Expected Loss to be Paid - Net Expected Loss to be Paid After Net Expected Recoveries for Breaches of R&W Rollforward (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jun. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		$ 1,432	$ 1,297	$ 1,303	$ 1,198
Economic Loss Development / (Benefit)		0	204	(5)	298
Accretion of discount		10	8	27	24
Changes in discount rates		(9)	(6)	(15)	28
Changes in timing and assumptions		(1)	202	(17)	246
(Paid) Recovered Losses		(241)	(209)	(238)	(225)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		1,191	1,292	$ 1,191	1,292
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)				45 days
Loss and LAE Reserve paid		6	7	$ 17	16
Expected LAE to be paid		15		15		$ 23
Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		1,082	1,086	1,203	904
Economic Loss Development / (Benefit)		39	229	50	427
(Paid) Recovered Losses		(251)	(222)	(384)	(251)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		870	1,093	870	1,093
Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		1,041	1,044	1,157	871
Economic Loss Development / (Benefit)		42	229	59	431
(Paid) Recovered Losses		(251)	(227)	(384)	(256)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		832	1,046	832	1,046
Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		41	42	46	33
Economic Loss Development / (Benefit)		(3)	0	(9)	(4)
(Paid) Recovered Losses		0	5	0	5
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		38	47	38	47
RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		326	182	73	206
Economic Loss Development / (Benefit)		(40)	(19)	(52)	(70)
(Paid) Recovered Losses		17	13	152	40
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		303	176	303	176
RMBS [Member] | First Lien [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W				123
Economic Loss Development / (Benefit)		13	(1)	(4)	22
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		237		237
RMBS [Member] | Second Lien [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W				50
Economic Loss Development / (Benefit)		27	20	56	48
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		66		66
Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		24	29	27	88
Economic Loss Development / (Benefit)		1	(6)	(3)	(59)
(Paid) Recovered Losses		(7)	0	(6)	(14)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		18	23	18	23
Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W		350	211	100	294
Economic Loss Development / (Benefit)		(39)	(25)	(55)	(129)
(Paid) Recovered Losses		10	13	146	26
End of Period, Net Expected Loss to be Paid After Recoveries for R&W		321	199	321	199
MBIA UK Insurance Limited [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity		0	0	0	21
MBIA UK Insurance Limited [Member] | Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					13
MBIA UK Insurance Limited [Member] | Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					0
MBIA UK Insurance Limited [Member] | Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					13
MBIA UK Insurance Limited [Member] | RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					0
MBIA UK Insurance Limited [Member] | Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					8
MBIA UK Insurance Limited [Member] | Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity					8
Reinsurance of SGI Insured Portfolio [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				131
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				1
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				0
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				1
Reinsurance of SGI Insured Portfolio [Member] | RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				130
Reinsurance of SGI Insured Portfolio [Member] | Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				0
Reinsurance of SGI Insured Portfolio [Member] | Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity				$ 130
AGC [Member] | Reinsurance of SGI Insured Portfolio [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	$ 131	$ 0	$ 0		$ 0