Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net Income Available for Common Shares [Abstract]
Net Income - Omnicom Group Inc.	$ 204.6	$ 201.9
Net income allocated to participating securities	(4.5)	(2.0)
Net income available for common shares	$ 200.1	$ 199.9
Weighted Average Shares [Abstract]
Basic	273.3	283.6
Dilutive stock options and restricted shares	4.2	5.6
Diluted	277.5	289.2
Anti-dilutive stock options and restricted shares	0.2	0.3
Net Income per Common Share - Omnicom Group Inc. [Abstract]
Basic	$ 0.73	$ 0.70
Diluted	$ 0.72	$ 0.69