Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 364.2	$ 328.6	$ 628.3	$ 570.4
Net income allocated to participating securities	(0.1)	(0.5)	(0.1)	(1.0)
Net income available for common shares	$ 364.1	$ 328.1	$ 628.2	$ 569.4
Weighted Average Shares:
Basic	226.8	232.1	228.5	233.3
Dilutive stock options and restricted shares	1.3	1.9	1.3	1.9
Diluted	228.1	234.0	229.8	235.2
Anti-dilutive stock options and restricted shares	1.0	1.0	1.0	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.61	$ 1.41	$ 2.75	$ 2.44
Diluted	$ 1.60	$ 1.40	$ 2.73	$ 2.42