Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segments:
Total revenue	$ 2,125.5	$ 2,438.1	$ 6,511.0	$ 7,319.5
Net revenue	1,954.6	2,061.4	5,780.1	6,192.1
Segment EBITA	269.9	302.0	429.4	659.3
Amortization of acquired intangibles	21.3	21.7	64.4	64.6
Depreciation	49.7	47.3	152.5	148.5
Capital expenditures	40.1	53.7	112.0	133.8
Total assets	16,074.7		16,074.7		$ 17,751.9
Operating income	248.6	280.3	365.0	594.7
Total (expenses) and other income	(56.0)	(47.6)	(177.3)	(144.2)
Income before income taxes	192.6	232.7	187.7	450.5
IAN
Segments:
Total revenue	1,783.6	1,896.3	5,274.9	5,718.1
Net revenue	1,685.5	1,739.2	4,935.7	5,246.4
Segment EBITA	260.0	262.3	459.4	640.7
Amortization of acquired intangibles	20.2	20.6	61.2	61.3
Depreciation	43.8	40.9	133.8	127.9
Capital expenditures	31.7	44.5	85.2	106.9
Total assets	13,307.4		13,307.4		15,155.2
CMG
Segments:
Total revenue	341.9	541.8	1,236.1	1,601.4
Net revenue	269.1	322.2	844.4	945.7
Segment EBITA	25.9	50.7	21.9	94.0
Amortization of acquired intangibles	1.1	1.1	3.2	3.3
Depreciation	5.0	5.2	15.5	15.2
Capital expenditures	1.2	4.0	4.1	7.9
Total assets	1,505.4		1,505.4		1,725.5
Corporate and Other
Segments:
Segment EBITA	(16.0)	(11.0)	(51.9)	(75.4)
Amortization of acquired intangibles	0.0	0.0	0.0	0.0
Depreciation	0.9	1.2	3.2	5.4
Capital expenditures	7.2	$ 5.2	22.7	$ 19.0
Total assets	$ 1,261.9		$ 1,261.9		$ 871.2