Expected Loss to be Paid - Net Expected Loss to be Paid After Recoveries for Breaches of R&W (Details) - USD ($) $ in Millions		12 Months Ended
	Jun. 01, 2018	Dec. 31, 2018	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 of R&W		$ 1,303	$ 1,198
Net expected loss to be paid		131	21
Accretion of discount		36	33
Changes in discount rates		(17)	25
Changes in timing and assumptions		(24)	255
Total economic loss development (benefit)		(5)	313
Net (paid) recovered losses		(246)	(229)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		$ 1,183	1,303
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		$ 28	24
Expected LAE to be paid		31	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 of R&W		1,203	904
Net expected loss to be paid		1	13
Total economic loss development (benefit)		56	549
Net (paid) recovered losses		(396)	(263)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		864	1,203
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 of R&W		1,157	871
Net expected loss to be paid		0	0
Total economic loss development (benefit)		70	554
Net (paid) recovered losses		(395)	(268)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		832	1,157
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 of R&W		46	33
Net expected loss to be paid		1	13
Total economic loss development (benefit)		(14)	(5)
Net (paid) recovered losses		(1)	5
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		32	46
Residential Mortgage-Backed Securities (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 of R&W		73	206
Net expected loss to be paid		130	0
Total economic loss development (benefit)		(69)	(181)
Net (paid) recovered losses		159	48
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		293	73
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 of R&W		27	88
Net expected loss to be paid		0	8
Total economic loss development (benefit)		8	(55)
Net (paid) recovered losses		(9)	(14)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		26	27
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 of R&W		100	294
Net expected loss to be paid		130	8
Total economic loss development (benefit)		(61)	(236)
Net (paid) recovered losses		150	34
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		319	100
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	$ 131		0
MBIA UK [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid		$ 0	$ 21