v3.26.1
Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2026
Dec. 31, 2025
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Dec. 31, 2024
Ceded Credit Risk [Line Items]            
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses $ 9,964 $ 9,526 $ 9,732 $ 9,044 $ 8,969 $ 8,260
Reinsurer concentration risk | Reinsurance recoverable | AM Best A- or better            
Ceded Credit Risk [Line Items]            
Concentration risk percentage 62.30% 62.10%        
Reinsurer concentration risk | Reinsurance recoverable | Not Rated | Ceded credit risk, secured            
Ceded Credit Risk [Line Items]            
Concentration risk percentage 95.00% 95.00%        
Reinsurer concentration risk | Reinsurance recoverable | Not Rated | All Other Carriers            
Ceded Credit Risk [Line Items]            
Concentration risk percentage [1] 37.70% 37.90%        
Reinsurer concentration risk | Stockholders' equity | Largest Balance Due From Any One Carrier            
Ceded Credit Risk [Line Items]            
Concentration risk percentage 8.40% 8.10%        
[1] At June 30, 2026 and December 31, 2025 over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other.