Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income available for common shares:
Net income - Omnicom Group Inc.	$ 415.0	$ 290.2	$ 370.7	$ 263.2	$ 399.2	$ 298.9	$ 364.2	$ 264.1	$ 1,339.1	$ 1,326.4	$ 1,088.4
Net income allocated to participating securities									0.0	(0.1)	(1.6)
Net income available for common shares									$ 1,339.1	$ 1,326.3	$ 1,086.8
Weighted average shares
Basic									219.8	226.6	232.3
Dilutive stock options and restricted shares									1.1	1.0	1.6
Diluted									220.9	227.6	233.9
Anti-dilutive stock options and restricted shares									0.0	1.0	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.90	$ 1.33	$ 1.69	$ 1.18	$ 1.78	$ 1.33	$ 1.61	$ 1.15	$ 6.09	$ 5.85	$ 4.68
Diluted	$ 1.89	$ 1.32	$ 1.68	$ 1.17	$ 1.77	$ 1.32	$ 1.60	$ 1.14	$ 6.06	$ 5.83	$ 4.65