Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 331.3	$ 286.9	$ 1,010.8	$ 904.4
Cash flow hedge:
Amortization of loss included in interest expense	1.4	1.4	4.2	4.2
Income tax effect	(0.4)	(0.6)	(1.2)	(1.8)
Other comprehensive income	1.0	0.8	3.0	2.4
Defined benefit pension plans and postemployment arrangements:
Amortization of prior service cost	2.0	1.9	5.9	6.1
Amortization of actuarial losses	2.0	2.0	6.2	5.6
Income tax effect	1.5	(1.4)	(0.9)	(5.1)
Other comprehensive income	5.5	2.5	11.2	6.6
Available-for-sale securities:
Unrealized gain for the period	0.0	0.0	0.0	0.5
Income tax effect	0.0	0.0	0.0	(0.2)
Reclassification	0.0	0.0	0.3	0.0
Other comprehensive income	0.0	0.0	0.3	0.3
Foreign currency translation adjustment	(30.3)	159.6	(267.4)	414.4
Other Comprehensive Income	(23.8)	162.9	(252.9)	423.7
Comprehensive Income	307.5	449.8	757.9	1,328.1
Comprehensive Income Attributed To Noncontrolling Interests	27.5	32.6	53.9	96.5
Comprehensive Income - Omnicom Group Inc.	$ 280.0	$ 417.2	$ 704.0	$ 1,231.6