v3.26.1
Acquisitions and Dispositions (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2026
USD ($)
acquisition
Mar. 31, 2025
USD ($)
Dec. 31, 2025
acquisition
insuranceBroker
Jan. 31, 2025
Business Combination [Line Items]        
Total consideration $ 45 $ 62    
Cash paid 43 37    
Estimated fair value of deferred/contingent consideration 2 10    
Fair value of previously-held equity method investment   15    
Deferred purchase consideration from prior years' acquisitions 14 27    
Acquisition related costs 48 78    
Number of insurance brokers acquired | insuranceBroker     3  
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh Management Consulting Celent Advisory Business        
Business Combination [Line Items]        
Cash proceeds from sale   25    
Net gain included in revenue consolidated statements of income   15    
Prior Fiscal Periods Acquisitions        
Business Combination [Line Items]        
Estimated fair value of deferred/contingent consideration 18      
Deferred purchase consideration from prior years' acquisitions   27    
Contingent consideration from prior year's acquisitions   19    
Mc Griff Insurance Services L L C        
Business Combination [Line Items]        
Acquisition related costs $ 42 69    
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    
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    
Risk and Insurance Services        
Business Combination [Line Items]        
Number of acquisitions made (in acquisitions) | acquisition 1   14  
Consulting        
Business Combination [Line Items]        
Number of acquisitions made (in acquisitions) | acquisition 1   6