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Debt (Estimated Fair Value Of Significant Financial Instruments) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt $ 11mmc_LongTermDebtandCapitalLeaseObligationCurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 $ 334mmc_LongTermDebtandCapitalLeaseObligationCurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Long-term debt 3,493mmc_LongTermDebtandCapitalLeaseObligationNoncurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 2,819mmc_LongTermDebtandCapitalLeaseObligationNoncurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt 11mmc_LongTermDebtandCapitalLeaseObligationCurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 334mmc_LongTermDebtandCapitalLeaseObligationCurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Long-term debt $ 3,376mmc_LongTermDebtandCapitalLeaseObligationNoncurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 $ 2,621mmc_LongTermDebtandCapitalLeaseObligationNoncurrentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember