Contracts Accounted for as Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	$ 674	$ 844
Variable Interest Entity, Primary Beneficiary [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	47	55
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	626	922
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	30	40
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	51	(74)
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	677	848
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	(3)	(4)
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	612	901
United States [Member] | Residential Mortgage-Backed Securities (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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	21	(114)
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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	$ 14	$ 21