Expected Loss to be Paid (Recovered) - Additional Information (Details) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($)	Mar. 31, 2023	Sep. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Sep. 30, 2023 USD ($) Payment Curve scenario	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) scenario	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Schedule of Expected Losses to be Paid [Line Items]
Period of insured credit performance of guaranteed obligations (in some cases over)					30 years
Discount factor (as a percent)					4.80%		4.08%
Net Expected Loss to be Paid (Recovered)	$ 490		$ 727	$ 560	$ 490	$ 727	$ 522	$ 442	$ 411
Loss and LAE Reserve paid	9		5		17	25
Expected LAE to be paid	29				29		11
Net economic loss development (benefit)	87		(72)		$ 147	(148)
Additional loss recovery assumption, recovery period					5 years
First Lien
Schedule of Expected Losses to be Paid [Line Items]
Effect of recovery					$ 7
Second Lien
Schedule of Expected Losses to be Paid [Line Items]
Effect of recovery					31
Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	408		632	443	408	632	412	217	209
Net economic loss development (benefit)	134		22		193	(22)
Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	44		43	44	44	43	$ 44	46	52
Net economic loss development (benefit)	1		1		$ 6	1
United States
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent					98.20%		98.50%
United States | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	399		626	433	$ 399	626	$ 403	210	197
Net economic loss development (benefit)	135		24		193	(16)
United States | RMBS
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	38		52	$ 73	38	52	$ 66	$ 179	$ 150
Net economic loss development (benefit)	(48)		(95)		(52)	(127)
United States | RMBS | First Lien
Schedule of Expected Losses to be Paid [Line Items]
Net economic loss development (benefit)	$ (12)		(38)		$ (12)	(34)
Number of delinquent payments | Payment					2
Projected loss assumptions, CDR, plateau rate, projection period					36 months
Projected loss assumptions, final CPR, period for voluntary prepayments to continue					12 months
Intermediate conditional default rate (as a percent)	5.00%				5.00%
Number of scenarios weighted in estimating expected losses | scenario					5		5
United States | RMBS | First Lien | Base Scenario
Schedule of Expected Losses to be Paid [Line Items]
Percent of deferred loan balances to be recovered	30.00%			20.00%
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		10.00%
United States | RMBS | First Lien | Least Stressful Environment
Schedule of Expected Losses to be Paid [Line Items]
Percent of deferred loan balances to be recovered		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
Period from plateau to intermediate conditional default rate (in months)					12 months
Final conditional default rate as a percentage of plateau conditional default rate					5.00%
Projected loss assumptions, final CPR, period for voluntary prepayments to continue					1 year
Default from delinquentor rate, term					36 months
Performing or projected to reperform, projection period					36 months
Projected loss assumptions, loss severity, subsequent period					18 months
Estimated loss severity rate, one through six months (as a percent)					18 months
Loss severity (as a percent)	40.00%				40.00%
Projected loss assumptions, period to reach final loss severity rate					2 years 6 months
Final CPR	15.00%				15.00%
United States | RMBS | First Lien | More Stressful Environment
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)					16 months
Projected loss assumptions, period to reach final loss severity rate					9 years
Percent of deferred loan balances to be recovered					10.00%
Projected loss assumptions, increase (decrease) in expected loss to be paid, net					$ 28
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
Percent of deferred loan balances to be recovered					50.00%
Projected loss assumptions, increase (decrease) in expected loss to be paid, net					$ (20)
Decrease in the plateau period used to calculate potential change in loss estimate (in months)					6 months
United States | RMBS | Second Lien
Schedule of Expected Losses to be Paid [Line Items]
Net economic loss development (benefit)	$ (36)		$ (57)		$ (40)	$ (93)
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%
Loss recovery assumption (as a percent)	2.00%				2.00%		2.00%
Liquidation rate	40.00%			30.00%	40.00%
Number of conditional default rate curves modeled in estimating losses | Curve					5
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					10.00%
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 | Home Equity Line of Credit and Closed-end Mortgage
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	15.00%				15.00%
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
Stress period (in months)					58 months
Increase in conditional default rate ramp down period					4 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					$ 89
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					$ 120
United States | Home Equity Line of Credit
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments					10 years
United States | Home Equity Line of Credit Insured
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments					15 years
Minimum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate					4.51%		3.82%
Maximum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate					5.45%		4.69%