Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segments:
Total revenue	$ 2,297.5	$ 2,208.2	$ 6,858.4	$ 6,457.8
Net revenue	1,895.7	1,832.5	5,617.9	5,342.4
Segment operating income (loss):	261.7	255.0	549.7	515.4
Interest Income (Expense), Net	(22.3)	(16.9)	(59.6)	(53.6)
Interest Expense	27.6	21.0	73.6	67.6
Other expense, net	(15.3)	(9.9)	(56.0)	(24.5)
Income before income taxes	224.1	228.2	434.1	437.3
Depreciation and amortization	44.0	42.2	134.0	124.5
Capital expenditures	44.2	39.8	105.7	108.7
IAN
Segments:
Total revenue	1,774.7	1,702.8	5,272.7	4,981.9
Net revenue	1,580.5	1,531.4	4,690.9	4,456.5
Segment operating income (loss):	236.3	216.9	531.5	462.9
Depreciation and amortization	36.9	35.1	110.1	101.7
Capital expenditures	35.2	29.3	82.9	77.9
CMG
Segments:
Total revenue	522.8	505.4	1,585.7	1,475.9
Net revenue	315.2	301.1	927.0	885.9
Segment operating income (loss):	48.0	53.2	109.4	126.8
Depreciation and amortization	6.1	5.6	18.2	17.6
Capital expenditures	4.2	6.2	7.2	12.9
Corporate and Other
Segments:
Segment operating income (loss):	(22.6)	(15.1)	(91.2)	(74.3)
Depreciation and amortization	1.0	1.5	5.7	5.2
Capital expenditures	$ 4.8	$ 4.3	$ 15.6	$ 17.9