Quarterly Results of Operations (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Results of Operations (Unaudited) [Abstract]
Revenue	$ 4,058.1	$ 3,490.5	$ 3,637.0	$ 3,398.9	$ 3,944.5	$ 3,406.6	$ 3,561.0	$ 3,307.3	$ 14,584.5	$ 14,219.4	$ 13,872.5
Operating expenses	3,506.9	3,111.1	3,114.0	3,027.2	3,396.5	3,019.3	3,054.6	2,944.8	12,759.2	12,415.2	12,201.4
Operating Income	551.2	379.4	523.0	371.7	548.0	387.3	506.4	362.5	1,825.3	1,804.2	1,671.1
Net income - Omnicom Group Inc.	$ 300.5	$ 196.0	$ 289.5	$ 205.1	$ 307.1	$ 203.9	$ 282.7	$ 204.6	$ 991.1	$ 998.3	$ 952.6
Net Income Per Share Omnicom Group Inc. - Basic	$ 1.14	$ 0.74	$ 1.09	$ 0.76	$ 1.13	$ 0.75	$ 1.03	$ 0.73	$ 3.73	$ 3.64	$ 3.38
Net Income Per Share Omnicom Group Inc. - Diluted	$ 1.13	$ 0.74	$ 1.09	$ 0.76	$ 1.13	$ 0.74	$ 1.02	$ 0.72	$ 3.71	$ 3.61	$ 3.33