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Expected Loss to be Paid (Recovered) - Additional Information (Details) € in Millions, £ in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 18, 2025 board_member | Aug. 31, 2025 board_member | Mar. 31, 2026 USD ($) Payment | Mar. 31, 2025 USD ($) | Mar. 31, 2026 GBP (£) | Mar. 31, 2026 EUR (€) | Dec. 31, 2025 USD ($)
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 |  |  | $ 2 | $ (61) |  |  |
Expected LAE to be paid |  |  | 13 |  |  |  | $ 8
Credit for estimated future recoveries of claims paid |  |  | 309 |  |  |  | 311
Net |  |  | 278,600 |  |  |  | 277,100
Insured financial obligations insured contractual payments outstanding, net |  |  | 441,500 |  |  |  | 440,800
Plaintiff, terminated board members | board_member |  | 6 |  |  |  |  |
Plaintiff, total board members | board_member |  | 7 |  |  |  |  |
Plaintiff, terminated board members suing for reinstatement | board_member | 3 |  |  |  |  |  |
BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 8,640 |  |  |  | 8,760
European Renewable Energy | BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 876 |  |  | € 758 |
Thames Water Utilities Finance PLC | BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 2,400 |  | £ 1,800 |  |
U.K. Student Accommodations | BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 386 |  | 292 |  |
U.K. National Health Service Trusts | BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 552 |  | £ 417 |  |
U.S. Transportation | BIG |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | $ 1,100 |  |  |  |
United States | RMBS | First Lien |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Number of delinquent payments | Payment |  |  | 2 |  |  |  |
Deferred loan balances to be recovered, least stressful environment, percent |  |  | 80.00% |  | 80.00% | 80.00% |
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 |  |  | $ 32 |  |  |  |
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 |  |  |  |
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 |  |  | $ (31) |  |  |  |
United States | RMBS | Second Lien |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  | 2.00% |  |  |  |
Projected loss assumptions, period of consistent conditional default rate |  |  | 5 years |  |  |  |
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 | Home Equity Line of Credit |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Change in estimate for increased conditional default rate plateau period |  |  | $ 46 |  |  |  |
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 |  |  | 47 |  |  |  |
Puerto Rico |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 553 |  |  |  | 553
Insured financial obligations insured contractual payments outstanding, net |  |  | 632 |  |  |  | 643
Puerto Rico | Other Puerto Rico Exposure |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |
Net |  |  | 76 |  |  |  | 76
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
Insured financial obligations insured contractual payments outstanding, net |  |  | $ 528 |  |  |  | $ 537

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