Quarterly Results of Operations (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Results of Operations (Unaudited) [Abstract]
Revenue	$ 3,852.9	$ 3,380.9	$ 3,487.4	$ 3,151.3	$ 3,586.7	$ 2,994.6	$ 3,041.2	$ 2,920.0	$ 13,872.5	$ 12,542.5	$ 11,720.7
Operating Expenses	3,365.5	3,007.5	2,999.3	2,829.2	3,147.0	2,680.5	2,625.8	2,629.0	12,201.4	11,082.3	10,345.8
Operating Income	487.4	373.4	488.1	322.1	439.7	314.1	415.4	291.0	1,671.1	1,460.2	1,374.9
Net Income - Omnicom Group Inc.	$ 271.9	$ 203.7	$ 275.1	$ 201.9	$ 246.4	$ 174.6	$ 243.3	$ 163.4	$ 952.6	$ 827.7	$ 793.0
Net Income Per Share Omnicom Group Inc. - Basic	$ 0.97	$ 0.73	$ 0.98	$ 0.70	$ 0.84	$ 0.58	$ 0.80	$ 0.53	$ 3.38	$ 2.74	$ 2.54
Net Income Per Share Omnicom Group Inc. - Diluted	$ 0.96	$ 0.72	$ 0.96	$ 0.69	$ 0.83	$ 0.57	$ 0.79	$ 0.52	$ 3.33	$ 2.70	$ 2.53