Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 253.8	$ 239.3	$ 798.3	$ 762.3
Net income allocated to participating securities	(1.2)	(2.5)	(4.8)	(9.0)
Net income available for common shares	$ 252.6	$ 236.8	$ 793.5	$ 753.3
Weighted Average Shares:
Basic	237.4	243.2	238.4	244.7
Dilutive stock options and restricted shares	1.3	1.2	1.2	1.1
Diluted	238.7	244.4	239.6	245.8
Anti-dilutive stock options and restricted shares	0.0	0.1	0.0	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.06	$ 0.97	$ 3.33	$ 3.08
Diluted	$ 1.06	$ 0.97	$ 3.31	$ 3.06