Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 4,141.2	$ 3,623.8	$ 3,719.8	$ 3,468.9	$ 4,086.7	$ 3,714.3	$ 3,859.6	$ 3,629.6	$ 14,953.7	$ 15,290.2	$ 15,273.6
Operating Expenses	3,494.8	3,150.5	3,146.1	3,040.0	3,459.5	3,212.0	3,277.3	3,207.9	12,831.4	13,156.7	13,189.8
Operating Profit	646.4	473.3	573.7	428.9	627.2	502.3	582.3	421.7	2,122.3	2,133.5	2,083.8
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 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
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.89	$ 1.32	$ 1.68	$ 1.17	$ 1.77	$ 1.32	$ 1.60	$ 1.14	$ 6.06	$ 5.83	$ 4.65