Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in Ownership Interests in Less Than 100% Owned Subsidiaries [Abstract]
Net income attributed to Omnicom Group Inc.	$ 254.4	$ 263.6	$ 328.6	$ 241.8	$ 350.3	$ 253.8	$ 326.1	$ 218.4	$ 1,088.4	$ 1,148.6	$ 1,093.9
Increase in additional paid-in capital from sale of shares in noncontrolling interests									1.8	2.0	1.7
Decrease in additional paid-in capital from purchase of shares in noncontrolling interests									(27.5)	(89.7)	(40.5)
Net transfers (to) from noncontrolling interests									(25.7)	(87.7)	(38.8)
Change from net income attributed to Omnicom Group Inc. and transfers (to) from noncontrolling interests									$ 1,062.7	$ 1,060.9	$ 1,055.1