Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 4,153.3	$ 3,706.6	$ 3,805.3	$ 3,469.2	$ 4,195.1	$ 3,749.6	$ 3,870.9	$ 3,502.2	$ 15,134.4	$ 15,317.8	$ 14,584.5
Operating expenses	3,577.8	3,278.3	3,266.7	3,091.5	3,615.7	3,316.0	3,322.5	3,119.5	13,214.3	13,373.7	12,759.2
Operating Income	575.5	428.3	538.6	377.7	579.4	433.6	548.4	382.7	1,920.1	1,944.1	1,825.3
Net income - Omnicom Group Inc.	$ 331.6	$ 239.3	$ 313.9	$ 209.1	$ 329.5	$ 243.8	$ 325.2	$ 205.5	$ 1,093.9	$ 1,104.0	$ 991.1
Net Income Per Share Omnicom Group Inc. - Basic	$ 1.35	$ 0.97	$ 1.27	$ 0.84	$ 1.30	$ 0.95	$ 1.24	$ 0.78	$ 4.43	$ 4.27	$ 3.73
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.35	$ 0.97	$ 1.26	$ 0.83	$ 1.30	$ 0.95	$ 1.23	$ 0.77	$ 4.41	$ 4.24	$ 3.71
Merger expenses							$ 1.8	$ 7.0
Operating Income, excluding merger expenses							550.2	389.7
Net Income - Omnicom Group Inc., excluding merger expenses							$ 315.8	$ 212.3
Net Income Per Share Omnicom Group Inc. - Diluted, excluding merger expenses							$ 1.20	$ 0.80