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Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 USD ($) Entity | Mar. 31, 2019 USD ($) | Dec. 31, 2019 USD ($) Entity
Variable Interest Entities [Line Items] |  |  |
Variable Interest Entity Change in Fair Value of Assets and Liabilities | $ 0 | $ 2 |
Realized Investment Gains (Losses) | 8 | 17 |
Equity Method Investments | 47 |  | $ 46
Deconsolidated VIE Assets |  | 292 |
Deconsolidated VIE Liabilities |  | 364 |
Fair value of special purpose entities | 3 |  | 3
variable interest entities deconsolidation gain loss | 0 | 15 |
Variable Interest Entity, Measure of Activity, Income or Loss before Tax | 3 | 16 |
Other investments | 363 |  | 478
Long-term debt | $ 2,760 |  | 2,822
Asset-Backed Securities and Utility Obligations [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Weighted average life | 10 months 24 days |  |
Average rating of assets held by sponsored special purpose entities | BBB+ |  |
Sponsored Variable Interest Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Total principal amount of debt outstanding | $ 378 |  | 403
Variable Interest Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax | (4) | 0 |
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change | (4) | 2 |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Interest Expense, Long-term Debt | (2) | (2) |
Other Expenses | 0 | $ 0 |
Long-term debt | $ 4,263 |  | $ 4,554
Consolidated Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number of Newly Consolidated Variable Interest Entities | 0 | 0 |
Number of DeConsolidated Variable Interest Entities | 0 | 0 |
Number Of Consolidated Variable Interest Entities | Entity | 7 |  | 7
Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number of DeConsolidated Variable Interest Entities | 0 | 0 |
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Long-term debt | $ 4,092 |  | $ 4,351
Ambac UK [Member] | Consolidated Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number Of Consolidated Variable Interest Entities | Entity | 6 |  | 6
Ambac Assurance [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Carry Value Secured Notes from LSNI | $ 495 |  | $ 535
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Gross Investment Income, Operating | 2 | $ 2 |
Realized Investment Gains (Losses) | 8 | $ 0 |
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Long-term debt | $ 1,685 |  | $ 1,763

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