Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Income per Share [Abstract]
Net Income - Omnicom Group Inc.	$ 313.3	$ 290.2	$ 547.3	$ 924.1
Weighted Average Shares:
Basic	215.4	218.2	215.6	220.3
Dilutive stock options and restricted shares	0.4	1.2	0.6	1.2
Diluted	215.8	219.4	216.2	221.5
Anti-dilutive stock options and restricted shares	0.8	0.9	0.8	0.9
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.45	$ 1.33	$ 2.54	$ 4.19
Diluted	$ 1.45	$ 1.32	$ 2.53	$ 4.17