Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Net Income Available for Common Shares:
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 allocated to participating securities									(12.4)	(20.4)	(25.1)
Net income available for common shares									$ 1,081.5	$ 1,083.6	$ 966.0
Weighted Average Shares:
Basic									244.2	253.9	258.9
Dilutive stock options and restricted shares									1.0	1.4	1.5
Diluted									245.2	255.3	260.4
Anti-dilutive stock options and restricted shares									0.1	0.6	0.7
Net Income per Common 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
Diluted	$ 1.35	$ 0.97	$ 1.26	$ 0.83	$ 1.30	$ 0.95	$ 1.23	$ 0.77	$ 4.41	$ 4.24	$ 3.71