Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 350.3	$ 253.8	$ 326.1	$ 218.4	$ 331.6	$ 239.3	$ 313.9	$ 209.1	$ 1,148.6	$ 1,093.9	$ 1,104.0
Net income allocated to participating securities									(6.5)	(12.4)	(20.4)
Net income available for common shares									$ 1,142.1	$ 1,081.5	$ 1,083.6
Weighted Average Shares:
Basic									237.9	244.2	253.9
Dilutive stock options and restricted shares									1.3	1.0	1.4
Diluted									239.2	245.2	255.3
Anti-dilutive stock options and restricted shares									0.0	0.1	0.6
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 1.47	$ 1.06	$ 1.36	$ 0.90	$ 1.35	$ 0.97	$ 1.27	$ 0.84	$ 4.80	$ 4.43	$ 4.27
Diluted	$ 1.47	$ 1.06	$ 1.36	$ 0.90	$ 1.35	$ 0.97	$ 1.26	$ 0.83	$ 4.78	$ 4.41	$ 4.24