Noncontrolling Interests (Details) (USD $) In Millions, 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
Changes in Ownership Interests in Less Than 100% Owned Subsidiaries [Abstract]
Net income attributed to 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
Increase in additional paid-in capital from sale of shares in noncontrolling interests									2.6	4.8	2.2
Decrease in additional paid-in capital from purchase of shares in noncontrolling interests									(30.7)	(37.6)	(26.0)
Net transfers (to) from noncontrolling interests									(28.1)	(32.8)	(23.8)
Changes in net income attributed to Omnicom Group Inc. and transfers (to) from noncontrolling interests									$ 970.2	$ 919.8	$ 803.9