v3.26.3
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 9 Months Ended 12 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 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 $ 192 $ 186 $ 101 $ 106 $ 106
Accretion of discount 2 2 6 5  
Changes in discount rates 16 0 10 6  
Changes in timing and assumptions (45) (40) 49 (28)  
Total economic loss development (benefit) (27) (38) 65 (17)  
Net (paid) recovered losses (52) (44) (53) 15  
Net expected loss to be paid (recovered), end of period 113 104 113 $ 104 101
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 174 192 95 $ 116 116
Total economic loss development (benefit) (23) (8) 72 87  
Net (paid) recovered losses (60) (81) (76) (100)  
Net expected loss to be paid (recovered), end of period 91 103 91 103 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 58 29 60 33 33
Total economic loss development (benefit) 0 1 0 (64)  
Net (paid) recovered losses 0 30 (2) 91  
Net expected loss to be paid (recovered), end of period 58 60 58 60 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 18 (6) 6 (10) (10)
Total economic loss development (benefit) (4) (30) (7) (104)  
Net (paid) recovered losses 8 37 23 115  
Net expected loss to be paid (recovered), end of period 22 1 $ 22 1 $ 6
Minimum          
Schedule of Expected Losses to be Paid [Line Items]          
Risk free discount rate     2.44%   1.93%
Maximum          
Schedule of Expected Losses to be Paid [Line Items]          
Risk free discount rate     6.07%   5.35%
Weighted Average          
Schedule of Expected Losses to be Paid [Line Items]          
Risk free discount rate     5.00%   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 42 53 $ (31) 18 $ 18
Total economic loss development (benefit) (20) (2) 69 51  
Net (paid) recovered losses (63) (81) (79) (99)  
Net expected loss to be paid (recovered), end of period (41) (30) (41) (30) (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 (40) (35) (54) (43) (43)
Changes in discount rates (3) 1 (3) 0  
Total economic loss development (benefit) (4) (31) (7) (40)  
Net (paid) recovered losses 8 7 25 24  
Net expected loss to be paid (recovered), end of period (36) (59) (36) (59) (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 132 139 126 98 98
Total economic loss development (benefit) (3) (6) 3 36  
Net (paid) recovered losses 3 0 3 (1)  
Net expected loss to be paid (recovered), end of period $ 132 $ 133 $ 132 $ 133 $ 126