Expected Loss to be Paid - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Curve Payment	Dec. 31, 2018 USD ($) scenario	Dec. 31, 2017 USD ($)	Nov. 26, 2012 USD ($)
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)	1.94%	2.74%
Net Par Outstanding, Total	$ 236,807	$ 241,802
Liability for unpaid claims and claims adjustment expense, net largest single loss	737	1,183	$ 1,303
Net expected loss to be paid		131
Total economic loss development (benefit)	$ (1)	$ (5)
Liquidation rate review period	12 months
Loss Recovery Assumption of Charged-Off Loans, Estimated Future Recoveries, Percent	20.00%	10.00%
Additional Loss Recovery Assumption, Recovery Period	5 years
Loss Recovery Assumption, Additional Increase in Recovery Projection, Percent	30.00%
Loss Recovery Assumption, Additional Increase in Recovery Projection, Economic Benefit	$ 57
Loss Recovery Assumption, Additional Decrease in Recovery Projection, Percent	10.00%
Loss Recovery Assumption, Additional Decrease in Recovery Projection, Economic Loss	$ 57
Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	0.00%	0.00%
Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	2.45%	3.06%
United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	$ 184,787
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	$ 4,270	$ 4,767
Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Percent of total non-U.S. net expected losses to paid	3.20%	2.70%
Net Par Outstanding, Total	$ 52,020
Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	3,546	$ 4,270
Liability for unpaid claims and claims adjustment expense, net largest single loss	146	293	73
Net expected loss to be paid		130
Total economic loss development (benefit)	$ (234)	(69)
Maximum number of payments behind to be considered performing borrower | Payment	1
Expected future recoverable (payable) for breached representations and warranties	$ 53	(5)
Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member] | Minimum [Member] | Home equity lines of credit (HELOCs) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	2.50%
Home equity lines of credit (HELOCs) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments	10 years
Loan Modification, Extended Period for Which Borrower Can Pay Only Interest Payments	5 years
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	$ 226,746	230,665
Liability for unpaid claims and claims adjustment expense, net largest single loss	554	864	1,203
Net expected loss to be paid		1
Total economic loss development (benefit)	215	56
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	175,487	186,562
Liability for unpaid claims and claims adjustment expense, net largest single loss	531	832	1,157
Net expected credit for estimated future recoveries of claims paid	819	586
Net expected loss to be paid		0
Total economic loss development (benefit)	224	70
Public Finance [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	4,270
Total economic loss development (benefit)	224
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	51,259	44,103
Liability for unpaid claims and claims adjustment expense, net largest single loss	23	32	46
Net expected loss to be paid		1
Total economic loss development (benefit)	(9)	(14)
Public Finance [Member] | Non United States [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	11,341	6,094
Public Finance Stockton General Fund [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	107
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	4,744	5,683
Liability for unpaid claims and claims adjustment expense, net largest single loss	37	26	$ 27
Net expected loss to be paid		0
Total economic loss development (benefit)	18	8
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	8,506	10,160
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	4,300
BIG [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	1,618	2,387
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	40
BIG [Member] | Life Insurance Transactions [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total		85
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	84
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	6,669	7,429
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	5,771	6,388
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	898	1,041
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total	$ 179	259
First Lien [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Guarantor Obligations, Projected Loss Assumption, Final Conditional Prepayment Rates, Final Rate, Term	12 months
Liability for unpaid claims and claims adjustment expense, net largest single loss	$ 166	243
Total economic loss development (benefit)	$ (77)	$ 16
Number of delinquent payments | Payment	2
Projected loss assumptions, CDR, plateau rate, projection period	36 months
Intermediate conditional default rate (as a percent)	5.00%
Projected loss assumptions, number of scenarios considered | scenario		5
First Lien [Member] | Base Scenario [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
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%
Period of constant intermediate conditional default rate (in months)	36 months
Final conditional default rate as a percentage of plateau conditional default rate	5.00%
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue	3 years 6 months
Period for which estimated defaults are attributed to loans currently delinquent or in foreclosure	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%
Projected loss assumptions, period to reach final loss severity rate	2 years 6 months
First Lien [Member] | More Stressful Environment [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
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	$ 43
First Lien [Member] | Least Stressful Environment [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
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	$ 38
Decrease in the plateau period used to calculate potential change in loss estimate (in months)	6 months
Second Lien [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liability for unpaid claims and claims adjustment expense, net largest single loss	$ 20	$ 50
Total economic loss development (benefit)	$ (157)	$ (85)
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
Monthly delinquency, threshold period	6 months
Period of consistent CDR	6 months
Stress period (in months)	34 months
Loss recovery assumption		2.00%
Final CPR		15.00%
Number of conditional default rate curves modeled in estimating losses | Curve	5
Second Lien [Member] | Base Scenario [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)	28 months
Current conditional prepayment rate used in alternate scenario for loss estimate (as a percent)	15.00%
Stress period (in months)	34 months
Period of constant conditional default rate (in months)	6 months
Second Lien [Member] | Base Scenario One [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
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 [Member] | Base Scenario One [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member] | Home equity lines of credit (HELOCs) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period	$ 6
Second Lien [Member] | Based Scenario Two [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
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
Ultimate prepayment rate	10.00%
Decreased conditional default rate ramp down period	25 months
Second Lien [Member] | Based Scenario Two [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member] | Home equity lines of credit (HELOCs) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for decreased conditional default rate ramp down period	$ 7
Financial Guarantee Accounted for as Credit Derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liability for unpaid claims and claims adjustment expense, net largest single loss	(4)	$ (2)
Total economic loss development (benefit)	$ 14	17
Southern District of Mississippi Vs Madison County, Mississippi [Member] | Parkway East [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net Par Outstanding, Total		$ 18
ACA 2005-2 Collateralized Debt Obligations [Member] | Credit Default Swap [Member] | CIFG Holding Inc. [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | Financial Guarantee Accounted for as Credit Derivatives [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liability for unpaid claims and claims adjustment expense, net largest single loss				$ 400
Libertas II Collateralized Debt Obligations [Member] | Credit Default Swap [Member] | CIFG Holding Inc. [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | Financial Guarantee Accounted for as Credit Derivatives [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liability for unpaid claims and claims adjustment expense, net largest single loss				$ 325