Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 370.7	$ 364.2	$ 633.9	$ 628.3
Weighted Average Shares:
Basic	219.6	226.8	221.4	228.5
Dilutive stock options and restricted shares	1.3	1.3	1.1	1.3
Diluted	220.9	228.1	222.5	229.8
Anti-dilutive stock options and restricted shares	0.9	1.0	0.9	1.0
Net Income per Share - Omnicom Group Inc.:
Basic	$ 1.69	$ 1.61	$ 2.86	$ 2.75
Diluted	$ 1.68	$ 1.60	$ 2.85	$ 2.73