v3.26.3
Acquisitions and Divestitures - Narrative (Details)
shares in Thousands, £ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended
Sep. 30, 2026
USD ($)
Apr. 30, 2026
USD ($)
Apr. 30, 2026
GBP (£)
Jan. 27, 2026
USD ($)
shares
Dec. 31, 2025
USD ($)
Sep. 30, 2026
USD ($)
Jun. 30, 2026
shares
Mar. 31, 2026
shares
Sep. 30, 2026
USD ($)
shares
Sep. 30, 2025
USD ($)
Business Combination [Line Items]                    
Acquisition,contingent consideration                 $ 7,000,000 $ 0
Acquisitions, cash payment                 20,000,000 16,000,000
Preliminary values of intangible assets                 456,000,000  
Cash proceeds from other disposal                 12,000,000  
Gain on disposal                   40,000,000
Customer Relationships [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets                 $ 318,000,000  
Restricted Stock Units [Member]                    
Business Combination [Line Items]                    
Number of shares issued | shares                 1,129  
Newfront Insurance Holdings, Inc                    
Business Combination [Line Items]                    
Acquisitions, cash payment       $ 900,000,000            
Acquisiiton , equity payment       150,000,000            
Newfront Insurance Holdings, Inc | Customer Relationships [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets       $ 209,000,000            
Weighted-average expected lives of intangible assets       8 years            
Newfront Insurance Holdings, Inc | Developed Software [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets       $ 55,000,000            
Weighted-average expected lives of intangible assets       7 years            
Newfront Insurance Holdings, Inc | Performance Targets                    
Business Combination [Line Items]                    
Contingent consideration, amount       $ 250,000,000            
Newfront Insurance Holdings, Inc | Restricted Stock Units [Member]                    
Business Combination [Line Items]                    
Number of shares issued | shares       187     3   187  
Newfront Insurance Holdings, Inc | Restricted Stock Awards [Member]                    
Business Combination [Line Items]                    
Number of shares issued | shares       225            
Number of ordinary shares vested | shares             26 199    
Newfront Insurance Holdings, Inc | Senior Notes                    
Business Combination [Line Items]                    
Proceeds from debt, net of issuance costs         $ 1,000,000,000          
Newfront Insurance Holdings, Inc | Up-Front                    
Business Combination [Line Items]                    
Contingent consideration fair value       $ 1,050,000,000.00            
Completed Acquisition                    
Business Combination [Line Items]                    
Non-cash revaluation gain           $ 37,000,000     $ 37,000,000  
Completed Acquisition | Al-Futtaim Willis                    
Business Combination [Line Items]                    
Acquisitions, cash payment $ 58,000,000                  
Remaining controlling interest 51.00%         51.00%     51.00%  
Non-cash revaluation gain $ 37,000,000                  
Cushon Acquisition                    
Business Combination [Line Items]                    
Acquisitions, cash payment | £     £ 150              
Contingent consideration, amount | £     £ 100              
Cushon Acquisition | Customer Relationships [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets   $ 58,000,000                
Weighted-average expected lives of intangible assets   20 years 20 years              
Cushon Acquisition | Developed Software [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets   $ 49,000,000                
Weighted-average expected lives of intangible assets   5 years 5 years              
Cushon Acquisition | Trade Names [Member]                    
Business Combination [Line Items]                    
Preliminary values of intangible assets   $ 4,000,000                
Weighted-average expected lives of intangible assets   10 years 10 years              
Other [Member]                    
Business Combination [Line Items]                    
Acquisitions, cash payment                 $ 55,000,000 14,000,000
Acquisitions, cash payment                 $ 23,000,000 $ 16,000,000
Equity Based Payment | Newfront Insurance Holdings, Inc | Above Target Performance                    
Business Combination [Line Items]                    
Contingent consideration, amount       $ 150,000,000