Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 239.3	$ 243.8	$ 762.3	$ 774.5
Net income allocated to participating securities	(2.5)	(4.3)	(9.0)	(14.7)
Net income available for common shares	$ 236.8	$ 239.5	$ 753.3	$ 759.8
Weighted Average Shares:
Basic	243.2	251.4	244.7	255.6
Dilutive stock options and restricted shares	1.2	1.0	1.1	1.7
Diluted	244.4	252.4	245.8	257.3
Anti-dilutive stock options and restricted shares	0.1	0.0	0.1	0.0
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.97	$ 0.95	$ 3.08	$ 2.97
Diluted	$ 0.97	$ 0.95	$ 3.06	$ 2.95