Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net Income	$ 1,440.5	$ 1,192.2	$ 1,246.7
Cash flow hedge:
Amortization of loss included in interest expense	5.6	5.3	4.0
Loss for the period	0.0	0.0	(48.9)
Income tax effect	(1.6)	(2.1)	18.7
Other comprehensive income (loss)	4.0	3.2	(26.2)
Defined benefit pension plans and postemployment arrangements:
Unrecognized actuarial gains (losses) and prior service cost for the period	10.4	(12.2)	(18.3)
Amortization of prior service cost and actuarial losses	16.6	16.1	14.0
Income tax effect	(7.9)	(1.7)	1.6
Other comprehensive income (loss)	19.1	2.2	(2.7)
Available-for-sale securities:
Unrealized gain for the period	0.0	0.8	0.2
Income tax effect	0.0	(0.3)	(0.1)
Reclassification	0.3	0.0	0.0
Other comprehensive income (loss)	0.3	0.5	0.1
Foreign currency translation adjustment	(319.1)	412.7	(319.4)
Other Comprehensive Income (Loss)	(295.7)	418.6	(348.2)
Comprehensive Income	1,144.8	1,610.8	898.5
Comprehensive Income Attributed To Noncontrolling Interests	83.9	129.4	90.5
Comprehensive Income - Omnicom Group Inc.	$ 1,060.9	$ 1,481.4	$ 808.0