Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 399.2	$ 298.9	$ 364.2	$ 264.1	$ 254.4	$ 263.6	$ 328.6	$ 241.8	$ 1,326.4	$ 1,088.4	$ 1,148.6
Net income allocated to participating securities									(0.1)	(1.6)	(6.5)
Net income available for common shares									$ 1,326.3	$ 1,086.8	$ 1,142.1
Weighted Average Shares:
Basic									226.6	232.3	237.9
Dilutive stock options and restricted shares									1.0	1.6	1.3
Diluted									227.6	233.9	239.2
Anti-dilutive stock options and restricted shares									1.0	1.0	0.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.78	$ 1.33	$ 1.61	$ 1.15	$ 1.10	$ 1.14	$ 1.41	$ 1.03	$ 5.85	$ 4.68	$ 4.80
Diluted	$ 1.77	$ 1.32	$ 1.60	$ 1.14	$ 1.09	$ 1.13	$ 1.40	$ 1.02	$ 5.83	$ 4.65	$ 4.78