Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 325.2	$ 289.5	$ 530.7	$ 494.7
Net income allocated to participating securities	(6.3)	(7.8)	(10.4)	(13.4)
Net income available for common shares	$ 318.9	$ 281.7	$ 520.3	$ 481.3
Weighted Average Shares:
Basic	256.2	257.7	257.7	259.4
Dilutive stock options, restricted shares and shares issuable for the conversion premium of convertible debt	2.0	1.5	2.1	1.3
Diluted	258.2	259.2	259.8	260.7
Anti-dilutive stock options and restricted shares	0.1	0.1	0.1	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.24	$ 1.09	$ 2.02	$ 1.86
Diluted	$ 1.23	$ 1.09	$ 2.00	$ 1.85