Expected Loss to be Paid - Net Expected Loss to be Paid (Recovered) and Net Economic Loss Development (Benefit) by Accounting Model (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 735	$ 960	$ 735	$ 960	$ 660	$ 737	$ 963	$ 1,183
Economic loss development after recoveries for R&W	(34)	37	(31)	39
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	572	772	572	772	519	554	697	864
Economic loss development after recoveries for R&W	(32)	(84)	(91)	(145)
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	543	749	543	749	493	531	666	832
Economic loss development after recoveries for R&W	(30)	(92)	(86)	(154)
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	29		29			23
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	128	162	128	162	104	146	237	293
Economic loss development after recoveries for R&W	(1)	118	62	183
RMBS [Member] | First Lien [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	122		122			166
Economic loss development after recoveries for R&W	4	(19)	(55)	(50)
RMBS [Member] | Second Lien [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	6		6			20
Economic loss development after recoveries for R&W	(3)	(99)	(7)	(133)
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	35	26	35	26	37	37	29	26
Economic loss development after recoveries for R&W	(1)	3	(2)	1
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	163	188	163	188	$ 141	183	$ 266	$ 319
Economic loss development after recoveries for R&W	(2)	121	60	184
Insurance Contracts [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	684		684			683
Economic loss development after recoveries for R&W	(32)	22	(31)	12
FG VIEs and other [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	65		65			58
Economic loss development after recoveries for R&W	(1)	14	(7)	24
Credit derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(14)		(14)			$ (4)
Economic loss development after recoveries for R&W	$ (1)	$ 1	$ 7	$ 3