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Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 USD ($) Entity | Mar. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) Entity
Variable Interest Entities [Line Items] |  |  |
Variable Interest Entity Change in Fair Value of Assets and Liabilities | $ (1,000,000) | $ 0 |
Realized Investment Gains (Losses) | 2,000,000 | 8,000,000 |
Other Operating Income (Expense), Net | 5,000,000 | 0 |
Fair value of special purpose entities | 1,000,000 |  | $ 1,000,000
Variable Interest Entity, Measure of Activity, Income or Loss before Tax | 0 | 3,000,000 |
Long-term debt | $ 2,661,000,000 |  | 2,739,000,000
Sponsored Variable Interest Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Total principal amount of debt outstanding |  |  | $ 410,000,000
Consolidated Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number Of Consolidated Variable Interest Entities | Entity | 6 |  | 6
Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax | $ 1,000,000 | (4,000,000) |
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change | (1,000,000) | (4,000,000) |
Interest Expense, Long-term Debt | (1,000,000) | $ (2,000,000) |
Long-term debt | $ 4,427,000,000 |  | $ 4,493,000,000
Consolidated Variable Interest Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number of DeConsolidated Variable Interest Entities | 0 | 0 |
Ambac UK [Member] | Consolidated Entities [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Number Of Consolidated Variable Interest Entities | Entity | 5 |  | 5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Long-term debt | $ 4,264,000,000 |  | $ 4,324,000,000
Ambac Assurance [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Carry Value Secured Notes from LSNI | 464,000,000 |  | 465,000,000
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Gross Investment Income, Operating | 2,000,000 | $ 2,000,000 |
Realized Investment Gains (Losses) | 1,000,000 | $ 8,000,000 |
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Long-term debt | $ 1,626,000,000 |  | $ 1,641,000,000
5.1% Junior Surplus Notes, General Account Due 2020 | Ambac Assurance Corporation [Member] |  |  |
Variable Interest Entities [Line Items] |  |  |
Note Holders | 1 |  |
5.1% Junior Surplus Notes, General Account Due 2020 | Ambac Assurance Corporation [Member] | Corolla Notes |  |  |
Variable Interest Entities [Line Items] |  |  |
Note Holders | 1 |  |

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