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, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 1,298	$ 1,244	$ 1,303	$ 1,198
Economic loss development after recoveries for R&W	(24)	47
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,050	1,011	1,203	904
Economic loss development after recoveries for R&W	(42)	119
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	1,007	970	1,157	871
Economic loss development after recoveries for R&W	(39)	124
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	43	41	46	33
Economic loss development after recoveries for R&W	(3)	(5)
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	219	197	73	206
Economic loss development after recoveries for R&W	16	(22)
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	152		123
Economic loss development after recoveries for R&W	(24)	(9)
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	67		50
Economic loss development after recoveries for R&W	(8)	(13)
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	36	27	88
Economic loss development after recoveries for R&W	2	(50)
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	248	233	100	$ 294
Economic loss development after recoveries for R&W	18	(72)
Financial Guarantee Accounted for as Insurance Contracts [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,211		1,226
Economic loss development after recoveries for R&W	(33)	66
Financial Guaranty Variable Interest Entities and Other [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	93		91
Economic loss development after recoveries for R&W	2	(4)
Financial Guarantee Accounted for as Credit Derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(6)		$ (14)
Economic loss development after recoveries for R&W	$ 7	$ (15)