Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 4,195.1	$ 3,749.6	$ 3,870.9	$ 3,502.2	$ 4,058.1	$ 3,490.5	$ 3,637.0	$ 3,398.9	$ 15,317.8	$ 14,584.5	$ 14,219.4
Operating expenses	3,615.7	3,316.0	3,322.5	3,119.5	3,506.9	3,111.1	3,114.0	3,027.2	13,373.7	12,759.2	12,415.2
Operating Income	579.4	433.6	548.4	382.7	551.2	379.4	523.0	371.7	1,944.1	1,825.3	1,804.2
Net income - Omnicom Group Inc.	329.5	243.8	325.2	205.5	300.5	196.0	289.5	205.1	1,104.0	991.1	998.3
Net Income Per Share Omnicom Group Inc. - Basic	$ 1.30	$ 0.95	$ 1.24	$ 0.78	$ 1.14	$ 0.74	$ 1.09	$ 0.76	$ 4.27	$ 3.73	$ 3.64
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.30	$ 0.95	$ 1.23	$ 0.77	$ 1.13	$ 0.74	$ 1.09	$ 0.76	$ 4.24	$ 3.71	$ 3.61
Merger expenses			1.8	7.0	13.3	28.1			8.8	41.4
Operating Income, excluding merger expenses			550.2	389.7	564.5	407.5
Net Income - Omnicom Group Inc., excluding merger expenses			$ 315.8	$ 212.3	$ 313.8	$ 217.7
Net Income Per Share Omnicom Group Inc. - Diluted, excluding merger expenses			$ 1.20	$ 0.80	$ 1.18	$ 0.82