Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 1,020.8	$ 1,435.9	$ 1,440.5
Cash flow hedge:
Amortization of loss included in interest expense	5.5	5.5	5.6
Income tax effect	(1.6)	(1.6)	(1.6)
Other comprehensive income (loss), Cash flow hedge	3.9	3.9	4.0
Defined benefit pension plans and postemployment arrangements:
Unrecognized actuarial gains (losses) and prior service cost for the period	(29.5)	(45.5)	10.4
Amortization of prior service cost and actuarial losses	13.9	8.7	16.6
Income tax effect	4.5	10.7	(7.9)
Other comprehensive income (loss), Defined benefit pension plans and postemployment arrangements	(11.1)	(26.1)	19.1
Available-for-sale securities:
Reclassification	0.0	0.0	0.3
Foreign currency translation adjustment	(5.0)	74.9	(319.1)
Other Comprehensive Income (Loss)	(12.2)	52.7	(295.7)
Comprehensive Income	1,008.6	1,488.6	1,144.8
Comprehensive Income Attributed To Noncontrolling Interests	79.5	96.3	83.9
Comprehensive Income - Omnicom Group Inc.	$ 929.1	$ 1,392.3	$ 1,060.9