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Expected Loss to be Paid (Recovered) - Additional Information (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 26, 2023 USD ($) | Mar. 31, 2025 USD ($) Payment | Mar. 31, 2024 USD ($) | Dec. 31, 2024 USD ($)
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 |  | $ (61) | $ 7 |
Expected LAE to be paid |  | 12 |  | $ 11
Credit for estimated future recoveries of claims paid |  | 260 |  | 262
Net |  | 263,591 |  | 261,552
Insured financial obligations insured contractual payments outstanding, net |  | $ 419,136 |  | 415,966
Additional loss recovery assumption, recovery period |  | 5 years |  |
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 |  |  |
United States | RMBS |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |
Net |  | $ 1,474 |  | $ 1,507
United States | RMBS | First Lien |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |
Number of delinquent payments | Payment |  | 2 |  |
Projected loss assumptions, CDR, plateau rate, projection period |  | 36 months |  |
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% |  |
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] |  |  |  |
Percent of deferred loan balances to be recovered |  | 20.00% |  |
Projected loss assumptions, period to reach final loss severity rate |  | 9 years |  |
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 |  |
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 |  | $ (29) |  |
United States | RMBS | Second Lien |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |
Loss recovery assumption (as a percent) |  | 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 |  | 20.00% |  |
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 |  | $ 77 |  |
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 |  | $ 77 |  |
Puerto Rico |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |
Net |  | 637 |  | $ 637
Insured financial obligations insured contractual payments outstanding, net |  | 744 |  | 756
Puerto Rico | Other Puerto Rico Exposure |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |
Net |  | 92 |  | 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 |  | 532 |  | 532
Insured financial obligations insured contractual payments outstanding, net |  | $ 619 |  | $ 629

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