Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Income per Share [Abstract]
Net income - Omnicom Group Inc.	$ 945.4	$ 1,339.1	$ 1,326.4
Weighted average shares:
Basic	215.6	219.8	226.6
Dilutive stock options and restricted shares	0.6	1.1	1.0
Diluted	216.2	220.9	227.6
Anti-dilutive stock options and restricted shares	0.8	0.0	1.0
Net income per share - Omnicom Group Inc.:
Basic	$ 4.38	$ 6.09	$ 5.85
Diluted	$ 4.37	$ 6.06	$ 5.83