Contracts Accounted for as Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	$ 1,119	$ 1,047
Salvage and Subrogation Recoverable, net	387	347
Net Reserve (Recoverable)	732	700
Financial Guarantee Insurance And Other Product Line [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	1,118
Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	1,177	1,111
Salvage and Subrogation Recoverable, net	387	347
Net Reserve (Recoverable)	790	764
Financial Guarantee Insurance And Other Product Line [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	(58)	(64)
Salvage and Subrogation Recoverable, net	0	0
Net Reserve (Recoverable)	(58)	(64)
Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	825	732
Salvage and Subrogation Recoverable, net	96	86
Net Reserve (Recoverable)	729	646
Triple-X Life Insurance Transaction [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	20	36
Salvage and Subrogation Recoverable, net	29	0
Net Reserve (Recoverable)	(9)	36
Other structured finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	59	60
Salvage and Subrogation Recoverable, net	0	0
Net Reserve (Recoverable)	59	60
Structured Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	352	379
Salvage and Subrogation Recoverable, net	289	262
Net Reserve (Recoverable)	63	117
Financial Guarantee [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	1,177	1,111
Salvage and Subrogation Recoverable, net	385	348
Net Reserve (Recoverable)	792	763
Other [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	0	0
Salvage and Subrogation Recoverable, net	2	(1)
Net Reserve (Recoverable)	(2)	1
United States [Member] | Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	809	711
Salvage and Subrogation Recoverable, net	96	86
Net Reserve (Recoverable)	713	625
United States [Member] | RMBS [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	273	283
Salvage and Subrogation Recoverable, net	260	262
Net Reserve (Recoverable)	13	21
Non United States [Member] | Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE Reserve, net	16	21
Salvage and Subrogation Recoverable, net	0	0
Net Reserve (Recoverable)	$ 16	$ 21