Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified
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Dec. 31, 2014
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Dec. 31, 2013
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Financial instruments owned: |
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Total assets measured at fair value |
$ 257us-gaap_AssetsFairValueDisclosure |
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$ 202us-gaap_AssetsFairValueDisclosure |
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Fiduciary Assets: |
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Fiduciary assets |
57mmc_FiduciaryAssetsFairValueDisclosure |
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0mmc_FiduciaryAssetsFairValueDisclosure |
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Liabilities: |
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Total liabilities measured at fair value |
207us-gaap_LiabilitiesFairValueDisclosure |
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357us-gaap_LiabilitiesFairValueDisclosure |
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Identical Assets (Level 1) [Member] |
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Financial instruments owned: |
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Total assets measured at fair value |
257us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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199us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Fiduciary Assets: |
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Fiduciary assets |
57mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Liabilities: |
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Total liabilities measured at fair value |
0us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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0us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Observable Inputs (Level 2) [Member] |
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Financial instruments owned: |
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Total assets measured at fair value |
0us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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3us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Fiduciary Assets: |
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Fiduciary assets |
0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Liabilities: |
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Total liabilities measured at fair value |
0us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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253us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Unobservable Inputs (Level 3) [Member] |
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Fiduciary Assets: |
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Fiduciary assets |
0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Liabilities: |
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Total liabilities measured at fair value |
207us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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104us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Other Assets [Member] |
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Financial instruments owned: |
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Mutual funds |
150us-gaap_OtherAssetsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[1] |
154us-gaap_OtherAssetsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[1] |
Other Assets [Member] | Identical Assets (Level 1) [Member] |
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Financial instruments owned: |
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Mutual funds |
150us-gaap_OtherAssetsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[1] |
154us-gaap_OtherAssetsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[1] |
Cash and Cash Equivalents [Member] |
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Financial instruments owned: |
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Money market funds |
107us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[2] |
45us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[2] |
Cash and Cash Equivalents [Member] | Identical Assets (Level 1) [Member] |
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Financial instruments owned: |
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Money market funds |
107us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[2] |
45us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[2] |
Other Receivables [Member] |
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Financial instruments owned: |
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Interest rate swap derivatives |
0us-gaap_InterestRateFairValueHedgeAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mmc_OtherReceivablesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[3] |
3us-gaap_InterestRateFairValueHedgeAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mmc_OtherReceivablesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
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Other Receivables [Member] | Observable Inputs (Level 2) [Member] |
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Financial instruments owned: |
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Interest rate swap derivatives |
0us-gaap_InterestRateFairValueHedgeAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mmc_OtherReceivablesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[3] |
3us-gaap_InterestRateFairValueHedgeAssetAtFairValue / us-gaap_BalanceSheetLocationAxis = mmc_OtherReceivablesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[3] |
Accounts Payable and Accrued Liabilities and Other Liabilities [Member] |
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Liabilities: |
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Contingent purchase consideration liability |
207mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[4] |
104mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[4] |
Accounts Payable and Accrued Liabilities and Other Liabilities [Member] | Unobservable Inputs (Level 3) [Member] |
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Liabilities: |
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Contingent purchase consideration liability |
207mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[4] |
104mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure / us-gaap_BalanceSheetLocationAxis = mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[4] |
Money Market Funds [Member] |
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Fiduciary Assets: |
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Fiduciary assets |
57mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MoneyMarketFundsMember |
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0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MoneyMarketFundsMember |
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Money Market Funds [Member] | Identical Assets (Level 1) [Member] |
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Fiduciary Assets: |
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Fiduciary assets |
57mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MoneyMarketFundsMember |
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0mmc_FiduciaryAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MoneyMarketFundsMember |
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Senior Debt Obligations Due 2014 [Member] | Long-term Debt [Member] |
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Liabilities: |
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Senior Notes due 2014 |
0us-gaap_DebtInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DebtInstrumentAxis = mmc_SeniorDebtObligationsDue2014Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[5] |
253us-gaap_DebtInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DebtInstrumentAxis = mmc_SeniorDebtObligationsDue2014Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[5] |
Senior Debt Obligations Due 2014 [Member] | Long-term Debt [Member] | Observable Inputs (Level 2) [Member] |
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Liabilities: |
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Senior Notes due 2014 |
$ 0us-gaap_DebtInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DebtInstrumentAxis = mmc_SeniorDebtObligationsDue2014Member / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[5] |
$ 253us-gaap_DebtInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DebtInstrumentAxis = mmc_SeniorDebtObligationsDue2014Member / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |
[5] |
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