Segment Information Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012		Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segments:
Revenue	$ 2,063.3	$ 1,670.4		$ 1,715.7	$ 1,506.8	$ 2,072.6	$ 1,726.5	$ 1,740.7	$ 1,474.8	$ 6,956.2	$ 7,014.6	$ 6,507.3
Segment operating income										678.3	687.2	548.7
Interest expense										(133.5)	(136.8)	(139.7)
Interest income										29.5	37.8	28.7
Other income, net	95.4	1.7	[1]	4.7	(1.3)	13.9	137.1	5.3	(6.1)	100.5	150.2	12.9
Income before income taxes										674.8	738.4	450.6
Depreciation and amortization of fixed assets and intangible assets										147.7	150.9	148.4
Capital expenditures										169.2	140.3	96.3
IAN
Segments:
Segment revenue										5,728.5	5,891.8	5,468.4
Segment operating income										701.1	728.0	615.2
Depreciation and amortization of fixed assets and intangible assets										119.7	125.7	116.7
Capital expenditures										97.5	94.1	83.5
CMG
Segments:
Segment revenue										1,227.7	1,122.8	1,038.9
Segment operating income										114.5	101.2	78.8
Depreciation and amortization of fixed assets and intangible assets										14.4	12.8	14.2
Capital expenditures										26.7	16.9	6.9
Corporate and other
Segments:
Segment operating income										(137.3)	(142.0)	(145.3)
Depreciation and amortization of fixed assets and intangible assets										13.6	12.4	17.5
Capital expenditures										$ 45.0	$ 29.3	$ 5.9

[1] The three months ended December 31, 2012 and the three months ended September 30, 2011, include a pre-tax gain of $93.6 and $132.2, respectively, related to the sale of our holdings in Facebook.