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, 2020	Mar. 31, 2019	Dec. 31, 2019
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	$ 737	$ 1,183
Economic Loss Development / (Benefit)	(3)	(2)
Accretion of discount	4	8
Changes in discount rates	31	(4)
Changes in timing and assumptions	(38)	(6)
Net (paid) recovered losses	(74)	(218)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 660	963
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	$ 3	7
Expected LAE to be paid	30		$ 33
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	554	864
Economic Loss Development / (Benefit)	59	61
Net (paid) recovered losses	(94)	(228)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	519	697
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	531	832
Economic Loss Development / (Benefit)	56	62
Net (paid) recovered losses	(94)	(228)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	493	666
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	32
Economic Loss Development / (Benefit)	3	(1)
Net (paid) recovered losses	0	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	26	31
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	146	293
Economic Loss Development / (Benefit)	(63)	(65)
Net (paid) recovered losses	21	9
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	104	237
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	166
Economic Loss Development / (Benefit)	59	31
Changes in discount rates	(25)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	110
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	20
Economic Loss Development / (Benefit)	4	34
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	6
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	37	26
Economic Loss Development / (Benefit)	1	2
Net (paid) recovered losses	(1)	1
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	37	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	183	319
Economic Loss Development / (Benefit)	(62)	(63)
Net (paid) recovered losses	20	10
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 141	$ 266