Quarterly Results of Operations (Unaudited) (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
Quarterly Results of Operations (Unaudited) [Abstract]
Revenue	$ 3,944.5	$ 3,406.6	$ 3,561.0	$ 3,307.3	$ 3,852.9	$ 3,380.9	$ 3,487.4	$ 3,151.3	$ 14,219.4	$ 13,872.5	$ 12,542.5
Operating Expenses	3,396.5	3,019.3	3,054.6	2,944.8	3,365.5	3,007.5	2,999.3	2,829.2	12,415.2	12,201.4	11,082.3
Operating Income	548.0	387.3	506.4	362.5	487.4	373.4	488.1	322.1	1,804.2	1,671.1	1,460.2
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 Per Share Omnicom Group Inc. - Basic	$ 1.13	$ 0.75	$ 1.03	$ 0.73	$ 0.97	$ 0.73	$ 0.98	$ 0.70	$ 3.64	$ 3.38	$ 2.74
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.13	$ 0.74	$ 1.02	$ 0.72	$ 0.96	$ 0.72	$ 0.96	$ 0.69	$ 3.61	$ 3.33	$ 2.70