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Supplemental Disclosures to the Consolidated Statements of Cash Flows (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Non-cash issuance of common stock $ 64us-gaap_StockIssued1 $ 92us-gaap_StockIssued1
Stock-based compensation expense, equity awards 16us-gaap_AllocatedShareBasedCompensationExpense 26us-gaap_AllocatedShareBasedCompensationExpense
Prior Fiscal Periods Acquisitions [Member]    
Deferred purchase consideration from prior years' acquisitions 26mmc_PaymentsForPreviousAcquisitionDeferredConsideration
/ us-gaap_BusinessAcquisitionAxis
= mmc_PriorFiscalPeriodsAcquisitionsMember		 6mmc_PaymentsForPreviousAcquisitionDeferredConsideration
/ us-gaap_BusinessAcquisitionAxis
= mmc_PriorFiscalPeriodsAcquisitionsMember
Putnam [Member]    
Reduction of net cash flow for operations by discontinued operations $ 82mmc_DiscontinuedOperationIndemnificationPaymentRelatedtoDivestitureofBusiness
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mmc_PutnamMember