Expected Loss to be Paid - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Curve Payment scenario	Dec. 31, 2017 USD ($)	Jun. 01, 2018 USD ($)	Dec. 31, 2016 USD ($)	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)	2.74%	2.38%
Net par amount outstanding	$ 241,802	$ 264,952
Liability for unpaid claims and claims adjustment expense, net largest single loss	1,183	1,303		$ 1,198
Net expected loss to be paid	131	21
Total economic loss development (benefit)	$ (5)	$ 313
Liquidation rate review period	12 months
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	3.06%	2.78%
United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 196,506
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 4,767	$ 4,966
Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Percent of total non-U.S. net expected losses to paid	2.70%	3.70%
Net par amount outstanding	$ 45,296
Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,270	$ 4,818
Liability for unpaid claims and claims adjustment expense, net largest single loss	293	73		206
Net expected loss to be paid	130	0
Total economic loss development (benefit)	$ (69)	(181)
Maximum number of payments behind to be considered performing borrower | Payment	1
Expected future recoverable (payable) for breached representations and warranties	$ 5	(117)
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%
Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 1,184	1,199
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
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 1,100
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	230,665	252,314
Liability for unpaid claims and claims adjustment expense, net largest single loss	864	1,203		904
Net expected loss to be paid	1	13
Total economic loss development (benefit)	56	549
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	186,562	209,392
Liability for unpaid claims and claims adjustment expense, net largest single loss	832	1,157		871
Net expected credit for estimated future recoveries of claims paid	586	385
Net expected loss to be paid	0	0
Total economic loss development (benefit)	70	554
Public Finance [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,767
Total economic loss development (benefit)	70
Public Finance [Member] | City of Hartford, Connecticut [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	326
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	44,103	42,922
Liability for unpaid claims and claims adjustment expense, net largest single loss	32	46		33
Net expected loss to be paid	1	13
Total economic loss development (benefit)	(14)	(5)
Public Finance Stockton General Fund [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	110
Non-Infrastructure Public Finance [Member] | Spain [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	432
Non-Infrastructure Public Finance [Member] | Portugal [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	71
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,730	5,272
Liability for unpaid claims and claims adjustment expense, net largest single loss	26	27		$ 88
Net expected loss to be paid	0	8
Total economic loss development (benefit)	8	(55)
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	10,160	12,238
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,800
BIG [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	2,387	2,761
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	85	85
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	96
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	7,429	8,871
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	6,388	7,140
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	1,041	1,731
BIG [Member] | Non-Infrastructure Public Finance [Member] | Hungary [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	177
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	259	360
First 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	243	123
Total economic loss development (benefit)	$ 16	1
Number of delinquent payments | Payment	2
Projected loss assumptions, CDR, plateau rate, projection period	36 months
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue	12 months
Intermediate conditional default rate (as a percent)	5.00%
Projected loss assumptions, number of scenarios considered | scenario	5
First Lien [Member] | Reinsurance of SGI Insured Portfolio [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			$ 113
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	4 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
Current conditional prepayment rate used in alternate scenario for loss estimate (as a percent)	15.00%
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	$ 54
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	$ 33
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	$ 50	50
Total economic loss development (benefit)	$ (85)	$ (182)
Period from plateau to intermediate conditional default rate (in months)	28 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	6 months
Liquidation rates assumed		15.00%
Period of consistent CDR	6 months
Stress period (in months)	34 months	34 months
Loss recovery assumption	2.00%
Loss Recovery Assumption of Charged-Off Loans, Estimated Future Recoveries, Percent	10.00%
Final CPR	15.00%
Number of conditional default rate curves modeled in estimating losses | Curve	5
Second Lien [Member] | Reinsurance of SGI Insured Portfolio [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			$ 17
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
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	$ 9
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	$ 10
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	(2)	$ (14)
Total economic loss development (benefit)	17	(34)
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 amount outstanding	$ 18
Remaining principle amount after tender offer	2 years
MBIA UK [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid	$ 0	$ 21
MBIA UK [Member] | Europe [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 198
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