Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 1,192.2	$ 1,246.7	$ 1,203.4
Cash flow hedge:
Loss for the period	0.0	(48.9)	(5.6)
Amortization of loss included in interest expense	5.3	4.0	0.0
Income tax effect	(2.1)	18.7	2.3
Other comprehensive income (loss)	3.2	(26.2)	(3.3)
Defined benefit pension plans and postemployment arrangements:
Unrecognized actuarial losses and prior service cost for the period	(12.2)	(18.3)	(7.8)
Amortization of prior service cost and actuarial losses included in periodic benefit expense	16.1	14.0	14.8
Income tax effect	(1.7)	1.6	(2.8)
Other comprehensive income (loss)	2.2	(2.7)	4.2
Available-for-sale securities:
Unrealized gain for the period	0.8	0.2	0.4
Income tax effect	(0.3)	(0.1)	(0.1)
Other comprehensive income (loss)	0.5	0.1	0.3
Foreign currency translation adjustment	412.7	(319.4)	(427.2)
Other Comprehensive Income (Loss)	418.6	(348.2)	(426.0)
Comprehensive Income	1,610.8	898.5	777.4
Comprehensive Income Attributed To Noncontrolling Interests	129.4	90.5	80.7
Comprehensive Income - Omnicom Group Inc.	$ 1,481.4	$ 808.0	$ 696.7