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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 1,297	$ 1,326	$ 1,297	$ 1,326	$ 1,244	$ 1,198	$ 1,337	$ 1,391
Economic loss development after recoveries for R&W	47	22	94	81
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,086	1,000	1,086	1,000	1,011	904	903	809
Economic loss development after recoveries for R&W	79	109	198	208
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	1,044	963	1,044	963	970	871	864	771
Economic loss development after recoveries for R&W	78	111	202	209
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	42	37	42	37	41	33	39	38
Economic loss development after recoveries for R&W	1	(2)	(4)	(1)
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	182	192	182	192	197	206	293	409
Economic loss development after recoveries for R&W	(29)	(81)	(51)	(112)
Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(4)	100	(4)	100	1	54	102	99
Economic loss development after recoveries for R&W	(2)	(2)	(55)	2
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	33		33
Economic loss development after recoveries for R&W	1		1
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	33	34	33	34	35	34	39	74
Economic loss development after recoveries for R&W	(1)	(4)	2	(17)
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	211	326	211	326	$ 233	294	$ 434	$ 582
Economic loss development after recoveries for R&W	(32)	(87)	(104)	(127)
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	1,203		1,203			1,083
Economic loss development after recoveries for R&W	55	40	121	101
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	98		98			105
Economic loss development after recoveries for R&W	0	(7)	(4)	(3)
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	(4)		(4)			$ 10
Economic loss development after recoveries for R&W	$ (8)	$ (11)	$ (23)	$ (17)