Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 4,086.7	$ 3,714.3	$ 3,859.6	$ 3,629.6	$ 4,176.6	$ 3,719.5	$ 3,790.1	$ 3,587.4	$ 15,290.2	$ 15,273.6	$ 15,416.9
Operating Expenses	3,459.5	3,212.0	3,277.3	3,207.9	3,550.1	3,249.3	3,218.6	3,171.7	13,156.7	13,189.8	13,386.4
Operating Profit	627.2	502.3	582.3	421.7	626.5	470.2	571.5	415.7	2,133.5	2,083.8	2,030.5
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 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
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.77	$ 1.32	$ 1.60	$ 1.14	$ 1.09	$ 1.13	$ 1.40	$ 1.02	$ 5.83	$ 4.65	$ 4.78