Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segments:
Total revenue	$ 2,169.1	$ 2,063.8
Net revenue	1,774.0	1,675.3
Segment operating income (loss):	38.8	34.7
Interest expense, net	(15.9)	(15.7)
Other (expense) income, net	(24.4)	0.8
(Loss) income before income taxes	(1.5)	19.8
Depreciation and amortization	46.0	41.0
Capital expenditures	22.8	24.8
IAN
Segments:
Total revenue	1,685.5	1,567.8
Net revenue	1,481.3	1,391.1
Segment operating income (loss):	57.0	50.1
Depreciation and amortization	37.4	32.7
Capital expenditures	15.8	17.2
CMG
Segments:
Total revenue	483.6	496.0
Net revenue	292.7	284.2
Segment operating income (loss):	19.4	22.1
Depreciation and amortization	6.1	6.0
Capital expenditures	1.1	2.7
Corporate and Other
Segments:
Segment operating income (loss):	(37.6)	(37.5)
Depreciation and amortization	2.5	2.3
Capital expenditures	$ 5.9	$ 4.9