Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 290.2	$ 298.9	$ 924.1	$ 927.2
Weighted Average Shares:
Basic	218.2	224.8	220.3	227.2
Dilutive stock options and restricted shares	1.2	1.1	1.2	1.3
Diluted	219.4	225.9	221.5	228.5
Anti-dilutive stock options and restricted shares	0.9	1.0	0.9	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.33	$ 1.33	$ 4.19	$ 4.08
Diluted	$ 1.32	$ 1.32	$ 4.17	$ 4.06