Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 264.1	$ 241.8
Net income allocated to participating securities	0.0	(0.5)
Net income available for common shares	$ 264.1	$ 241.3
Weighted Average Shares:
Basic	230.2	234.6
Dilutive stock options and restricted shares	1.3	1.9
Diluted	231.5	236.5
Anti-dilutive stock options and restricted shares	1.0	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.15	$ 1.03
Diluted	$ 1.14	$ 1.02