Operating Expenses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating Expenses [Abstract]
Salary and service costs	$ 2,620.8omc_SalaryAndServiceCosts	$ 2,622.8omc_SalaryAndServiceCosts
Office and general expenses	470.7omc_OfficeAndGeneralExpenses	496.7omc_OfficeAndGeneralExpenses
Operating expenses	3,091.5us-gaap_OperatingExpenses	3,119.5us-gaap_OperatingExpenses
Merger expenses		$ 7.0

Office and general expenses are primarily comprised of rent and occupancy costs, technology related costs and depreciation and amortization.

No definition available.

+ Details

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Namespace Prefix:	omc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Salary and service costs are primarily comprised of employee compensation related costs.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 23

-URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e1043-128460

+ Details

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- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Details

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