Pension and Other Postemployment Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plans [Abstract]
Defined contribution plans, contribution expense	$ 115.2	$ 118.8	$ 112.9
Defined Benefit Pension Plans [Member]
Components of Net Periodic Benefit Expense
Service cost	7.9	7.7	10.1
Interest cost	8.2	7.9	7.9
Expected return on plan assets	(3.5)	(2.8)	(3.6)
Amortization of prior service cost	0.8	4.4	4.6
Amortization of actuarial losses	2.5	6.9	7.0
Net periodic benefit expense	15.9	24.1	$ 26.0
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	93.9	70.0
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	58.3	$ 42.0
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ 7.3
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	2.90%	3.60%	3.50%
Compensation increases	2.50%	2.50%	2.00%
Expected return on plan assets	5.50%	5.80%	5.30%
Defined benefit pension plans, contributions by employer	$ 7.1	$ 8.0
Benefit Obligation [Rollforward]
January 1	258.4	277.0
Service cost	7.9	7.7	$ 10.1
Interest cost	8.2	7.9	7.9
Amendments, curtailments and settlements	1.4	0.1
Actuarial losses (gains)	28.8	(16.0)
Benefits paid	(9.2)	(23.1)
Foreign currency translation	(2.0)	4.8
December 31	293.5	258.4	277.0
Fair Value of Plan Assets [Rollforward]
January 1	57.4	80.3
Actual return on plan assets	8.7	(4.7)
Employer contributions	7.1	8.0
Benefits paid	(9.2)	(23.1)
Foreign currency translation and other	0.3	(3.1)
December 31	64.3	57.4	80.3
Funded Status [Abstract]
Funded Status	(229.2)	(201.0)
Amounts Recorded in Balance Sheet, as Classified [Abstract]
Other assets	2.8	2.5
Other current liabilities	(5.1)	(5.1)
Long-term liabilities	(226.9)	(198.4)
Amounts Recorded in Balance Sheet	(229.2)	(201.0)
Accumulated benefit obligation	268.9	219.9
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Benefit obligation	(280.7)	(247.0)
Plan assets	48.3	43.5
Plans with benefit obligations in excess of plan assets, liabilities	$ (232.4)	$ (203.5)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	2.80%	3.60%
Compensation increases	2.70%	2.70%
Estimated Future Benefit Payments [Abstract]
2020	$ 10.4
2021	14.2
2022	15.6
2023	17.0
2024	23.6
2025 - 2029	$ 129.3
Postemployment Arrangements [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	We have executive retirement agreements under which benefits will be paid to participants or to their beneficiaries over periods up to ten years beginning after cessation of full-time employment. Our postemployment arrangements are unfunded and benefits are paid when due.
Components of Net Periodic Benefit Expense
Service cost	$ 4.4	$ 4.7	4.4
Interest cost	4.3	3.6	3.7
Amortization of prior service cost	4.5	3.5	3.5
Amortization of actuarial losses	0.9	1.8	1.0
Net periodic benefit expense	14.1	13.6	$ 12.6
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	60.8	48.0
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	37.2	$ 28.0
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ 6.4
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	2.90%	3.40%	3.90%
Compensation increases	3.50%	3.50%	3.50%
Benefit Obligation [Rollforward]
January 1	$ 126.5	$ 127.7
Service cost	4.4	4.7	$ 4.4
Interest cost	4.3	3.6	3.7
Amendments, curtailments and settlements	4.5	6.5
Actuarial losses (gains)	14.1	(7.9)
Benefits paid	(7.8)	(8.1)
December 31	146.0	126.5	$ 127.7
Amounts Recorded in Balance Sheet, as Classified [Abstract]
Other current liabilities	(8.6)	(7.7)
Long-term liabilities	(137.4)	(118.8)
Amounts Recorded in Balance Sheet	$ (146.0)	$ (126.5)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	2.90%	4.00%
Compensation increases	3.50%	3.50%
Estimated Future Benefit Payments [Abstract]
2020	$ 8.7
2021	9.0
2022	9.6
2023	8.9
2024	8.8
2025 - 2029	$ 46.0
Retention Plan [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	We have a Senior Executive Restrictive Covenant and Retention Plan, or Retention Plan, for certain executive officers selected by the Compensation Committee. The Retention Plan is a non-qualified deferred compensation severance plan that was adopted to secure non-competition, non-solicitation, non-disparagement and ongoing consulting services from such executive officers and to strengthen the retention aspect of executive officer compensation. The Retention Plan provides annual payments upon termination following at least seven years of service with Omnicom or its subsidiaries to the participants or to their beneficiaries. A participant’s annual benefit is payable for 15 consecutive calendar years following termination, but in no event prior to age 55. The annual benefit is equal to the lesser of (i) the participant’s final average pay times an applicable percentage, which is based upon the executive’s years of service as an executive officer, not to exceed 35% or (ii) $1.5 million adjusted for cost-of-living, beginning with the second annual payment, not to exceed 2.5% per year. The Retention Plan is not funded and benefits are paid when due.