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, 2017	Mar. 31, 2016	Dec. 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	$ 1,244	$ 1,337	$ 1,198	$ 1,391
Economic loss development after recoveries for R&W	47	59
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	1,011	903	904	809
Economic loss development after recoveries for R&W	119	99
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	970	864	871	771
Economic loss development after recoveries for R&W	124	98
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	41	39	33	38
Economic loss development after recoveries for R&W	(5)	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	197	293	206	409
Economic loss development after recoveries for R&W	(22)	(31)
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	1	102	54	99
Economic loss development after recoveries for R&W	(53)	4
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	34
Economic loss development after recoveries for R&W	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	35	39	34	74
Economic loss development after recoveries for R&W	3	(13)
Structured Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	233	434	294	$ 582
Economic loss development after recoveries for R&W	(72)	(40)
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	1,141		1,083
Economic loss development after recoveries for R&W	66	61
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	99		105
Economic loss development after recoveries for R&W	(4)	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	4		$ 10
Economic loss development after recoveries for R&W	$ (15)	$ (6)