Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 300.5	$ 196.0	$ 289.5	$ 205.1	$ 307.1	$ 203.9	$ 282.7	$ 204.6	$ 991.1	$ 998.3	$ 952.6
Net income allocated to participating securities									(25.1)	(22.5)	(10.7)
Net income available for common shares									$ 966.0	$ 975.8	$ 941.9
Weighted Average Shares:
Basic									258.9	268.3	279.0
Dilutive stock options, restricted shares and shares issuable for the conversion premium of convertible debt									1.5	1.7	4.3
Diluted									260.4	270.0	283.3
Anti-dilutive stock options and restricted shares									0.7	0.3	1.7
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.14	$ 0.74	$ 1.09	$ 0.76	$ 1.13	$ 0.75	$ 1.03	$ 0.73	$ 3.73	$ 3.64	$ 3.38
Diluted	$ 1.13	$ 0.74	$ 1.09	$ 0.76	$ 1.13	$ 0.74	$ 1.02	$ 0.72	$ 3.71	$ 3.61	$ 3.33