Noncontrolling Interests (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in Ownership Interests in Less Than 100% Owned Subsidiaries [Abstract]
Net income attributed to Omnicom Group Inc.	$ 945.4	$ 1,339.1	$ 1,326.4
Net transfers (to) from noncontrolling interests	5.8	(22.3)	(39.7)
Change from net income attributed to Omnicom Group Inc. and transfers (to) from noncontrolling interests	$ 951.2	$ 1,316.8	$ 1,286.7