Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 15,290.2	$ 15,273.6	$ 15,416.9
Salary and service costs	11,306.1	11,227.2	11,419.0
Occupancy and other costs	1,309.6	1,240.8	1,230.6
Net gain on disposition of subsidiaries	(178.4)	0.0	0.0
Cost of services	12,437.3	12,468.0	12,649.6
Selling, general and administrative expenses	455.4	439.7	443.9
Depreciation and amortization	264.0	282.1	292.9
Operating Expenses	13,156.7	13,189.8	13,386.4
Operating Profit	2,133.5	2,083.8	2,030.5
Interest Expense	266.4	248.6	231.3
Interest Income	57.2	49.7	42.6
Income Before Income Taxes and Income From Equity Method Investments	1,924.3	1,884.9	1,841.8
Income Tax Expense	492.7	696.2	600.5
Income From Equity Method Investments	8.9	3.5	5.4
Net Income	1,440.5	1,192.2	1,246.7
Net Income Attributed To Noncontrolling Interests	114.1	103.8	98.1
Net Income - Omnicom Group Inc.	$ 1,326.4	$ 1,088.4	$ 1,148.6
Net Income Per Share - Omnicom Group Inc.:
Basic	$ 5.85	$ 4.68	$ 4.80
Diluted	5.83	4.65	4.78
Dividends Declared Per Common Share	$ 2.40	$ 2.25	$ 2.15