Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Payment Curve scenario	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) scenario	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
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)			0.54%		1.94%
Total net exposure	$ 233,075		$ 233,075		$ 236,807
Net expected loss to be paid after recoveries for R&W	471	$ 718	471	$ 718	$ 737	$ 735	$ 960	$ 1,183
Economic loss development after recoveries for R&W	70	25	101	(14)
Changes in discount rates	$ (2)	1	$ 30	(4)
Additional loss recovery assumption, recovery period			5 years
Loss recovery assumption, additional increase in recovery projection, percent	30.00%		30.00%
Loss recovery assumption, additional increase in recovery projection, economic benefit	$ 50		$ 50
Loss recovery assumption, additional decrease in recovery projection, percent	10.00%		10.00%
Loss recovery assumption, additional decrease in recovery projection, economic loss	$ 50		$ 50
Minimum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate			0.00%		0.00%
Maximum
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate			1.52%		2.45%
Puerto Rico
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,096		$ 4,096		$ 4,270
Non United States
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent			7.00%		3.20%
RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	3,132		$ 3,132		$ 3,546
Net expected loss to be paid after recoveries for R&W	137	135	137	135	146	128	162	293
Economic loss development after recoveries for R&W	1	(40)	(61)	(223)
Future net R&W benefit	$ 82		$ 82		53
RMBS | U.S. | Minimum | HELOCs
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	2.50%		2.50%
HELOCs | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments			10 years
Extended period for which borrow can pay only interest payments			5 years
Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 6,131		$ 6,131		6,515
Net expected loss to be paid after recoveries for R&W	38	35	38	35	37	35	26	26
Economic loss development after recoveries for R&W	9	10	11	9
Public finance
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	223,812		223,812		226,746
Net expected loss to be paid after recoveries for R&W	296	548	296	548	554	572	772	864
Economic loss development after recoveries for R&W	60	55	151	200
Public finance | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	172,570		172,570		175,487
Net expected loss to be paid after recoveries for R&W	263	520	263	520	531	$ 543	$ 749	$ 832
Net expected credit for estimated future recoveries of claims paid	1,025		1,025		819
Economic loss development after recoveries for R&W	56	50	142	204
Public finance | Non United States
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	33		33		23
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	104		104
BIG
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	7,967		7,967		8,506
BIG | Puerto Rico
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,100		4,100
BIG | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,524		1,524		1,618
BIG | Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	232		232		219
BIG | Life insurance transactions
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	40		40
BIG | Student Loan
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	73		73
BIG | Public finance
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	6,211		6,211		6,669
BIG | Public finance | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 5,351		$ 5,351		5,771
First Lien | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Liquidation rate	20.00%		20.00%
Net expected loss to be paid after recoveries for R&W	$ 121		$ 121		$ 166
Economic loss development after recoveries for R&W	$ (6)	27	$ 49	77
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
Changes in discount rates	$ 1		$ (24)
First Lien | Base Scenario | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period			36 months
Performing or projected to reperform, 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			2 years 9 months
Default from delinquentor rate, term			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%		15.00%
First Lien | More Stressful Environment | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)			15 months
Projected loss assumptions, period to reach final loss severity rate			9 years
Projected loss assumptions, increase (decrease) in expected loss to be paid, net			$ 44
First Lien | Least Stressful Environment | RMBS | U.S.
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			$ 45
Decrease in the plateau period used to calculate potential change in loss estimate (in months)			6 months
Second Lien | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Liquidation rate	20.00%		20.00%
Net expected loss to be paid after recoveries for R&W	$ 16		$ 16		$ 20
Economic loss development after recoveries for R&W	$ 5	13	$ 12	146
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
Loss recovery assumption (as a percent)	2.00%		2.00%		2.00%
Number of conditional default rate curves modeled in estimating losses | Curve			5
Guarantor obligations, recoveries received	$ 15		$ 48
Monthly delinquency threshold			6 months
Second Lien | RMBS | U.S. | Home Equity Line of Credit and Closed-end Mortgage
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	15.00%		15.00%
Second Lien | Base Scenario | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)			34 months
Second Lien | Most Stressful | RMBS | U.S.
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
Second Lien | Most Stressful | RMBS | U.S. | HELOCs
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period			$ 8
Second Lien | Least Stressful | RMBS | U.S.
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
Change in estimate for decreased prepayment rate, Percent			10.00%
Decreased conditional default rate ramp down period			25 months
Second Lien | Least Stressful | RMBS | U.S. | HELOCs
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for decreased conditional default rate ramp down period			$ 9
Second Lien | RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)			28 months
Period of constant conditional default rate (in months)			6 months
Credit derivatives
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ (13)		$ (13)		$ (4)
Economic loss development after recoveries for R&W	$ 7	$ 10	$ 0	$ 7