Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 205.1	$ 204.6
Net income allocated to participating securities	(5.4)	(4.5)
Net income available for common shares	$ 199.7	$ 200.1
Weighted Average Shares:
Basic	261.0	273.3
Dilutive stock options and restricted shares and shares issuable for the conversion value of convertible debt	2.2	4.2
Diluted	263.2	277.5
Anti-dilutive stock options and restricted shares	0.1	0.2
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.76	$ 0.73
Diluted	$ 0.76	$ 0.72