Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) scenario	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) scenario	Sep. 30, 2018 USD ($) Curve Payment scenario	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) scenario	Jun. 01, 2018 USD ($)	Jun. 30, 2017 USD ($)	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)				3.03%		2.38%
Total net exposure	$ 246,940			$ 246,940		$ 264,952
Net expected loss to be paid after recoveries for R&W	1,191	$ 1,292	$ 1,432	1,191	$ 1,292	$ 1,303		$ 1,297	$ 1,198
Economic loss development after recoveries for R&W	0	204		$ (5)	298
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				3.20%		2.78%
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,767			$ 4,767		$ 4,966
Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent				3.20%		3.70%
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,566			$ 4,566		$ 4,818
Net expected loss to be paid after recoveries for R&W	303	176	326	303	176	73		182	206
Economic loss development after recoveries for R&W	(40)	(19)		(52)	(70)
Future net R&W benefit	$ 13			$ 13		117
RMBS [Member] | United States [Member] | Minimum [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	2.50%			2.50%
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
Extended period for which borrow can pay only interest payments				5 years
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 5,002			$ 5,002		5,272
Net expected loss to be paid after recoveries for R&W	18	23	24	18	23	27		29	88
Economic loss development after recoveries for R&W	1	(6)		(3)	(59)
Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,185			1,185		1,199
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,200			1,200
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	235,153			235,153		252,314
Net expected loss to be paid after recoveries for R&W	870	1,093	1,082	870	1,093	1,203		1,086	904
Economic loss development after recoveries for R&W	39	229		50	427
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	190,418			190,418		209,392
Net expected loss to be paid after recoveries for R&W	832	1,046	1,041	832	1,046	1,157		1,044	871
Economic loss development after recoveries for R&W	42	229		59	431
Public Finance [Member] | City of Hartford, Connecticut [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	331			331
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	44,735			44,735		42,922
Net expected loss to be paid after recoveries for R&W	38	47	$ 41	38	47	46		$ 42	$ 33
Economic loss development after recoveries for R&W	(3)	0		(9)	(4)
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	110			110
Non-Infrastructure Public Finance [Member] | Spain [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	437			437
Non-Infrastructure Public Finance [Member] | Portugal [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	71			71
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	10,293			10,293		12,238
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,800			4,800
BIG [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	2,560			2,560		2,761
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	325			325		360
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	85			85		85
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	111			111
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	7,242			7,242		8,871
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	6,171			6,171		7,140
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,071			1,071		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]
Total net exposure	179			179
First Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	237			237		$ 123
Economic loss development after recoveries for R&W	$ 13	(1)		$ (4)	22
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%			5.00%
Number of scenarios weighted in estimating expected losses | scenario	5		5	5		5
First Lien [Member] | Reinsurance of SGI Insured Portfolio [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W							$ 113
First Lien [Member] | Base Scenario [Member] | 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
Period of constant intermediate conditional default rate (in months)				36 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 CDR, Period for voluntary prepayments to continue				4 years 9 months
Default from delinquentor rate, term				36 months
Guarantor obligations, default period currently performing				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%		15.00%
First Lien [Member] | More Stressful Environment [Member] | 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				$ 57
First Lien [Member] | Least Stressful Environment [Member] | 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				$ 35
Decrease in the plateau period used to calculate potential change in loss estimate (in months)				6 months
Second Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 66			$ 66		$ 50
Economic loss development after recoveries for R&W	$ 27	20		$ 56	48
Period from plateau to intermediate conditional default rate (in months)				28 months
Number of scenarios weighted in estimating expected losses | scenario						5
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%		2.00%
Number of conditional default rate curves modeled in estimating losses | Curve				5
Monthly delinquency threshold				6 months
Second Lien [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit and Closed-end Mortgage [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	15.00%		15.00%	15.00%		15.00%
Second Lien [Member] | Reinsurance of SGI Insured Portfolio [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W							$ 17
Second Lien [Member] | Base Scenario [Member] | 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] | 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] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period				$ 11
Second Lien [Member] | Based Scenario Two [Member] | 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
Change in estimate for decreased prepayment rate, Percent				10.00%
Decreased conditional default rate ramp down period				25 months
Second Lien [Member] | Based Scenario Two [Member] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for decreased conditional default rate ramp down period				$ 12
Southern District of Mississippi Vs Madison County, Mississippi [Member] | Parkway East [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 18			$ 18
Payment time period on annual debt service				2 years
MBIA UK Insurance Limited [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity	0	0		$ 0	21
MBIA UK Insurance Limited [Member] | Europe [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	201			201
MBIA UK Insurance Limited [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity					0
MBIA UK Insurance Limited [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity					8
MBIA UK Insurance Limited [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity					13
MBIA UK Insurance Limited [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity					0
MBIA UK Insurance Limited [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on acquired portfolio activity					13
Credit derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(4)			(4)		$ (14)
Economic loss development after recoveries for R&W	$ 2	$ (1)		$ 11	$ (24)
ACA 2005-2 Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [Member] | Credit derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W										$ 400
Libertas II Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [Member] | Credit derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W										$ 325