Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ 371.6	$ (14.4)	$ 677.5	$ 257.4
Cash flow hedge:
Amortization of loss included in interest expense	1.4	1.4	2.8	2.8
Income tax effect	(0.4)	(0.4)	(0.8)	(0.8)
Other comprehensive income (loss), Cash flow hedge	1.0	1.0	2.0	2.0
Defined benefit pension plans and postemployment arrangements:
Amortization of prior service cost	1.3	1.2	2.5	2.5
Amortization of actuarial losses	3.2	1.9	6.5	3.8
Income tax effect	(1.6)	(1.0)	(3.4)	(2.0)
Other comprehensive income (loss), Defined benefit pension plans and postemployment arrangements	2.9	2.1	5.6	4.3
Foreign currency translation adjustment	58.1	82.8	11.2	(288.4)
Other Comprehensive Income (Loss)	62.0	85.9	18.8	(282.1)
Comprehensive Income (Loss)	433.6	71.5	696.3	(24.7)
Comprehensive Income (Loss) Attributed To Noncontrolling Interests	26.7	24.0	34.0	0.3
Comprehensive Income (Loss) - Omnicom Group Inc.	$ 406.9	$ 47.5	$ 662.3	$ (25.0)