Expected Loss to be Paid - Net Expected Loss to be Paid and Net Economic Loss Development by Accounting Model (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 963	$ 1,298	$ 1,183	$ 1,303
Economic loss development after recoveries for R&W	(2)	(24)
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	697	1,050	864	1,203
Economic loss development after recoveries for R&W	61	(42)
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	666	1,007	832	1,157
Economic loss development after recoveries for R&W	62	(39)
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	31	43	32	46
Economic loss development after recoveries for R&W	(1)	(3)
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	237	219	293	73
Economic loss development after recoveries for R&W	(65)	16
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	209		243
Economic loss development after recoveries for R&W	31	(24)
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	28		50
Economic loss development after recoveries for R&W	34	8
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	29	29	26	27
Economic loss development after recoveries for R&W	2	2
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	266	248	319	$ 100
Economic loss development after recoveries for R&W	(63)	18
Insurance Contracts [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	904		1,110
Economic loss development after recoveries for R&W	10	(33)
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		75
Economic loss development after recoveries for R&W	(10)	2
Credit derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(6)		$ (2)
Economic loss development after recoveries for R&W	$ (2)	$ 7