v3.8.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Feb. 19, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 8-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol MMC    
Entity Registrant Name MARSH & MCLENNAN COMPANIES, INC.    
Entity Central Index Key 0000062709    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   507,621,360  
Entity Public Float     $ 39,350,425,982