Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net Income Available for Common Shares [Abstract]
Net Income - Omnicom Group Inc.	$ 275.1	$ 243.3	$ 477.0	$ 406.7
Net income allocated to participating securities	(2.9)	(2.2)	(4.9)	(4.0)
Net income available for common shares	$ 272.2	$ 241.1	$ 472.1	$ 402.7
Weighted Average Shares [Abstract]
Basic	278.7	302.3	281.2	304.3
Dilutive stock options and restricted shares	5.0	4.7	4.9	4.4
Diluted	283.7	307.0	286.1	308.7
Anti-dilutive stock options and restricted shares	0.3	11.5	0.3	11.5
Net Income per Common Share - Omnicom Group Inc. [Abstract]
Basic	$ 0.98	$ 0.80	$ 1.68	$ 1.32
Diluted	$ 0.96	$ 0.79	$ 1.65	$ 1.30