Expected Loss to be Paid (Recovered) - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended	15 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) Payment scenario Curve	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) scenario	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
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)			0.78%		0.60%
Net par outstanding	$ 235,668		$ 235,668		$ 234,153
Net Expected Loss to be Paid (Recovered)	199	$ 471	199	$ 471	529	$ 466	$ 735	$ 737
Net economic loss development (benefit)	(94)	70	$ (101)	101
Increase in expected recoveries	$ 56
Additional loss recovery assumption, recovery period			5 years
Loss recovery assumption, additional increase in recovery projection, percent	40.00%		40.00%
Loss recovery assumption, additional increase in recovery projection, economic benefit	$ 45		$ 45
Loss recovery assumption, additional decrease in recovery projection, percent	20.00%		20.00%
Loss recovery assumption, additional decrease in recovery projection, economic loss	$ 45		$ 45
Public Finance Stockton Pension Obligation Bonds
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	100		100
Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	226,257		226,257		224,625
Net Expected Loss to be Paid (Recovered)	10	296	10	296	341	243	572	554
Net economic loss development (benefit)	(31)	60	(28)	151
Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	6,901		6,901		6,538
Net Expected Loss to be Paid (Recovered)	47	38	47	38	40	45	35	37
Net economic loss development (benefit)	2	9	9	11
BIG
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	7,346		7,346		7,975
BIG | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	5,920		5,920		6,334
BIG | Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	124		124		$ 161
BIG | Student Loan
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	60		60
BIG | Life insurance transactions
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	40		$ 40
United States
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent			89.60%		93.20%
United States | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	175,952		$ 175,952		$ 171,597
Net Expected Loss to be Paid (Recovered)	(9)	263	(9)	263	305	221	543	531
Net economic loss development (benefit)	(29)	56	(13)	142
United States | RMBS
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	2,510		2,510		2,990
Net Expected Loss to be Paid (Recovered)	142	137	142	137	148	$ 178	$ 128	$ 146
Net economic loss development (benefit)	(65)	1	$ (82)	(61)
United States | RMBS | Second Lien
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)			28 months
United States | RMBS | First Lien
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	160		$ 160		$ 133
Net economic loss development (benefit)	$ (10)	6	$ 1	(49)
Number of delinquent payments | Payment			2
Liquidation rate						20.00%
Projected loss assumptions, CDR, plateau rate, projection period			36 months
Projected loss assumptions, final CPR, period for voluntary prepayments to continue			12 months
Percent of deferred loan balances to be recovered	20.00%
Economic benefit	$ 19
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]
Projected loss assumptions, CDR, plateau rate, projection period			36 months
Period from plateau to intermediate conditional default rate (in months)			12 months
Intermediate conditional default rate as a percentage of plateau conditional default rate			20.00%
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 9 months
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)			15 months
Projected loss assumptions, increase (decrease) in expected loss to be paid, net			$ (30)
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)			9 months
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 | First Lien | Most Stressful
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, period to reach final loss severity rate			9 years
United States | RMBS | Second Lien
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	$ (18)		$ (18)		$ 15
Net economic loss development (benefit)	(55)	$ (5)	$ (83)	$ (12)
Liquidation rate						20.00%
Expected loss for second lien transactions	$ 14
Period from plateau to intermediate conditional default rate (in months)			28 months
Period of loan default estimate			6 months
Number of preceding months average liquidation rates used to estimate loan default rate			6 months
Projected loss assumptions, period of consistent conditional default rate			6 months
Stress period (in months)			34 months
Monthly delinquency threshold			6 months
Loss recovery assumption (as a percent)	2.00%		2.00%		2.00%
Number of conditional default rate curves modeled in estimating losses | Curve			5
Liquidation rate	30.00%		30.00%
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 | Base Scenario
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)			34 months
United States | RMBS | Second Lien | Most Stressful
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period			8 months
Period from plateau to intermediate conditional default rate (in months)			31 months
Stress period (in months)			39 months
Increase in conditional default rate ramp down period			3 months
United States | RMBS | Second Lien | Most Stressful | Home Equity Line of Credit
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period			$ 7
United States | RMBS | Second Lien | Least Stressful
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)			29 months
Period of constant conditional default rate (in months)			4 months
Decreased conditional default rate ramp down period			25 months
Change in estimate for decreased prepayment rate, Percent			10.00%
United States | RMBS | Second Lien | Least Stressful | 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			$ 8
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 | BIG | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	$ 5,443		$ 5,443		$ 5,439
United States | BIG | RMBS
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	1,302		1,302		1,480
Puerto Rico
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	3,572		3,572		3,725
Puerto Rico | BIG
Schedule of Expected Losses to be Paid [Line Items]
Net par outstanding	3,600		3,600
Non United States | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	19		19		$ 36
Net economic loss development (benefit)	$ (2)		$ (15)
Minimum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate			0.00%		0.00%
Minimum | United States | RMBS | Home Equity Line of Credit
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	1.00%		1.00%			2.50%
Maximum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate			2.13%		1.72%