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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 1,191	$ 1,292	$ 1,191	$ 1,292	$ 1,432	$ 1,303	$ 1,297	$ 1,198
Economic loss development after recoveries for R&W	0	204	(5)	298
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	870	1,093	870	1,093	1,082	1,203	1,086	904
Economic loss development after recoveries for R&W	39	229	50	427
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	832	1,046	832	1,046	1,041	1,157	1,044	871
Economic loss development after recoveries for R&W	42	229	59	431
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	38	47	38	47	41	46	42	33
Economic loss development after recoveries for R&W	(3)	0	(9)	(4)
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	303	176	303	176	326	73	182	206
Economic loss development after recoveries for R&W	(40)	(19)	(52)	(70)
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	237		237			123
Economic loss development after recoveries for R&W	13	(1)	(4)	22
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	66		66			50
Economic loss development after recoveries for R&W	27	20	56	48
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	18	23	18	23	24	27	29	88
Economic loss development after recoveries for R&W	1	(6)	(3)	(59)
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	321	199	321	199	$ 350	100	$ 211	$ 294
Economic loss development after recoveries for R&W	(39)	(25)	(55)	(129)
Financial Guarantee Insurance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,112		1,112			1,226
Economic loss development after recoveries for R&W	1	207	(9)	328
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	83		83			91
Economic loss development after recoveries for R&W	(3)	(2)	(7)	(6)
Credit derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(4)		(4)			$ (14)
Economic loss development after recoveries for R&W	$ 2	$ (1)	$ 11	$ (24)