Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net Income	$ 286.9	$ 278.2	$ 904.4	$ 866.3
Cash flow hedge:
Loss for the period	0.0	0.0	0.0	(48.8)
Amortization of loss included in interest expense	1.4	1.4	4.2	2.6
Income tax effect	(0.6)	(0.6)	(1.8)	19.2
Other comprehensive income	0.8	0.8	2.4	(27.0)
Defined benefit pension plans and postemployment arrangements:
Amortization of prior service cost included in periodic benefit expense	1.9	1.8	6.1	5.5
Amortization of actuarial losses included in periodic benefit expense	2.0	1.4	5.6	4.4
Income tax effect	(1.4)	(1.3)	(5.1)	(3.9)
Other comprehensive income	2.5	1.9	6.6	6.0
Available-for-sale securities:
Unrealized gain for the period	0.0	0.1	0.5	0.1
Income tax effect	0.0	0.0	(0.2)	0.0
Other comprehensive income	0.0	0.1	0.3	0.1
Foreign currency translation adjustment	159.6	(31.1)	414.4	(36.2)
Other Comprehensive Income	162.9	(28.3)	423.7	(57.1)
Comprehensive Income	449.8	249.9	1,328.1	809.2
Comprehensive Income Attributed To Noncontrolling Interests	32.6	24.3	96.5	85.2
Comprehensive Income - Omnicom Group Inc.	$ 417.2	$ 225.6	$ 1,231.6	$ 724.0