Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 4,241.8	$ 3,791.1	$ 3,884.9	$ 3,499.1	$ 4,153.3	$ 3,706.6	$ 3,805.3	$ 3,469.2	$ 15,416.9	$ 15,134.4	$ 15,317.8
Operating Expenses	3,639.9	3,338.0	3,323.1	3,107.0	3,577.8	3,278.3	3,266.7	3,091.5	13,408.0	13,214.3	13,373.7
Operating Profit	601.9	453.1	561.8	392.1	575.5	428.3	538.6	377.7	2,008.9	1,920.1	1,944.1
Net income - Omnicom Group Inc.	$ 350.3	$ 253.8	$ 326.1	$ 218.4	$ 331.6	$ 239.3	$ 313.9	$ 209.1	$ 1,148.6	$ 1,093.9	$ 1,104.0
Net Income Per Share Omnicom Group Inc. - Basic	$ 1.47	$ 1.06	$ 1.36	$ 0.90	$ 1.35	$ 0.97	$ 1.27	$ 0.84	$ 4.80	$ 4.43	$ 4.27
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.47	$ 1.06	$ 1.36	$ 0.90	$ 1.35	$ 0.97	$ 1.26	$ 0.83	$ 4.78	$ 4.41	$ 4.24