Derivative Instruments and Hedging Activities (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Long-Term Debt, fixed rate, percentage	50.00%
Long-Term Debt, floating rate, percentage	50.00%
Forward Foreign Exchange Contracts [Member]
Forward foreign exchange contracts, terms	The terms of our forward contracts are generally less than 90 days.
Foreign Exchange Risk of Intercompany Borrowing and Investment Activities [Member] \| Fair Value Hedge [Member] \| Designated as Hedging Instrument [Member] \| Forward Foreign Exchange Contracts [Member]
Forward foreign exchange contracts, notional amount	113,100,000	$ 207,000,000
Foreign Exchange Risk of Revenue and Expenses Denominated in Different Currencies [Member] \| Fair Value Hedge [Member] \| Designated as Hedging Instrument [Member] \| Forward Foreign Exchange Contracts [Member]
Forward foreign exchange contracts, notional amount	44,000,000	56,200,000
Interest Rate Swaps on 2022 Notes [Member]
Interest rate swaps, hedged amount	1,250,000,000
Interest Rate Swaps on 2020 Notes [Member]
Interest rate swaps, hedged amount	1,000,000,000
LIBOR [Member] \| Interest Rate Swaps on 2022 Notes [Member]
Interest rate swaps, spread	1.05%
LIBOR [Member] \| Interest Rate Swaps on 2020 Notes [Member]
Interest rate swaps, spread	2.16%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of the hedged item as of the balance sheet date related to the derivative. For example, the hedged balance on a debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAmountOfHedgedItem
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the relevant terms of the derivative. Includes the type of instrument, risk being hedged, notional amount, counterparty, inception date, maturity date, relevant interest rates, strike price, cap price, and floor price.

+ References

No definition available.

+ Details

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

- Definition

The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtPercentageBearingFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

+ References

+ Details

Name:	us-gaap_LongTermDebtPercentageBearingVariableInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	omc_DerivativeUnderlyingRiskMitigatedAxis=omc_ForeignExchangeRiskofIntercompanyBorrowingandInvestmentActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	omc_DerivativeUnderlyingRiskMitigatedAxis=omc_ForeignExchangeRiskofRevenueandExpensesDenominatedinDifferentCurrenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=omc_InterestRateSwapson2020NotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=omc_InterestRateSwapson2022NotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: