Pension and Other Postemployment Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined contribution plans, contribution expense	$ 87.9	$ 127.9	$ 123.2
Defined Benefit Pension Plans [Member]
Components of Net Periodic Benefit Expense
Service cost	1.6	2.4	2.8
Interest cost	8.5	11.1	5.6
Expected return on plan assets	(2.2)	(0.3)	(1.4)
Amortization of prior service cost	0.4	0.3	0.4
Amortization of actuarial loss	0.8	0.7	4.0
Net periodic benefit expense	9.1	14.2	$ 11.4
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	6.5	15.9
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	4.5	$ 11.0
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ (0.4)
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	4.50%	4.70%	2.10%
Compensation increases	3.40%	2.60%	2.60%
Expected return on plan assets	2.20%	1.50%	1.60%
Defined benefit pension plans, contributions by employer	$ 10.6	$ 8.8
Benefit obligation:
Benefit obligation, January 1	224.3	228.6
Service cost	1.6	2.4	$ 2.8
Interest cost	8.5	11.1	5.6
Amendments, curtailments and settlements	16.1	(0.4)
Actuarial (gain) loss	(23.1)	(2.1)
Benefits paid	(11.7)	(12.4)
Foreign currency translation	0.3	(2.9)
Benefit obligation, December 31	216.0	224.3	228.6
Fair value of plan assets:
Fair value of plan assets, January 1	45.5	44.0
Actual return on plan assets	2.0	3.9
Employer contributions	10.6	8.8
Benefits paid	(11.7)	(12.4)
Foreign currency translation and other	(0.5)	1.2
Fair value of plan assets, December 31	45.9	45.5	44.0
Funded Status [Abstract]
Funded status, December 31	(170.1)	(178.8)
Funded status recognized in the balance sheet:
Other assets	1.5	2.1
Other current liabilities	(10.7)	(8.6)
Long-term liabilities	(160.9)	(172.3)
Amounts Recorded in Balance Sheet	$ (170.1)	$ (178.8)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	5.20%	4.60%
Compensation increases	2.50%	3.50%
Accumulated benefit obligation	$ 169.1	$ 179.5
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Benefit obligation	(208.3)	(215.5)
Plan assets	36.7	34.7
Plans with benefit obligations in excess of plan assets	(171.6)	(180.8)
Estimated Future Benefit Payments [Abstract]
2025	12.3
2026	11.8
2027	12.6
2028	16.0
2029	22.2
2030 - 2034	102.0
Postemployment Arrangements [Member]
Components of Net Periodic Benefit Expense
Service cost	2.9	3.4	4.5
Interest cost	5.9	5.7	2.6
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost	4.4	3.8	3.8
Amortization of actuarial loss	0.2	0.0	2.5
Net periodic benefit expense	13.4	12.9	$ 13.4
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	25.3	41.1
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	17.6	$ 28.8
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ 3.7
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	4.50%	4.70%	1.80%
Compensation increases	3.50%	3.50%	3.50%
Defined benefit pension plans, contributions by employer	$ 0.0	$ 0.0
Benefit obligation:
Benefit obligation, January 1	142.2	130.8
Service cost	2.9	3.4	$ 4.5
Interest cost	5.9	5.7	2.6
Amendments, curtailments and settlements	(0.6)	6.9
Actuarial (gain) loss	(12.4)	6.3
Benefits paid	(11.4)	(10.9)
Foreign currency translation	0.0	0.0
Benefit obligation, December 31	126.6	142.2	130.8
Fair value of plan assets:
Fair value of plan assets, January 1	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	0.0	0.0
Benefits paid	0.0	0.0
Foreign currency translation and other	0.0	0.0
Fair value of plan assets, December 31	0.0	0.0	$ 0.0
Funded Status [Abstract]
Funded status, December 31	(126.6)	(142.2)
Funded status recognized in the balance sheet:
Other assets	0.0	0.0
Other current liabilities	(13.5)	(11.9)
Long-term liabilities	(113.1)	(130.3)
Amounts Recorded in Balance Sheet	$ (126.6)	$ (142.2)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	5.30%	4.60%
Compensation increases	3.50%	3.50%
Estimated Future Benefit Payments [Abstract]
2025	$ 13.5
2026	12.2
2027	13.7
2028	13.4
2029	12.7
2030 - 2034	$ 48.0
Senior Executive Retention Plan [Member]
Defined benefit plans, plan information	The Senior Executive Retention Plan provides annual payments to the participants or to their beneficiaries upon termination following at least seven years of service with Omnicom or its subsidiaries. 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 generally 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, not to exceed 2.5% per year. The Senior Executive Retention Plan is not funded, and benefits are paid when due.
Key Executive Retention Plan [Member]
Defined benefit plans, plan information	The Key Executive Retention Plan provides annual payments to the participants or to their beneficiaries upon termination following at least six years of service from the date of the participant’s award agreement with Omnicom or its subsidiaries. A participant’s annual benefit is payable for 12 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 employee’s years of service, not to exceed 65% or (ii) $1.0 million. The annual benefit vests 100% after six years of service from the date of the award agreement. The Key Executive Retention Plan is not funded, and benefits are paid when due.