v3.26.3
Acquisitions and Dispositions (Narrative) (Details)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jul. 31, 2026
USD ($)
Jun. 30, 2026
USD ($)
Sep. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Sep. 30, 2026
USD ($)
acquisition
Sep. 30, 2025
USD ($)
Dec. 31, 2025
acquisition
insuranceBroker
Jan. 31, 2025
Business Combination [Line Items]                  
Total consideration           $ 550 $ 337    
Cash paid           318 254    
Estimated fair value of deferred/contingent consideration           90 68    
Fair value of previously-held equity method investment             15    
Deferred purchase consideration from prior years' acquisitions           101 53    
Acquisition related costs     $ 41 $ 81   144 218    
Number of insurance brokers acquired | insuranceBroker               3  
Payment to acquire non-controlling interest   $ 54       $ 54 $ 0    
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh Agency's Technology Consulting And Administrative Solutions                  
Business Combination [Line Items]                  
Cash proceeds from sale         $ 25        
Net gain included in revenue consolidated statements of income         15        
Minimum                  
Business Combination [Line Items]                  
Revenue target period (in years)           2 years 2 years    
Maximum                  
Business Combination [Line Items]                  
Revenue target period (in years)           4 years 4 years    
Deasterra Partners, S.L.                  
Business Combination [Line Items]                  
Ownership share (as a percent) 60.00%                
Gain on previously-held equity method investment, fair value $ 126   126     $ 126      
Current Fiscal Period Acquisitions                  
Business Combination [Line Items]                  
Total consideration           550      
Cash paid           318      
Estimated fair value of deferred/contingent consideration           90      
Fair value of previously-held equity method investment           142      
Prior Fiscal Periods Acquisitions                  
Business Combination [Line Items]                  
Estimated fair value of deferred/contingent consideration           103      
Deferred purchase consideration from prior years' acquisitions             $ 53    
Contingent consideration from prior year's acquisitions             38    
Mc Griff Insurance Services L L C                  
Business Combination [Line Items]                  
Acquisition related costs     $ 35 $ 52   $ 124 $ 166    
Carpenter Turner Cyprus Ltd.                  
Business Combination [Line Items]                  
Ownership share (as a percent)                 51.50%
Gain on previously-held equity method investment, fair value         $ 13        
Risk and Insurance Services                  
Business Combination [Line Items]                  
Number of acquisitions made (in acquisitions) | acquisition           6   14  
Consulting                  
Business Combination [Line Items]                  
Number of acquisitions made (in acquisitions) | acquisition           2   6