Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segments:
Total revenue	$ 2,025.7	$ 2,520.2	$ 4,385.5	$ 4,881.4
Net revenue	1,853.4	2,125.9	3,825.5	4,130.7
Segment EBITA	62.3	285.5	159.5	357.3
Amortization of acquired intangibles	(21.8)	(21.3)	(43.1)	(42.9)
Depreciation	51.3	51.7	102.8	101.2
Capital expenditures	27.3	47.3	71.9	80.1
Total assets	14,896.9		14,896.9		$ 17,751.9
Operating income	40.5	264.2	116.4	314.4
Total (expenses) and other income	(65.4)	(47.7)	(121.3)	(96.6)
Income before income taxes	(24.9)	216.5	(4.9)	217.8
IAN
Segments:
Total revenue	1,672.5	1,959.6	3,491.3	3,821.8
Net revenue	1,585.7	1,801.1	3,250.2	3,507.2
Segment EBITA	100.4	262.4	199.4	378.4
Amortization of acquired intangibles	(20.8)	(20.2)	(41.0)	(40.7)
Depreciation	45.3	44.9	90.0	87.0
Capital expenditures	19.8	35.9	53.5	62.4
Total assets	12,667.1		12,667.1		15,155.2
CMG
Segments:
Total revenue	353.2	560.6	894.2	1,059.6
Net revenue	267.7	324.8	575.3	623.5
Segment EBITA	(26.3)	42.9	(4.0)	43.3
Amortization of acquired intangibles	(1.0)	(1.1)	(2.1)	(2.2)
Depreciation	5.4	5.2	10.5	10.0
Capital expenditures	1.3	2.9	2.9	3.9
Total assets	1,528.2		1,528.2		1,725.5
Corporate and Other
Segments:
Segment EBITA	(11.8)	(19.8)	(35.9)	(64.4)
Amortization of acquired intangibles	(0.0)	(0.0)	(0.0)	(0.0)
Depreciation	0.6	1.6	2.3	4.2
Capital expenditures	6.2	$ 8.5	15.5	$ 13.8
Total assets	$ 701.6		$ 701.6		$ 871.2