Contracts Accounted for as Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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	$ 295	$ 674
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	33	47
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	333	626
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	40	30
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	(38)	51
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	295	677
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	0	(3)
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	328	612
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	(78)	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]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	$ 5	$ 14