Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 263.2	$ 264.1
Weighted Average Shares:
Basic	223.2	230.2
Dilutive stock options and restricted shares	1.0	1.3
Diluted	224.2	231.5
Anti-dilutive stock options and restricted shares	0.9	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.18	$ 1.15
Diluted	$ 1.17	$ 1.14