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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 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	$ 735	$ 960	$ 737	$ 1,183
Economic Loss Development / (Benefit)	70	25	101	(14)
Accretion of discount	1	5	7	19
Changes in discount rates	(2)	1	30	(4)
Changes in timing and assumptions	71	19	64	(29)
Net (paid) recovered losses	(334)	(267)	(367)	(451)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	471	718	$ 471	718
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	11	7	$ 20	23
Expected LAE to be paid	22		22		$ 33
Public finance
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	572	772	554	864
Economic Loss Development / (Benefit)	60	55	151	200
Net (paid) recovered losses	(336)	(279)	(409)	(516)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	296	548	296	548
Public finance | U.S.
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	543	749	531	832
Economic Loss Development / (Benefit)	56	50	142	204
Net (paid) recovered losses	(336)	(279)	(410)	(516)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	263	520	263	520
Public finance | Non United States
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	29	23	23	32
Economic Loss Development / (Benefit)	4	5	9	(4)
Net (paid) recovered losses	0	0	1	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	33	28	33	28
RMBS | U.S.
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	128	162	146	293
Economic Loss Development / (Benefit)	1	(40)	(61)	(223)
Net (paid) recovered losses	8	13	52	65
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	137	135	137	135
RMBS | First Lien | U.S.
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)	(6)	27	49	77
Changes in discount rates	1		(24)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	121		121
RMBS | Second Lien | U.S.
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)	5	13	12	146
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	16		16
Other structured finance
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	35	26	37	26
Economic Loss Development / (Benefit)	9	10	11	9
Net (paid) recovered losses	(6)	(1)	(10)	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	38	35	38	35
Structured finance
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	163	188	183	319
Economic Loss Development / (Benefit)	10	(30)	(50)	(214)
Net (paid) recovered losses	2	12	42	65
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 175	$ 170	$ 175	$ 170