Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Income per Share [Abstract]
Net Income - Omnicom Group Inc.	$ 287.8	$ 258.1
Weighted Average Shares:
Basic	215.6	216.6
Dilutive stock options and restricted shares	1.2	0.9
Diluted	216.8	217.5
Anti-dilutive stock options and restricted shares	0.7	0.9
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.33	$ 1.19
Diluted	$ 1.33	$ 1.19