Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Income (Loss) per Share [Abstract]
Net Income (Loss) - Omnicom Group Inc.	$ (24.2)	$ 370.7	$ 234.0	$ 633.9
Weighted Average Shares:
Basic	214.9	219.6	215.8	221.4
Dilutive stock options and restricted shares	0.5	1.3	0.7	1.1
Diluted	215.4	220.9	216.5	222.5
Anti-dilutive stock options and restricted shares	0.8	0.9	0.8	0.9
Net Income (Loss) per Share - Omnicom Group Inc.:
Basic	$ (0.11)	$ 1.69	$ 1.08	$ 2.86
Diluted	$ (0.11)	$ 1.68	$ 1.08	$ 2.85