Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 243.8	$ 196.0	$ 774.5	$ 690.7
Net income allocated to participating securities	(4.3)	(4.8)	(14.7)	(18.1)
Net income available for common shares	$ 239.5	$ 191.2	$ 759.8	$ 672.6
Weighted Average Shares:
Basic	251.4	258.2	255.6	259.0
Dilutive stock options, restricted shares and shares issuable for the conversion premium of convertible debt	1.0	1.7	1.7	1.4
Diluted	252.4	259.9	257.3	260.4
Anti-dilutive stock options and restricted shares	0	0	0	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.95	$ 0.74	$ 2.97	$ 2.60
Diluted	$ 0.95	$ 0.74	$ 2.95	$ 2.58