Pension and Other Postemployment Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Contribution Plans [Abstract]
Defined contribution plans, contribution expense	$ 113.5	$ 102.1	$ 83.9
Level 3
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	1.7	6.1
Actual return on plan assets	0.1	0.1
Purchases, sales and settlements, net	1.1	(4.5)
Fair value of plan assets December 31	2.9	1.7
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 cover approximately 6,900 participants, are not covered by ERISA and include plans required by local law.
Components of Net Periodic Benefit Cost [Abstract]
Service cost	6.6	5.8	3.9
Interest cost	7.6	6.5	6.1
Expected return on plan assets	(3.5)	(3.5)	(3.5)
Amortization of prior service cost	3.1	3.1	2.5
Amortization of actuarial (gains) losses	1.0	0.3	0.6
Curtailments and settlements	0	0	1.2
Net periodic benefit cost	14.8	12.2	10.8
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	90.3	66.2
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	55.5	41.2
Unrecognized actuarial gains and losses expected to be amortized in the next year	4.6
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Cost [Abstract]
Discount rate	4.60%	5.10%	5.60%
Compensation increases	1.80%	1.60%	2.30%
Expected return on plan assets	6.00%	6.20%	6.70%
Defined benefit pension plans, contributions by employer	9.1	7.8	7.5
Benefit Obligation [Rollforward]
Benefit obligation January 1	146.3	130.0
Service cost	6.6	5.8	3.9
Interest cost	7.6	6.5	6.1
Amendments, curtailments, and settlements	0	(13.6)
Actuarial (gains) losses	30.5	23.1
Benefits paid	(4.7)	(5.1)
Foreign currency translation and other	2.1	(0.4)
Benefit obligation December 31	188.4	146.3	130.0
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	50.5	53.9
Actual return on plan assets	6.5	(0.6)
Employer contributions	9.1	7.8	7.5
Benefits paid	(4.7)	(5.1)
Settlements	0	(5.4)
Foreign currency translation and other	1.3	(0.1)
Fair value of plan assets December 31	62.7	50.5	53.9
Funded Status [Abstract]
Funded Status December 31	(125.7)	(95.8)
Amounts Recorded in Balance Sheet [Abstract]
Other assets	3.5	1.9
Other current liabilities	(1.4)	(1.3)
Long-term liabilities	(127.8)	(96.4)
Total amount recognized in balance sheet	(125.7)	(95.8)
Accumulated benefit obligation	176.5	135.8
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Benefit obligation	168.8	136.4
Plan assets	39.6	38.7
Plans with benefit obligations in excess of plan assets, Liabilities	129.2	97.7
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.70%	4.60%
Compensation increases	1.70%	1.40%
Estimated Future Benefit Payments [Abstract]
2013	4.8
2014	6.2
2015	5.9
2016	6.6
2017	7.3
2018 - 2022	54.6
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]
Fair value of plan assets December 31	59.7	48.6
Defined Benefit Pension Plans [Member] | Level 2
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	0.1	0.2
Defined Benefit Pension Plans [Member] | Level 3
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	2.9	1.7
Defined Benefit Pension Plans [Member] | Cash and Cash Equivalents [Member]
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	1.6
Fair value of plan assets December 31	2.3
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	2.00%
Percentage of Plan Assets, Actual Allocation	3.00%	3.00%
Defined Benefit Pension Plans [Member] | Cash and Cash Equivalents [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	2.3	1.6
Defined Benefit Pension Plans [Member] | Mutual Funds [Member]
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	29.7
Fair value of plan assets December 31	36.8
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	56.00%
Percentage of Plan Assets, Actual Allocation	59.00%	59.00%
Defined Benefit Pension Plans [Member] | Mutual Funds [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	36.8	29.7
Defined Benefit Pension Plans [Member] | Unit Trusts [Member]
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	17.3
Fair value of plan assets December 31	20.6
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	35.00%
Percentage of Plan Assets, Actual Allocation	33.00%	34.00%
Defined Benefit Pension Plans [Member] | Unit Trusts [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	20.6	17.3
Defined Benefit Pension Plans [Member] | Insurance Contracts [Member]
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	1.7
Fair value of plan assets December 31	2.9
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	5.00%
Percentage of Plan Assets, Actual Allocation	5.00%	3.00%
Defined Benefit Pension Plans [Member] | Insurance Contracts [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31
Defined Benefit Pension Plans [Member] | Insurance Contracts [Member] | Level 3
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	2.9	1.7
Defined Benefit Pension Plans [Member] | Other Plan Assets [Member]
Fair Value of Plan Assets [Rollforward]
Fair value of assets January 1	0.2
Fair value of plan assets December 31	0.1
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	2.00%
Percentage of Plan Assets, Actual Allocation	0.00%	1.00%
Defined Benefit Pension Plans [Member] | Other Plan Assets [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31
Defined Benefit Pension Plans [Member] | Other Plan Assets [Member] | Level 2
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31	0.1	0.2
Defined Benefit Pension Plans [Member] | Other Plan Assets [Member] | Level 3
Fair Value of Plan Assets [Rollforward]
Fair value of plan assets December 31
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 not to exceed 2.5% per year. The Retention Plan is unfunded and benefits will be 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	4.1	3.9	1.8
Interest cost	4.7	4.7	3.9
Amortization of prior service cost	0.7	0.6	0.6
Amortization of actuarial (gains) losses	2.0	2.1	1.0
Net periodic benefit cost	11.5	11.3	7.3
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	57.2	44.9
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	34.3	26.9
Unrecognized actuarial gains and losses expected to be amortized in the next year	3.8
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Cost [Abstract]
Discount rate	4.90%	5.00%	5.00%
Compensation increases	3.50%	3.50%	3.50%
Benefit Obligation [Rollforward]
Benefit obligation January 1	105.2	104.7
Service cost	4.1	3.9	1.8
Interest cost	4.7	4.7	3.9
Actuarial (gains) losses	(15.0)	(2.6)
Benefits paid	(10.3)	(10.7)
Benefit obligation December 31	118.7	105.2	104.7
Fair Value of Plan Assets [Rollforward]
Benefits paid	(10.3)	(10.7)
Amounts Recorded in Balance Sheet [Abstract]
Other current liabilities	(9.9)	(10.2)
Long-term liabilities	(108.8)	(95.0)
Total amount recognized in balance sheet	(118.7)	(105.2)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.70%	4.90%
Compensation increases	3.50%	3.50%
Estimated Future Benefit Payments [Abstract]
2013	9.9
2014	9.5
2015	8.2
2016	8.4
2017	7.4
2018 - 2022	$ 28.5