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
	Mar. 31, 2019	Mar. 31, 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	$ 1,183	$ 1,303
Economic Loss Development / (Benefit)	(2)	(24)
Accretion of discount	8	8
Changes in discount rates	(4)	(6)
Changes in timing and assumptions	(6)	(26)
(Paid) Recovered Losses	(218)	19
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 963	1,298
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	5
Expected LAE to be paid	28		$ 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	864	1,203
Economic Loss Development / (Benefit)	61	(42)
(Paid) Recovered Losses	(228)	(111)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	697	1,050
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	832	1,157
Economic Loss Development / (Benefit)	62	(39)
(Paid) Recovered Losses	(228)	(111)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	666	1,007
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	32	46
Economic Loss Development / (Benefit)	(1)	(3)
(Paid) Recovered Losses	0	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	31	43
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	293	73
Economic Loss Development / (Benefit)	(65)	16
(Paid) Recovered Losses	9	130
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	237	219
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)	31	(24)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	209
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)	34	8
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	28
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	27
Economic Loss Development / (Benefit)	2	2
(Paid) Recovered Losses	1	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	29	29
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	319	100
Economic Loss Development / (Benefit)	(63)	18
(Paid) Recovered Losses	10	130
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 266	$ 248