Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in Ownership Interests in Less Than 100% Owned Subsidiaries [Abstract]
Net income attributed to Omnicom Group Inc.	$ 331.6	$ 239.3	$ 313.9	$ 209.1	$ 329.5	$ 243.8	$ 325.2	$ 205.5	$ 1,093.9	$ 1,104.0	$ 991.1
Increase in additional paid-in capital from sale of shares in noncontrolling interests									1.7	6.3	5.2
Decrease in additional paid-in capital from purchase of shares in noncontrolling interests									(40.5)	(70.8)	(22.0)
Net transfers (to) from noncontrolling interests									(38.8)	(64.5)	(16.8)
Change from net income attributed to Omnicom Group Inc. and transfers (to) from noncontrolling interests									$ 1,055.1	$ 1,039.5	$ 974.3