Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segments:
Total revenue	$ 2,361.2	$ 2,169.1
Net revenue	2,004.8	1,774.0
EBITA	71.8	44.1
Amortization	21.6	5.3
Depreciation	49.5	40.7
Capital expenditures	32.8	22.8
Total assets	16,050.0		$ 15,620.3
Operating income	50.2	38.8
Total (expenses) and other income	(48.9)	(40.3)
Income (loss) before income taxes	1.3	(1.5)
IAN
Segments:
Total revenue	1,872.2	1,685.5
Net revenue	1,711.2	1,481.3
EBITA	114.5	61.0
Amortization	20.5	4.1
Depreciation	42.3	33.3
Capital expenditures	26.5	15.8
Total assets	13,888.1		13,867.9
CMG
Segments:
Total revenue	489.0	483.6
Net revenue	293.6	292.7
EBITA	1.9	20.7
Amortization	1.1	1.2
Depreciation	4.6	4.9
Capital expenditures	1.0	1.1
Total assets	1,660.5		1,516.7
Corporate and Other
Segments:
EBITA	(44.6)	(37.6)
Amortization	0.0	0.0
Depreciation	2.6	2.5
Capital expenditures	5.3	$ 5.9
Total assets	$ 501.4		$ 235.7