Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 196.0	$ 203.9	$ 690.7	$ 691.2
Net income allocated to participating securities	(4.8)	(4.5)	(18.1)	(15.4)
Net income available for common shares	$ 191.2	$ 199.4	$ 672.6	$ 675.8
Weighted Average Shares:
Basic	258.2	266.6	259.0	269.6
Dilutive stock options and restricted shares and shares issuable for the conversion value of convertible debt	1.7	1.9	1.4	1.9
Diluted	259.9	268.5	260.4	271.5
Anti-dilutive stock options and restricted shares	0	0.2	0.1	0.2
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.74	$ 0.75	$ 2.60	$ 2.51
Diluted	$ 0.74	$ 0.74	$ 2.58	$ 2.49