Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net Income Available for Common Shares [Abstract]
Net Income - Omnicom Group Inc.	$ 203.7	$ 174.6	$ 680.7	$ 581.2
Net income allocated to participating securities	(2.3)	(1.7)	(7.3)	(5.6)
Net income available for common shares	$ 201.4	$ 172.9	$ 673.4	$ 575.6
Weighted Average Shares [Abstract]
Basic	277.1	299.3	279.8	302.7
Dilutive stock options and restricted shares	4.3	4.2	4.5	4.2
Diluted	281.4	303.5	284.3	306.9
Anti-dilutive stock options and restricted shares	2.1	9.1	1.6	9.0
Net Income per Common Share - Omnicom Group Inc. [Abstract]
Basic	$ 0.73	$ 0.58	$ 2.41	$ 1.90
Diluted	$ 0.72	$ 0.57	$ 2.37	$ 1.88