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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Jan. 10, 2017	Dec. 31, 2016	Jul. 01, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 1,292	$ 1,090	$ 1,292	$ 1,090	$ 1,297	$ 21	$ 1,198	$ 22	$ 1,326	$ 1,391
Economic loss development after recoveries for R&W	204	(44)	298	37
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,093	854	1,093	854	1,086		904		1,000	809
Economic loss development after recoveries for R&W	229	8	427	216
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	1,046	816	1,046	816	1,044		871		963	771
Economic loss development after recoveries for R&W	229	9	431	218
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	47	38	47	38	42		33		37	38
Economic loss development after recoveries for R&W	0	(1)	(4)	(2)
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	176	148	176	148	182		206		192	409
Economic loss development after recoveries for R&W	(19)	(27)	(70)	(139)
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	(7)	54	(7)	54	(4)		54		100	99
Economic loss development after recoveries for R&W	(1)	(23)	(56)	(21)
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	34		34
Economic loss development after recoveries for R&W	1		2
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	30	34	30	34	33	8	34	2	34	74
Economic loss development after recoveries for R&W	(5)	(2)	(3)	(19)
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	199	236	199	236	$ 211	$ 8	294	$ (20)	$ 326	$ 582
Economic loss development after recoveries for R&W	(25)	(52)	(129)	(179)
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	1,205		1,205				1,083
Economic loss development after recoveries for R&W	207	(35)	328	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		93				105
Economic loss development after recoveries for R&W	(2)	(3)	(6)	(6)
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)		(6)				$ 10
Economic loss development after recoveries for R&W	$ (1)	$ (6)	$ (24)	$ (23)