Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenue	$ 3,435.0	$ 3,206.5	$ 10,433.6	$ 9,414.1
Salary and service costs	2,461.8	2,287.1	7,609.9	6,851.5
Occupancy and other costs	285.5	273.1	871.0	872.6
Gain on disposition of subsidiary	0.0	0.0	(50.5)	0.0
COVID-19 repositioning costs	0.0	0.0	0.0	277.9
Cost of services	2,747.3	2,560.2	8,430.4	8,002.0
Selling, general and administrative expenses	95.0	90.2	269.9	259.2
Depreciation and amortization	51.1	54.7	157.9	168.8
Operating Expenses	2,893.4	2,705.1	8,858.2	8,430.0
Operating Profit	541.6	501.4	1,575.4	984.1
Interest Expense	50.7	54.4	184.8	166.6
Interest Income	7.0	5.9	20.1	25.1
Income Before Income Taxes and Income (Loss) From Equity Method Investments	497.9	452.9	1,410.7	842.6
Income Tax Expense	120.0	120.9	355.1	240.2
Income (Loss) From Equity Method Investments	2.2	2.9	2.1	(10.1)
Net Income	380.1	334.9	1,057.7	592.3
Net Income Attributed To Noncontrolling Interests	24.5	21.6	66.1	45.0
Net Income - Omnicom Group Inc.	$ 355.6	$ 313.3	$ 991.6	$ 547.3
Net Income Per Share - Omnicom Group Inc.:
Basic	$ 1.66	$ 1.45	$ 4.61	$ 2.54
Diluted	$ 1.65	$ 1.45	$ 4.58	$ 2.53