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Expected Loss to be Paid (Recovered) - Additional Information (Details) € in Millions, $ in Millions, £ in Billions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 18, 2025 board_member | Jun. 26, 2023 USD ($) | Aug. 31, 2025 board_member | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) Payment | Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) | Sep. 30, 2025 GBP (£) | Sep. 30, 2025 EUR (€)
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 |  |  |  | $ 5 | $ 5 | $ (51) | $ 25 |  |  |
Expected LAE to be paid |  |  |  | 4 |  | 4 |  | $ 11 |  |
Credit for estimated future recoveries of claims paid |  |  |  | 310 |  | 310 |  | 265 |  |
Net |  |  |  | 274,875 |  | 274,875 |  | 261,552 |  |
Insured financial obligations insured contractual payments outstanding, net |  |  |  | 438,764 |  | $ 438,764 |  | 415,966 |  |
Plaintiff, terminated board members | board_member |  |  | 6 |  |  |  |  |  |  |
Plaintiff, total board members | board_member |  |  | 7 |  |  |  |  |  |  |
Plaintiff, terminated board members suing for reinstatement | board_member | 3 |  |  |  |  |  |  |  |  |
Additional loss recovery assumption, recovery period |  |  |  |  |  | 5 years |  |  |  |
BIG |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 11,378 |  | $ 11,378 |  | 10,182 |  |
Puerto Rico Electric Power Authority | Guarantee Obligations |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Damages sought, estimate |  | $ 2,400 |  |  |  |  |  |  |  |
Damages sought |  | $ 8,500 |  |  |  |  |  |  |  |
UK Regulated Utilities | BIG |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 5,200 |  | 5,200 |  |  | £ 3.8 |
European Renewable Energy | BIG |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 930 |  | 930 |  |  |  | € 792
Thames Water Utilities Finance PLC | BIG |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 2,300 |  | 2,300 |  |  | £ 1.7 |
United States | RMBS |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 1,400 |  | 1,400 |  | 1,507 |  |
United States | RMBS | BIG |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | $ 780 |  | $ 780 |  | $ 819 |  |
United States | RMBS | Home Equity Line of Credit |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Projected future recoveries on previously charged-off loans |  |  |  | 60.00% |  | 60.00% |  | 50.00% | 60.00% | 60.00%
Effect of recovery |  |  |  | $ 26 |  |  |  |  |  |
United States | 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 |  |  |  |
United States | RMBS | First Lien | Base Scenario |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  | 50.00% |  |  |  | 50.00% |  |
United States | 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 CPR, 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% |  | 40.00% |  |  | 40.00% | 40.00%
Projected loss assumptions, period to reach final loss severity rate |  |  |  |  |  | 2 years 6 months |  |  |  |
United States | RMBS | First Lien | More Stressful Environment |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Projected loss assumptions, period to reach final loss severity rate |  |  |  |  |  | 9 years |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  |  | 20.00% |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  |  | 16 months |  |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  |  |  |  |  | $ 33 |  |  |  |
United States | 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 |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  |  | 80.00% |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  |  | 8 months |  |  |  |
Decrease in the plateau period used to calculate potential change in loss estimate (in months) |  |  |  |  |  | 6 months |  |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  |  |  |  |  | $ (32) |  |  |  |
United States | RMBS | Second Lien |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Loss recovery assumption (as a percent) |  |  |  | 2.00% |  | 2.00% |  | 2.00% | 2.00% | 2.00%
Projected loss assumptions, period of consistent conditional default rate |  |  |  |  |  | 36 months |  |  |  |
Period of conditional default rate downward trend |  |  |  |  |  | 1 year |  |  |  |
Percent of original period of consistent conditional default rate |  |  |  |  |  | 5.00% |  |  |  |
United States | RMBS | Second Lien | More Stressful Environment |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  |  | 40.00% |  |  |  |
United States | RMBS | Second Lien | Least Stressful Environment |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  |  | 80.00% |  |  |  |
United States | 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 from plateau to intermediate conditional default rate (in months) |  |  |  |  |  | 16 months |  |  |  |
Increase in conditional default rate ramp down period |  |  |  |  |  | 4 months |  |  |  |
Stress period (in months) |  |  |  |  |  | 58 months |  |  |  |
United States | 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 |  |  |  |  |  | $ 49 |  |  |  |
United States | 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 |  |  |  |
Change in estimate for decreased prepayment rate, percent |  |  |  |  |  | 10.00% |  |  |  |
United States | 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 |  |  |  |  |  | $ 49 |  |  |  |
Puerto Rico |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | $ 553 |  | 553 |  | $ 637 |  |
Insured financial obligations insured contractual payments outstanding, net |  |  |  | 645 |  | 645 |  | 756 |  |
Puerto Rico | Other Puerto Rico Exposure |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 76 |  | 76 |  | 92 |  |
Puerto Rico | Puerto Rico Defaulted Exposures Subject to a Plan Support Agreement | Puerto Rico Electric Power Authority |  |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |  |
Net |  |  |  | 464 |  | 464 |  | 532 |  |
Insured financial obligations insured contractual payments outstanding, net |  |  |  | $ 539 |  | $ 539 |  | $ 629 |  |

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