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