Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 218.4	$ 209.1
Net income allocated to participating securities	(1.5)	(2.8)
Net income available for common shares	$ 216.9	$ 206.3
Weighted Average Shares:
Basic	240.0	246.4
Dilutive stock options and restricted shares	1.1	1.0
Diluted	241.1	247.4
Anti-dilutive stock options and restricted shares	0.1	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.90	$ 0.84
Diluted	$ 0.90	$ 0.83