Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 284.7	$ 262.4
Cash flow hedge:
Amortization of loss included in interest expense	1.4	1.4
Income tax effect	(0.4)	(0.6)
Other comprehensive income	1.0	0.8
Defined benefit pension plans and postemployment arrangements:
Amortization of prior service cost included in periodic benefit expense	2.0	2.0
Amortization of actuarial losses included in periodic benefit expense	2.0	1.9
Income tax effect	(1.1)	(1.8)
Other comprehensive income	2.9	2.1
Available-for-sale securities:
Unrealized gain for the period	0.0	0.2
Income tax effect	0.0	(0.1)
Reclassification	0.3	0.0
Other comprehensive income	0.3	0.1
Foreign currency translation adjustment	88.7	114.7
Other Comprehensive Income	92.9	117.7
Comprehensive Income	377.6	380.1
Comprehensive Income Attributed To Noncontrolling Interests	25.6	35.4
Comprehensive Income - Omnicom Group Inc.	$ 352.0	$ 344.7