Comprehensive Income (Loss) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net Income	$ 228.9	$ 195.6	$ 753.2	$ 645.2
Foreign currency transaction and translation adjustments, net of income taxes of $(136.7) and $126.2 for the three months and $(38.5) and $(23.7) for the nine months ended September 30, 2011 and 2010, respectively	(265.1)	235.0	(74.4)	(43.3)
Defined benefit plans adjustment, net of income taxes of $0.6 and $0.5 for the three months and $1.8 and $1.6 for the nine months ended September 30, 2011 and 2010, respectively	0.9	0.7	2.7	2.3
Comprehensive income (loss)	(35.3)	431.3	681.5	604.2
Less: Comprehensive income (loss) attributed to noncontrolling interests	(0.5)	36.0	59.1	73.2
Comprehensive income (loss) - Omnicom Group Inc.	(34.8)	395.3	622.4	531.0
Foreign currency transaction and translation adjustments, income taxes	(136.7)	126.2	(38.5)	(23.7)
Defined benefit plans adjustment, income taxes	$ 0.6	$ 0.5	$ 1.8	$ 1.6