Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenue	$ 3,859.6	$ 3,790.1	$ 7,489.2	$ 7,377.6
Salary and service costs	2,772.9	2,731.7	5,485.7	5,420.2
Occupancy and other costs	319.6	305.7	639.9	607.6
Cost of services	3,092.5	3,037.4	6,125.6	6,027.8
Selling, general and administrative expenses	117.4	110.1	222.9	218.8
Depreciation and amortization	67.4	71.1	136.8	143.8
Operating Expenses	3,277.3	3,218.6	6,485.3	6,390.4
Operating Profit	582.3	571.5	1,003.9	987.2
Interest Expense	66.4	62.8	128.6	121.9
Interest Income	13.9	11.5	29.3	25.3
Income Before Income Taxes and Income From Equity Method Investments	529.8	520.2	904.6	890.6
Income Tax Expense	136.7	166.7	227.7	274.7
Income From Equity Method Investments	1.7	1.6	2.6	1.6
Net Income	394.8	355.1	679.5	617.5
Net Income Attributed To Noncontrolling Interests	30.6	26.5	51.2	47.1
Net Income - Omnicom Group Inc.	$ 364.2	$ 328.6	$ 628.3	$ 570.4
Net Income Per Share - Omnicom Group Inc.:
Basic	$ 1.61	$ 1.41	$ 2.75	$ 2.44
Diluted	1.60	1.40	2.73	2.42
Dividends Declared Per Common Share	$ 0.60	$ 0.55	$ 1.20	$ 1.10