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Income Taxes (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Disclosure [Abstract]        
Investments in foreign subsidiaries $ 6,300,000,000us-gaap_UndistributedEarningsOfForeignSubsidiaries      
Effective tax rate 28.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations 30.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations 29.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Valuation allowances 15,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 10,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 23,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount  
Valuation allowances, beginning balance (9,000,000)mmc_DeferredTaxAssetsValuationAllowanceAssumptionChangesEffectOnIncomeTaxExpenseBenefit (3,000,000)mmc_DeferredTaxAssetsValuationAllowanceAssumptionChangesEffectOnIncomeTaxExpenseBenefit 16,000,000mmc_DeferredTaxAssetsValuationAllowanceAssumptionChangesEffectOnIncomeTaxExpenseBenefit  
Net Operating Loss Carryforwards 55.00%mmc_OperatingLossCarryforwardsPercentageSubjecttoExpiration      
Unrecognized tax benefits that would impact effective tax rate 51,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 71,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 96,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
Accrued interest and penalties 7,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 10,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 13,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Unrecognized tax benefits minimum 0us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeLowerBound      
Unrecognized tax benefits maximum 16,000,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound      
Tax Credit Carryforward [Line Items]        
Putnam and issues included in consolidated mmc tax returns 97,000,000us-gaap_UnrecognizedTaxBenefits 128,000,000us-gaap_UnrecognizedTaxBenefits 117,000,000us-gaap_UnrecognizedTaxBenefits 143,000,000us-gaap_UnrecognizedTaxBenefits
Federal [Member]        
Tax Credit Carryforward [Line Items]        
Operating loss carryforwards tax benefits 3,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
     
State and Local [Member]        
Tax Credit Carryforward [Line Items]        
Operating loss carryforwards tax benefits 40,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
     
Non-U.S. [Member]        
Tax Credit Carryforward [Line Items]        
Operating loss carryforwards tax benefits 96,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
     
Putnam and Kroll [Member]        
Tax Credit Carryforward [Line Items]        
Putnam and issues included in consolidated mmc tax returns $ 2,000,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mmc_PutnamAndKrollMember
$ 2,000,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mmc_PutnamAndKrollMember
$ 6,000,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mmc_PutnamAndKrollMember