Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 263.6	$ 253.8	$ 834.0	$ 798.3
Net income allocated to participating securities	(0.3)	(1.2)	(1.4)	(4.8)
Net income available for common shares	$ 263.3	$ 252.6	$ 832.6	$ 793.5
Weighted Average Shares:
Basic	231.2	237.4	232.6	238.4
Dilutive stock options and restricted shares	1.5	1.3	1.8	1.2
Diluted	232.7	238.7	234.4	239.6
Anti-dilutive stock options and restricted shares	1.0	0.0	1.0	0.0
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.14	$ 1.06	$ 3.58	$ 3.33
Diluted	$ 1.13	$ 1.06	$ 3.55	$ 3.31