Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segments:
Revenue	$ 1,884.9	$ 1,917.9	$ 3,638.8	$ 3,659.9
Segment operating income (loss):	206.5	224.3	236.2	247.3
Interest expense	(25.7)	(24.5)	(46.6)	(47.1)
Interest income	4.7	5.6	9.9	11.4
Other (expense) income, net	(15.4)	0.4	(14.6)	(18.8)
Income before income taxes	170.1	205.8	184.9	192.8
Depreciation and amortization of fixed assets and intangible assets	41.3	39.8	82.3	77.8
Capital expenditures	44.1	36.0	68.9	63.0
IAN
Segments:
Revenue	1,537.8	1,548.5	2,945.4	2,950.1
Segment operating income (loss):	174.4	198.2	218.2	240.0
Depreciation and amortization of fixed assets and intangible assets	30.6	29.2	60.1	56.8
Capital expenditures	31.4	26.9	48.6	47.2
CMG
Segments:
Revenue	347.1	369.4	693.4	709.8
Segment operating income (loss):	53.9	50.0	77.3	70.4
Depreciation and amortization of fixed assets and intangible assets	5.3	4.9	10.6	9.7
Capital expenditures	4.0	2.2	6.7	3.7
Corporate and Other
Segments:
Segment operating income (loss):	(21.8)	(23.9)	(59.3)	(63.1)
Depreciation and amortization of fixed assets and intangible assets	5.4	5.7	11.6	11.3
Capital expenditures	$ 8.7	$ 6.9	$ 13.6	$ 12.1