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Goodwill and Other Intangibles (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting Information [Line Items]        
Goodwill $ 7,160us-gaap_Goodwill $ 7,159us-gaap_Goodwill $ 7,241us-gaap_Goodwill $ 6,893us-gaap_Goodwill
Aggregate amortization expense 24us-gaap_AmortizationOfIntangibleAssets 22us-gaap_AmortizationOfIntangibleAssets    
Risk and Insurance Services Segment [Member]        
Segment Reporting Information [Line Items]        
Goodwill 5,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mmc_RiskandInsuranceServicesSegmentMember		      
Consulting Segment [Member]        
Segment Reporting Information [Line Items]        
Goodwill $ 2,200us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mmc_ConsultingSegmentMember