.7)	(8.0)
Depreciation	45.2	31.9	87.5	65.2
Capital expenditures	35.9	31.9	62.4	47.7
Total assets	14,323.1		14,323.1		13,867.9
CMG
Segments:
Net revenue	320.1	319.1	613.7	611.8
Total revenue	551.6	579.3	1,040.6	1,062.9
EBITA	43.6	43.2	45.5	63.9
Amortization	(1.1)	(1.3)	(2.2)	(2.5)
Depreciation	4.9	4.7	9.5	9.6
Capital expenditures	2.9	1.9	3.9	3.0
Total assets	1,695.1		1,695.1		1,516.7
Corporate and Other
Segments:
EBITA	(19.8)	(31.0)	(64.4)	(68.6)
Amortization	(0.0)	(0.0)	(0.0)	(0.0)
Depreciation	1.6	2.2	4.2	4.7
Capital expenditures	8.5	$ 4.9	13.8	$ 10.8
Total assets	$ 508.7		$ 508.7		$ 235.7