Net Income per Common Share (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
Net Income Available for Common Shares:
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 allocated to participating securities									(20.4)	(25.1)	(22.5)
Net income available for common shares									$ 1,083.6	$ 966.0	$ 975.8
Weighted Average Shares:
Basic									253.9	258.9	268.3
Dilutive stock options and restricted shares									1.4	1.5	1.7
Diluted									255.3	260.4	270.0
Anti-dilutive stock options and restricted shares									0.6	0.7	0.3
Net Income per Common 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
Diluted	$ 1.30	$ 0.95	$ 1.23	$ 0.77	$ 1.13	$ 0.74	$ 1.09	$ 0.76	$ 4.24	$ 3.71	$ 3.61