Pension and Other Postemployment Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Contribution Plans [Abstract]
Defined contribution plans, contribution expense	$ 108.4	$ 109.3	$ 113.5
Defined Benefit Pension Plans [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	Certain of our subsidiaries sponsor noncontributory defined benefit pension plans. Two of our U.S. businesses and several of our non-U.S. businesses sponsor pension plans. These plans provide benefits to employees based on formulas recognizing length of service and earnings. The U.S. plans cover approximately 1,200 participants, are closed to new participants and do not accrue future benefit credits. The non-U.S. plans, which include plans required by local law, cover approximately 6,500 participants and are not covered by ERISA.
Components of Net Periodic Benefit Cost [Abstract]
Service cost	6.8	4.6	6.6
Interest cost	8.2	7.3	7.6
Expected return on plan assets	(4.2)	(3.7)	(3.5)
Amortization of prior service cost	4.3	3.6	3.1
Amortization of actuarial (gains) losses	2.3	3.5	1.0
Net periodic benefit cost	17.4	15.3	14.8
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	91.0	68.6
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	56.0	42.3
Unrecognized actuarial gains and losses expected to be amortized in the next year	10.1
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Cost [Abstract]
Discount rate	4.50%	3.90%	4.60%
Compensation increases	1.80%	1.60%	1.80%
Expected return on plan assets	5.80%	5.30%	6.00%
Defined benefit pension plans, contributions by employer	3.2	5.5	9.1
Benefit Obligation [Rollforward]
January 1	185.7	188.4
Service cost	6.8	4.6	6.6
Interest cost	8.2	7.3	7.6
Amendments, curtailments and settlements	0	7.6
Actuarial (gains) losses	31.3	(18.0)
Benefits paid	(5.0)	(5.1)
Foreign currency translation and other	(4.3)	0.9
December 31	222.7	185.7	188.4
Fair Value of Plan Assets [Rollforward]
January 1	72.2	62.7
Actual return on plan assets	6.5	8.1
Employer contributions	3.2	5.5	9.1
Benefits paid	(5.0)	(5.1)
Foreign currency translation and other	(2.5)	1.0
December 31	74.4	72.2	62.7
Funded Status [Abstract]
Funded Status December 31:	(148.3)	(113.5)
Amounts Recorded in Balance Sheet [Abstract]
Other assets	4.6	5.6
Other current liabilities	(4.4)	(1.1)
Long-term liabilities	(148.5)	(118.0)
Total amount recognized in balance sheet	(148.3)	(113.5)
Accumulated benefit obligation	210.0	173.0
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Benefit obligation	212.0	143.1
Plan assets	59.0	24.0
Plans with benefit obligations in excess of plan assets, Liabilities	153.0	119.1
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.50%	4.40%
Compensation increases	1.80%	1.60%
Estimated Future Benefit Payments [Abstract]
2015	7.6
2016	8.2
2017	9.0
2018	9.3
2019	10.4
2020 - 2024	72.0
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	100.00%
Percentage of Plan Assets, Actual Allocation	100.00%	100.00%
Defined Benefit Pension Plans [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
December 31	70.4	67.9
Defined Benefit Pension Plans [Member] | Level 2
Fair Value of Plan Assets [Rollforward]
December 31	0.3	0.9
Defined Benefit Pension Plans [Member] | Level 3
Fair Value of Plan Assets [Rollforward]
January 1	3.4	2.9
Actual return on plan assets	0.1	0.1
Purchases, sales and settlements, net	0.2	0.4
December 31	3.7	3.4
Defined Benefit Pension Plans [Member] | Cash
Fair Value of Plan Assets [Rollforward]
January 1	2.1
December 31	2.6
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	5.00%
Percentage of Plan Assets, Actual Allocation	3.00%	3.00%
Defined Benefit Pension Plans [Member] | Cash | Level 1
Fair Value of Plan Assets [Rollforward]
December 31	2.6	2.1
Defined Benefit Pension Plans [Member] | Mutual funds
Fair Value of Plan Assets [Rollforward]
January 1	42.5
December 31	43.4
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	54.00%
Percentage of Plan Assets, Actual Allocation	58.00%	59.00%
Defined Benefit Pension Plans [Member] | Mutual funds | Level 1
Fair Value of Plan Assets [Rollforward]
December 31	43.4	42.5
Defined Benefit Pension Plans [Member] | Unit trusts
Fair Value of Plan Assets [Rollforward]
January 1	23.3
December 31	24.4
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	33.00%
Percentage of Plan Assets, Actual Allocation	33.00%	32.00%
Defined Benefit Pension Plans [Member] | Unit trusts | Level 1
Fair Value of Plan Assets [Rollforward]
December 31	24.4	23.3
Defined Benefit Pension Plans [Member] | Insurance contracts
Fair Value of Plan Assets [Rollforward]
January 1	3.4
December 31	3.7
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	5.00%
Percentage of Plan Assets, Actual Allocation	5.00%	5.00%
Defined Benefit Pension Plans [Member] | Insurance contracts | Level 3
Fair Value of Plan Assets [Rollforward]
December 31	3.7	3.4
Defined Benefit Pension Plans [Member] | Other
Fair Value of Plan Assets [Rollforward]
January 1	0.9
December 31	0.3
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	3.00%
Percentage of Plan Assets, Actual Allocation	1.00%	1.00%
Defined Benefit Pension Plans [Member] | Other | Level 2
Fair Value of Plan Assets [Rollforward]
December 31	0.3	0.9
Retention Plan [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	In addition, we have a Senior Executive Restrictive Covenant and Retention Plan (“Retention Plan”) for certain executive officers of Omnicom selected to participate 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 unfunded and benefits are paid when due.
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 Cost [Abstract]
Service cost	3.8	4.1	4.1
Interest cost	4.5	4.3	4.7
Amortization of prior service cost	2.1	1.9	0.7
Amortization of actuarial (gains) losses	0.9	1.5	2.0
Net periodic benefit cost	11.3	11.8	11.5
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	60.0	44.2
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	36.0	26.5
Unrecognized actuarial gains and losses expected to be amortized in the next year	4.8
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Cost [Abstract]
Discount rate	4.70%	4.10%	4.90%
Compensation increases	3.50%	3.50%	3.50%
Benefit Obligation [Rollforward]
January 1	104.2	118.7
Service cost	3.8	4.1	4.1
Interest cost	4.5	4.3	4.7
Amendments, curtailments and settlements	7.5	2.8
Actuarial (gains) losses	11.3	(12.3)
Benefits paid	(9.2)	(13.4)
December 31	122.1	104.2	118.7
Fair Value of Plan Assets [Rollforward]
Benefits paid	(9.2)	(13.4)
Amounts Recorded in Balance Sheet [Abstract]
Other current liabilities	(8.7)	(9.3)
Long-term liabilities	(113.4)	(94.9)
Total amount recognized in balance sheet	(122.1)	(104.2)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.80%	4.70%
Compensation increases	3.50%	3.50%
Estimated Future Benefit Payments [Abstract]
2015	8.7
2016	9.0
2017	7.6
2018	7.4
2019	5.7
2020 - 2024	$ 26.9