Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net Income - Omnicom Group Inc.	$ 282.7	$ 275.1	$ 487.3	$ 477.0
Net income allocated to participating securities	(6.4)	(2.9)	(10.9)	(4.9)
Net income available for common shares	$ 276.3	$ 272.2	$ 476.4	$ 472.1
Basic	269.0	278.7	271.2	281.2
Dilutive stock options and restricted shares	2.8	5.0	2.7	4.9
Diluted	271.8	283.7	273.9	286.1
Anti-dilutive stock options and restricted shares	0.2	0.3	0.3	0.3
Basic	$ 1.03	$ 0.98	$ 1.76	$ 1.68
Diluted	$ 1.02	$ 0.96	$ 1.74	$ 1.65