Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Income per Share [Abstract]
Net Income - Omnicom Group Inc.	$ 355.6	$ 313.3	$ 991.6	$ 547.3
Weighted Average Shares:
Basic	214.0	215.4	215.0	215.6
Dilutive stock options and restricted shares	1.4	0.4	1.4	0.6
Diluted	215.4	215.8	216.4	216.2
Anti-dilutive stock options and restricted shares	0.7	0.8	0.7	0.8
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.66	$ 1.45	$ 4.61	$ 2.54
Diluted	$ 1.65	$ 1.45	$ 4.58	$ 2.53