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, 2019	Dec. 31, 2018
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		$ 1,183	$ 1,303
Net expected loss to be paid			131
Accretion of discount		22	36
Changes in discount rates		(11)	(17)
Changes in timing and assumptions		(12)	(24)
Total economic loss development (benefit)		(1)	(5)
Net (paid) recovered losses		(445)	(246)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		$ 737	1,183
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		$ 35	28
Expected LAE to be paid		33	31
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		864	1,203
Net expected loss to be paid			1
Total economic loss development (benefit)		215	56
Net (paid) recovered losses		(525)	(396)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		554	864
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		832	1,157
Net expected loss to be paid			0
Total economic loss development (benefit)		224	70
Net (paid) recovered losses		(525)	(395)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		531	832
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		32	46
Net expected loss to be paid			1
Total economic loss development (benefit)		(9)	(14)
Net (paid) recovered losses		0	(1)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		23	32
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		293	73
Net expected loss to be paid			130
Total economic loss development (benefit)		(234)	(69)
Net (paid) recovered losses		87	159
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		146	293
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		26	27
Net expected loss to be paid			0
Total economic loss development (benefit)		18	8
Net (paid) recovered losses		(7)	(9)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		37	26
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		319	100
Net expected loss to be paid			130
Total economic loss development (benefit)		(216)	(61)
Net (paid) recovered losses		80	150
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W		$ 183	$ 319
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