Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 307.1	$ 203.9	$ 282.7	$ 204.6	$ 271.9	$ 203.7	$ 275.1	$ 201.9	$ 998.3	$ 952.6	$ 827.7
Net income allocated to participating securities									(22.5)	(10.7)	(8.0)
Net income available for common shares									$ 975.8	$ 941.9	$ 819.7
Weighted Average Shares:
Basic									268.3	279.0	299.6
Dilutive stock options and restricted shares									1.7	4.3	3.9
Diluted									270.0	283.3	303.5
Anti-dilutive stock options and restricted shares									0.3	1.7	3.0
Net Income per Common Share - Omnicom Group Inc. [Abstract]
Basic	$ 1.13	$ 0.75	$ 1.03	$ 0.73	$ 0.97	$ 0.73	$ 0.98	$ 0.70	$ 3.64	$ 3.38	$ 2.74
Diluted	$ 1.13	$ 0.74	$ 1.02	$ 0.72	$ 0.96	$ 0.72	$ 0.96	$ 0.69	$ 3.61	$ 3.33	$ 2.70