Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segments:
Total revenue	$ 2,391.8	$ 2,185.8	$ 4,560.9	$ 4,249.6
Net Revenue	1,948.2	1,834.6	3,722.2	3,509.9
Segment operating income (loss):	249.2	225.7	288.0	260.4
Interest Income (Expense), Net	(21.4)	(21.0)	(37.3)	(36.7)
Interest Expense	26.1	25.7	46.0	46.6
Other expense, net	(16.3)	(15.4)	(40.7)	(14.6)
Income before income taxes	211.5	189.3	210.0	209.1
Depreciation and amortization	44.0	41.3	90.0	82.3
Capital expenditures	38.7	44.1	61.5	68.9
IAN
Segments:
Total revenue	1,812.5	1,711.3	3,498.0	3,279.1
Net Revenue	1,629.1	1,534.0	3,110.4	2,925.1
Segment operating income (loss):	238.2	195.9	295.2	246.0
Depreciation and amortization	35.8	33.9	73.2	66.6
Capital expenditures	31.9	31.4	47.7	48.6
CMG
Segments:
Total revenue	579.3	474.5	1,062.9	970.5
Net Revenue	319.1	300.6	611.8	584.8
Segment operating income (loss):	42.0	51.5	61.4	73.6
Depreciation and amortization	6.0	6.0	12.1	12.0
Capital expenditures	1.9	4.0	3.0	6.7
Corporate and Other
Segments:
Segment operating income (loss):	(31.0)	(21.7)	(68.6)	(59.2)
Depreciation and amortization	2.2	1.4	4.7	3.7
Capital expenditures	$ 4.9	$ 8.7	$ 10.8	$ 13.6