Segment Operations (Details) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segments:
Revenue	$ 2,196.2		$ 1,865.5		$ 1,876.1	$ 1,676.0	$ 2,207.1	$ 1,841.1	$ 1,851.4		$ 1,637.5	$ 7,613.8	$ 7,537.1	$ 7,122.3
Segment operating income												871.1	788.6	658.9
Restructuring and other reorganization-related reversals (charges), net	0.0		0.7		0.0	0.1	(0.1)	0.1	(0.7)		0.5	0.8	(0.2)	(60.6)
Interest expense												85.8	84.9	122.7
Interest income												22.8	27.4	24.7
Other expense, net	10.3	[1]	$ 37.2	[1]	$ (0.5)	$ (0.3)	0.1	$ 0.6	$ 11.2	[1]	$ (1.7)	46.7	10.2	32.3
Income before income taxes												762.2	720.7	468.0
Depreciation and amortization of fixed assets and intangible assets												157.0	163.0	157.4
Capital expenditures												161.1	148.7	173.0
Assets	12,585.1						12,736.6					12,585.1	12,736.6
IAN
Segments:
Revenue												6,144.9	6,076.3	5,772.8
Segment operating income												846.6	774.7	660.0
Depreciation and amortization of fixed assets and intangible assets												117.5	124.5	125.8
Capital expenditures												117.5	92.0	108.9
Assets	10,738.2						11,080.2					10,738.2	11,080.2
CMG
Segments:
Revenue												1,468.9	1,460.8	1,349.5
Segment operating income												166.3	163.4	139.7
Depreciation and amortization of fixed assets and intangible assets												18.4	18.5	15.8
Capital expenditures												12.7	11.7	18.6
Assets	1,338.6						1,347.5					1,338.6	1,347.5
Corporate and Other
Segments:
Segment operating income												(141.8)	(149.5)	(140.8)
Depreciation and amortization of fixed assets and intangible assets												21.1	20.0	15.8
Capital expenditures												30.9	45.0	$ 45.5
Assets	$ 508.3						$ 308.9					$ 508.3	$ 308.9

[1] The three months ended December 31, 2015 included pre-tax losses of $12.0 on sales of businesses. The three months ended September 30, 2015 included pre-tax losses of $38.0 on sales of businesses. The three months ended June 30, 2014 included a pre-tax loss of $10.4 related to our early extinguishment of debt.