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Stock Benefit Plans (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
May 31, 2007
Jul. 31, 2002
May 19, 2011
equity_incentive_plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options, weighted-average grant-date fair value (in dollars per share) $ 9.66us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 6.21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 6.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue      
Total intrinsic value of options exercised $ 174.3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 198.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 57.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue      
Cash received from the exercise of stock options 178.1us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions 281.1us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions 179.3us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions      
Performance-based restricted stock unit payable range maximum 200.00%mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardPerformanceBasedRestrictedStockPayableRangeMaximum          
Restricted Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-average grant-date fair value (in dollars per share) $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
         
Total fair value of deferred stock units   1.1mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
0.6mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Restricted Stock [Member] | Certain grants in 2005 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period, years 3 years          
Restricted Stock Units And Performance Stock Units [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period, years 3 years          
Total fair value of deferred stock units 165.3mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= mmc_RestrictedStockUnitsAndPerformanceStockUnitsMember
205.5mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= mmc_RestrictedStockUnitsAndPerformanceStockUnitsMember
262.6mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= mmc_RestrictedStockUnitsAndPerformanceStockUnitsMember
     
Restricted Stock Units (RSUs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-average grant-date fair value (in dollars per share) $ 48.16us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 36.70us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 31.96us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
     
Performance Shares [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Performance-based restricted stock unit payable range minimum 0.00%mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardPerformanceBasedRestrictedStockPayableRangeMinimum
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
         
Performance-based restricted stock unit payable range maximum 200.00%mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardPerformanceBasedRestrictedStockPayableRangeMaximum
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
         
Performance period 3 years          
Weighted-average grant-date fair value (in dollars per share) $ 48.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 36.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 31.89us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Target payout percentage 100.00%mmc_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceBasedPayoutTargetPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
         
Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Daily closing price observation period 10 years          
Average Volatility Periods Ending On Fifteen Anniversaries Prior To Valuation Date 10 years          
Unrecognized compensation cost related to option awards 16us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
         
Weighted-average period over which that cost is expected to be recognized, years 1 year 0 months 19 days          
Restricted Stock Units, Restricted Stock and Performance Units [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-average period over which that cost is expected to be recognized, years 1 year          
Unrecognized compensation cost $ 55.8us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= mmc_RestrictedStockUnitsRestrictedStockandPerformanceUnitsMember
         
Award Plans 2000 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of previous equity plans replaced           2mmc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberOfPreviousEquityPlansReplaced
/ us-gaap_PlanNameAxis
= mmc_AwardPlans2000Member
Incentive and Stock Award Plan 2011 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan) 23,200,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= mmc_IncentiveandStockAwardPlan2011Member
         
Incentive and Stock Award Plan 2011 [Member] | Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options vesting percentage 25.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= mmc_IncentiveandStockAwardPlan2011Member
         
Contractual term of stock option awards 10 years          
Employee Stock Purchase Plan 1999 [Member] | Employee Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan) 35,600,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
Number of share purchase times per plan year 4mmc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharePurchaseTimesPerPlanYear
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
Stock option price percent 95.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
Reduction in the shares available 10,000,000mmc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofReductioninSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
Shares purchased by employees 608,453us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
Shares available for issuance under the plan 2,779,195us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_EmployeeStockPurchasePlan1999Member
         
International Plan [Member] | Employee Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan) 12,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_InternationalPlanMember
         
Shares purchased by employees 93,490us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_InternationalPlanMember
         
Shares available for issuance under the plan 2,893,986us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_InternationalPlanMember
         
Shares due to shareholder action       4,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_InternationalPlanMember
5,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
/ us-gaap_PlanNameAxis
= mmc_InternationalPlanMember