Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segments:
Revenue	$ 2,207.1	$ 1,841.1	$ 1,851.4		$ 1,637.5	$ 2,122.7	$ 1,700.4		$ 1,756.2	$ 1,543.0	$ 7,537.1	$ 7,122.3	$ 6,956.2
Segment operating income											788.6	658.9	677.1
Restructuring and other reorganization-related (charges) reversals, net	(0.1)	0.1	(0.7)		0.5	(60.6)	0.2		(0.3)	0.1	(0.2)	(60.6)	1.2
Interest expense											84.9	122.7	133.5
Interest income											27.4	24.7	29.5
Other (expense) income, net	(0.1)	(0.6)	(11.2)	[1]	1.7	7.7	(46.6)	[1]	4.8	1.8	10.2	32.3	(100.5)
Income before income taxes											720.7	468.0	674.8
Depreciation and amortization of fixed assets and intangible assets											163.0	157.4	147.7
Capital expenditures											148.7	173.0	169.2
Assets	12,747.2					12,905.0					12,747.2	12,905.0
IAN
Segments:
Revenue											6,097.3	5,795.6	5,728.5
Segment operating income											777.1	662.1	700.2
Restructuring and other reorganization-related (charges) reversals, net												(0.3)
Depreciation and amortization of fixed assets and intangible assets											124.6	126.0	119.7
Capital expenditures											(92.1)	(109.0)	(97.5)
Assets	11,111.2					11,425.1					11,111.2	11,425.1
CMG
Segments:
Revenue											1,439.8	1,326.7	1,227.7
Segment operating income											161.0	137.6	114.2
Restructuring and other reorganization-related (charges) reversals, net												0.2
Depreciation and amortization of fixed assets and intangible assets											18.4	15.6	14.4
Capital expenditures											(11.6)	(18.5)	(26.7)
Assets	1,316.5					1,203.8					1,316.5	1,203.8
Corporate and Other
Segments:
Segment operating income											(149.5)	(140.8)	(137.3)
Depreciation and amortization of fixed assets and intangible assets											20.0	15.8	13.6
Capital expenditures											(45.0)	(45.5)	(45.0)
Assets	$ 319.5					$ 276.1					$ 319.5	$ 276.1

[1] The three months ended June 30, 2014 included a pre-tax loss of $10.4, related to our early extinguishment of debt. The three months ended September 30, 2013 included a pre-tax loss of $45.2, related to our early extinguishment of debt.