v3.25.4
Acquisitions and Dispositions (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2024
USD ($)
Jan. 01, 2024
USD ($)
Mar. 31, 2025
USD ($)
Mar. 31, 2024
USD ($)
Dec. 31, 2025
USD ($)
insuranceBroker
acquisition
Dec. 31, 2024
USD ($)
acquisition
Dec. 31, 2023
USD ($)
Jan. 31, 2025
Business Combination [Line Items]                
Number of insurance brokers acquired | insuranceBroker         3      
Business combination, consideration transferred         $ 857 $ 9,400    
Cash paid         726      
Estimated fair value of deferred/contingent consideration         116      
Fair value of previously-held equity method investment         15      
Deferred purchase consideration         67 113 $ 202  
Acquisition related expense         $ 282 $ 119    
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions                
Business Combination [Line Items]                
Proceeds from sale of business     $ 25          
Pre-tax gain (loss) on sale     15          
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh Management Consulting Celent Advisory Business                
Business Combination [Line Items]                
Proceeds from sale of business $ 24              
Pre-tax gain (loss) on sale $ 20              
Disposal Group, Disposed of by Sale, Not Discontinued Operations | U.S. Health And Benefits And U.K Pension Administration Businesses                
Business Combination [Line Items]                
Exit costs       $ 18        
Discontinued Operations, Held-for-Sale | U.S. Health And Benefits And U.K Pension Administration Businesses                
Business Combination [Line Items]                
Proceeds from sale of business   $ 120            
Pre-tax gain (loss) on sale   $ 35            
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    
Carpenter Turner Cyprus Ltd.                
Business Combination [Line Items]                
Ownership share (percent)               51.50%
Gain on previously-held equity method investment, fair value     $ 13          
Acquisitions Made In Prior Years                
Business Combination [Line Items]                
Estimated fair value of deferred/contingent consideration         $ 54 $ 39    
Deferred purchase consideration         41 166    
Mc Griff Insurance Services L L C                
Business Combination [Line Items]                
Acquisition related expense         $ 215 63    
Acquisitions Made In 2024                
Business Combination [Line Items]                
Cash paid           9,200    
Estimated fair value of deferred/contingent consideration           $ 190    
Risk and Insurance Services                
Business Combination [Line Items]                
Number of acquisitions made (in acquisitions) | acquisition         14 10    
Consulting                
Business Combination [Line Items]                
Number of acquisitions made (in acquisitions) | acquisition         6 7