v3.26.1
Expected Loss to be Paid (Recovered) - Net Expected Loss to be Paid After Net Expected Recoveries for Breaches of R&W Rollforward (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period $ 101 $ 106 $ 106
Accretion of discount 2 1  
Changes in discount rates (4) 5  
Changes in timing and assumptions 46 (21)  
Total economic loss development (benefit) 44 (15)  
Net (paid) recovered losses (4) 59  
Net expected loss to be paid (recovered), end of period 141 $ 150 101
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)   45 days  
Public finance      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period 95 $ 116 116
Total economic loss development (benefit) 47 53  
Net (paid) recovered losses (11) (12)  
Net expected loss to be paid (recovered), end of period 131 157 95
Other structured finance      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period 60 33 33
Total economic loss development (benefit) (1) (65)  
Net (paid) recovered losses (1) 62  
Net expected loss to be paid (recovered), end of period 58 30 60
Structured finance      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period 6 (10) (10)
Total economic loss development (benefit) (3) (68)  
Net (paid) recovered losses 7 71  
Net expected loss to be paid (recovered), end of period $ 10 (7) $ 6
Minimum      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Risk free discount rate 1.93%   1.93%
Maximum      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Risk free discount rate 5.68%   5.35%
Weighted Average      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Risk free discount rate 4.16%   3.92%
United States | Public finance      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period $ (31) 18 $ 18
Total economic loss development (benefit) 45 29  
Net (paid) recovered losses (11) (12)  
Net expected loss to be paid (recovered), end of period 3 35 (31)
United States | RMBS      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period (54) (43) (43)
Changes in discount rates 0 1  
Total economic loss development (benefit) (2) (3)  
Net (paid) recovered losses 8 9  
Net expected loss to be paid (recovered), end of period (48) (37) (54)
Non-U.S. public finance | Public finance      
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]      
Net expected loss to be paid (recovered), beginning of period 126 98 98
Total economic loss development (benefit) 2 24  
Net (paid) recovered losses 0 0  
Net expected loss to be paid (recovered), end of period $ 128 $ 122 $ 126