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Expected Loss to be Paid (Recovered) - Narrative (Details) $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Jun. 26, 2023 USD ($) | Dec. 31, 2024 USD ($) Payment | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Sep. 30, 2024
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Period of insured credit performance of guaranteed obligations (in some cases over) |  | 30 years |  |  |
Loss and LAE reserve paid |  | $ 30 | $ 25 | $ 33 |
Expected LAE to be paid |  | 11 | 22 |  |
Public finance estimated future recoveries of claims paid |  | 262 | 193 |  |
Net Par Outstanding |  | 261,552 | 249,153 |  |
Net |  | $ 415,966 | 397,636 |  |
Recovery period |  | 5 years |  |  |
PREPA | Guarantee Obligations |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Damages sought, estimate | $ 2,400 |  |  |  |
Damages sought, value | $ 8,500 |  |  |  |
First Lien |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Effect of recovery |  | $ 15 |  |  |
U.S. |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | 209,628 |  |  |
U.S. | RMBS |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | $ 1,507 | $ 1,774 |  |
U.S. | RMBS | First Lien |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  | 36 months |  |  |
Number of delinquent payments | Payment |  | 2 |  |  |
U.S. | RMBS | First Lien | Base Scenario |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Future recovery for deferred principal balances |  | 50.00% | 30.00% |  |
U.S. | RMBS | First Lien | Base Scenario |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  | 36 months |  |  |
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue |  | 1 year |  |  |
Estimated loss severity rate, one through six months (as a percent) |  | 18 months |  |  |
Loss severity (as a percent) |  | 40.00% |  |  |
Projected loss assumptions, period to reach final loss severity rate |  | 2 years 6 months |  |  |
U.S. | RMBS | First Lien | More Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Future recovery for deferred principal balances |  | 20.00% |  |  |
Projected loss assumptions, period to reach final loss severity rate |  | 9 years |  |  |
Period to reach intermediate conditional default rate |  | 16 months |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  | $ 31 |  |  |
U.S. | RMBS | First Lien | Least Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  | 30 months |  |  |
Future recovery for deferred principal balances |  | 80.00% |  |  |
Period to reach intermediate conditional default rate |  | 8 months |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  | $ 28 |  |  |
Decrease in the plateau period used to calculate potential change in loss estimate (in months) |  | 6 months |  |  |
U.S. | RMBS | Second Lien |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Loss recovery assumption |  | 2.00% | 2.00% |  |
Liquidation rate |  | 40.00% |  |  | 50.00%
Charge-off loan balances to be recovered, benefit |  | $ 29 |  |  |
Period of conditional default rate downward trend |  | 1 year |  |  |
Percent of original period of consistent conditional default rate |  | 5.00% |  |  |
U.S. | RMBS | Second Lien | More Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Future recovery for deferred principal balances |  | 20.00% |  |  |
U.S. | RMBS | Second Lien | Least Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Future recovery for deferred principal balances |  | 80.00% |  |  |
U.S. | RMBS | Second Lien | More Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  | 42 months |  |  |
Period to reach intermediate conditional default rate |  | 16 months |  |  |
Increase in conditional default rate ramp down period |  | 4 months |  |  |
Stress period (in months) |  | 58 months |  |  |
Ultimate prepayment rate |  | 15.00% |  |  |
U.S. | RMBS | Second Lien | More Stressful Environment | Home Equity Line of Credit |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Change in estimate for increased conditional default rate plateau period |  | $ 78 |  |  |
U.S. | RMBS | Second Lien | Least Stressful Environment |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Stress period (in months) |  | 38 months |  |  |
Period of constant conditional default rate (in months) |  | 30 months |  |  |
Decreased conditional default rate ramp down period |  | 8 months |  |  |
Ultimate prepayment rate |  | 10.00% |  |  |
U.S. | RMBS | Second Lien | Least Stressful Environment | Home Equity Line of Credit |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Change in estimate for decreased conditional default rate ramp down period |  | $ 78 |  |  |
Non U.S. |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | 51,924 |  |  |
Puerto Rico |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | 637 | $ 1,105 |  |
Net |  | 756 | 1,508 |  |
Puerto Rico | Puerto Rico Defaulted Exposures Subject to a Plan Support Agreement | PREPA |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | 532 | 624 |  |
Net |  | 629 | 751 |  |
Puerto Rico | Other Puerto Rico Exposures |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |
Net Par Outstanding |  | $ 92 | $ 109 |  |

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