Operating Expenses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating Expenses [Abstract]
Salary and service costs	$ 2,813.2	$ 2,585.2	$ 8,230.7	$ 7,733.3
Office and general expenses	502.8	497.8	1,518.5	1,490.8
Merger expenses	0	28.1	8.8	28.1
Operating expenses	$ 3,316.0	$ 3,111.1	$ 9,758.0	$ 9,252.2

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

No definition available.

+ Details

Name:	omc_OfficeAndGeneralExpenses
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

Name:	omc_SalaryAndServiceCosts
Namespace Prefix:	omc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Name:	us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	us-gaap_OperatingExpenses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_OperatingExpensesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration