Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net Income per Share [Abstract]
Net income - Omnicom Group Inc.	$ 1,407.8	$ 945.4	$ 1,339.1
Weighted average shares:
Basic	214.3	215.6	219.8
Dilutive stock options and restricted shares	1.3	0.6	1.1
Diluted	215.6	216.2	220.9
Anti-dilutive stock options and restricted shares	4.7	0.8	0.9
Net income per share - Omnicom Group Inc.:
Basic	$ 6.57	$ 4.38	$ 6.09
Diluted	$ 6.53	$ 4.37	$ 6.06