v3.25.4
Summary of Significant Accounting Policies (Narrative) (Details)
 12 Months Ended
Dec. 31, 2025
USD ($)
segment
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Number of business segments | segment 2    
Restricted cash, current $ 553,000,000 $ 455,000,000  
Equity method investment, Lag period 3 months    
Investment income $ 34,000,000 12,000,000 $ 5,000,000
Indefinite lived identified intangible assets 0 0  
Capitalized computer software costs 466,000,000 474,000,000  
Accumulated amortization 2,300,000,000 2,100,000,000  
Liability for errors and omissions 379,000,000 391,000,000  
Net uncollected premiums and claims and related payables 14,600,000,000 15,100,000,000  
Mc Griff Insurance Services L L C      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Increase due to acquisition   465,000,000  
Risk and Insurance Services      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Interest income, fiduciary assets 403,000,000 497,000,000 $ 453,000,000
Accounts Payable and Accrued Liabilities      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Liabilities for errors and omissions, current $ 91,000,000 $ 86,000,000  
Office Space | Lease Obligations | Lease Concentration Risk      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Concentration risk (as a percent) 98.00%    
Minimum | Customer Lists      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Intangible assets, useful life 10 years    
Minimum | Furniture and equipment      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 3 years    
Minimum | Building      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 30 years    
Minimum | Software Development      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 3 years    
Maximum | Customer Lists      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Intangible assets, useful life 15 years    
Maximum | Furniture and equipment      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 10 years    
Maximum | Building      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 40 years    
Maximum | Software Development      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Property, plant and equipment, useful life 10 years