Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net Income per Share [Abstract]
Net Income - Omnicom Group Inc.	$ 341.3	$ 385.9	$ 886.6	$ 1,032.6
Weighted Average Shares (millions):
Basic	193.8	195.6	195.2	196.5
Dilutive stock options and restricted shares	1.1	2.6	1.2	2.4
Diluted	194.9	198.2	196.4	198.9
Anti-dilutive stock options and restricted shares (millions)	5.8	0.0	5.8	0.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.76	$ 1.97	$ 4.54	$ 5.25
Diluted	$ 1.75	$ 1.95	$ 4.51	$ 5.19