Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 1,435.9	$ 1,440.5	$ 1,192.2
Cash flow hedge:
Amortization of loss included in interest expense	5.5	5.6	5.3
Income tax effect	1.6	1.6	2.1
Other comprehensive income (loss), Cash flow hedge	3.9	4.0	3.2
Defined benefit pension plans and postemployment arrangements:
Unrecognized actuarial gains (losses) and prior service cost for the period	(45.5)	10.4	(12.2)
Amortization of prior service cost and actuarial losses	8.7	16.6	16.1
Income tax effect	10.7	(7.9)	(1.7)
Other comprehensive income (loss), Defined benefit pension plans and postemployment arrangements	(26.1)	19.1	2.2
Available-for-sale securities:
Unrealized gain for the period	0.0	0.0	0.8
Income tax effect	0.0	0.0	(0.3)
Reclassification	0.0	0.3	0.0
Other comprehensive income (loss), Available-for-sale securities	0.0	0.3	0.5
Foreign currency translation adjustment	74.9	(319.1)	412.7
Other Comprehensive Income (Loss)	52.7	(295.7)	418.6
Comprehensive Income	1,488.6	1,144.8	1,610.8
Comprehensive Income Attributed To Noncontrolling Interests	96.3	83.9	129.4
Comprehensive Income - Omnicom Group Inc.	$ 1,392.3	$ 1,060.9	$ 1,481.4