Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 328.6	$ 326.1	$ 570.4	$ 544.5
Net income allocated to participating securities	(0.5)	(2.0)	(1.0)	(3.6)
Net income available for common shares	$ 328.1	$ 324.1	$ 569.4	$ 540.9
Weighted Average Shares:
Basic	232.1	237.7	233.3	238.9
Dilutive stock options and restricted shares	1.9	1.3	1.9	1.2
Diluted	234.0	239.0	235.2	240.1
Anti-dilutive stock options and restricted shares	1.0	0.0	1.0	0.0
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.41	$ 1.36	$ 2.44	$ 2.26
Diluted	$ 1.40	$ 1.36	$ 2.42	$ 2.25