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
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	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 for R&W	$ 960	$ 1,432	$ 1,183	$ 1,303
Economic Loss Development / (Benefit)	25	0	(14)	(5)
Accretion of discount	5	10	19	27
Changes in discount rates	1	(9)	(4)	(15)
Changes in timing and assumptions	19	(1)	(29)	(17)
(Paid) Recovered Losses	(267)	(241)	(451)	(238)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	718	1,191	$ 718	1,191
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	7	6	$ 23	17
Expected LAE to be paid	34		34		$ 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 for R&W	772	1,082	864	1,203
Economic Loss Development / (Benefit)	55	39	200	50
(Paid) Recovered Losses	(279)	(251)	(516)	(384)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	548	870	548	870
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	749	1,041	832	1,157
Economic Loss Development / (Benefit)	50	42	204	59
(Paid) Recovered Losses	(279)	(251)	(516)	(384)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	520	832	520	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 for R&W	23	41	32	46
Economic Loss Development / (Benefit)	5	(3)	(4)	(9)
(Paid) Recovered Losses	0	0	0	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	28	38	28	38
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	162	326	293	73
Economic Loss Development / (Benefit)	(40)	(40)	(223)	(52)
(Paid) Recovered Losses	13	17	65	152
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	135	303	135	303
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			243
Economic Loss Development / (Benefit)	27	13	77	(4)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	164		164
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)	13	27	146	56
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	29		29
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	26	24	26	27
Economic Loss Development / (Benefit)	10	1	9	(3)
(Paid) Recovered Losses	(1)	(7)	0	(6)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	35	18	35	18
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	188	350	319	100
Economic Loss Development / (Benefit)	(30)	(39)	(214)	(55)
(Paid) Recovered Losses	12	10	65	146
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	170	321	170	$ 321
Reinsurance of SGI Insured Portfolio [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	$ 0	$ 0	$ 0