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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 471	$ 718	$ 471	$ 718	$ 735	$ 737	$ 960	$ 1,183
Economic loss development after recoveries for R&W	(70)	(25)	(101)	14
Public finance
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	296	548	296	548	572	554	772	864
Economic loss development after recoveries for R&W	(60)	(55)	(151)	(200)
Public finance | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	263	520	263	520	543	531	749	832
Economic loss development after recoveries for R&W	(56)	(50)	(142)	(204)
Public finance | Non United States
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	33		33			23
RMBS | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	137	135	137	135	128	146	162	293
Economic loss development after recoveries for R&W	(1)	40	61	223
RMBS | First Lien | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	121		121			166
Economic loss development after recoveries for R&W	6	(27)	(49)	(77)
RMBS | Second Lien | U.S.
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	16		16			20
Economic loss development after recoveries for R&W	(5)	(13)	(12)	(146)
Other structured finance
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	38	35	38	35	35	37	26	26
Economic loss development after recoveries for R&W	(9)	(10)	(11)	(9)
Structured finance
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	175	170	175	170	$ 163	183	$ 188	$ 319
Economic loss development after recoveries for R&W	(10)	30	50	214
Insurance Contracts [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	421		421			683
Economic loss development after recoveries for R&W	(65)	(17)	(96)	(5)
FG VIEs and other
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	63		63			58
Economic loss development after recoveries for R&W	2	2	(5)	26
Credit derivatives
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(13)		(13)			$ (4)
Economic loss development after recoveries for R&W	$ (7)	$ (10)	$ 0	$ (7)