Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 326.1	$ 313.9	$ 544.5	$ 523.0
Net income allocated to participating securities	(2.0)	(3.9)	(3.6)	(6.7)
Net income available for common shares	$ 324.1	$ 310.0	$ 540.9	$ 516.3
Weighted Average Shares:
Basic	237.7	244.5	238.9	245.5
Dilutive stock options and restricted shares	1.3	1.2	1.2	1.1
Diluted	239.0	245.7	240.1	246.6
Anti-dilutive stock options and restricted shares	0.0	0.1	0.0	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.36	$ 1.27	$ 2.26	$ 2.10
Diluted	$ 1.36	$ 1.26	$ 2.25	$ 2.09