Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 298.9	$ 263.6	$ 927.2	$ 834.0
Net income allocated to participating securities	0.0	(0.3)	(0.1)	(1.4)
Net income available for common shares	$ 298.9	$ 263.3	$ 927.1	$ 832.6
Weighted Average Shares:
Basic	224.8	231.2	227.2	232.6
Dilutive stock options and restricted shares	1.1	1.5	1.3	1.8
Diluted	225.9	232.7	228.5	234.4
Anti-dilutive stock options and restricted shares	1.0	1.0	1.0	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.33	$ 1.14	$ 4.08	$ 3.58
Diluted	$ 1.32	$ 1.13	$ 4.06	$ 3.55