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Variable Interest Entities - Additional Information (Detail) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) Entity | Dec. 31, 2020 USD ($) Entity | Dec. 31, 2019 USD ($) Entity | Dec. 31, 2018 USD ($) | Feb. 12, 2018 USD ($)
Variable Interest Entities [Line Items] |  |  |  |  |
Fixed income securities, amortized cost | $ 2,807,000,000 | $ 2,807,000,000 | $ 3,187,000,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 149,000,000 | 149,000,000 | 132,000,000 |  |
satisfaction and discharge of segregated account deferred amounts |  |  |  |  | $ 3,000,000,000
satisfaction and discharge of segregated account deferred interest amounts |  |  |  |  | 857,000,000
Available-for-sale Securities | 2,949,000,000 | 2,949,000,000 | 3,314,000,000 |  |
Restricted Cash | 13,000,000 | 13,000,000 | 55,000,000 | $ 19,000,000 |
Fair value of special purpose entities | 1,000,000 | 1,000,000 | 3,000,000 |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets | 292,000,000 | 292,000,000 |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities | 364,000,000 | 364,000,000 |  |  |
Equity Method Investments | 51,000,000 | 51,000,000 | 46,000,000 |  |
Other Investments | 595,000,000 | 595,000,000 | 478,000,000 |  |
Deconsolidated VIE Assets | 0 | 0 |  |  |
Deconsolidated VIE Liabilities | 0 | 0 |  |  |
Long-term debt | 2,739,000,000 | 2,739,000,000 | 2,822,000,000 |  |
Derivative assets | 93,000,000 | 93,000,000 | 75,000,000 |  |
Other assets | 114,000,000 | 114,000,000 | 95,000,000 |  |
Assets | 13,220,000,000 | 13,220,000,000 | 13,320,000,000 |  |
Accrued interest payable | 517,000,000 | 517,000,000 | 441,000,000 |  |
Derivative liabilities | 114,000,000 | 114,000,000 | 90,000,000 |  |
Other liabilities | 106,000,000 | 106,000,000 | 93,000,000 |  |
Liabilities | 12,074,000,000 | $ 12,074,000,000 | 11,783,000,000 |  |
Asset-Backed Securities and Utility Obligations [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Weighted average life |  | 2 months 12 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 | $ 410,000,000 | $ 410,000,000 | $ 403,000,000 |  |
Consolidated Entities [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of DeConsolidated Variable Interest Entities |  | 1 | 1 | 4 |
Number of consolidated Variable Interest Entities | Entity | 6 | 6 | 7 |  |
Ambac UK [Member] | Consolidated Entities [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of consolidated Variable Interest Entities | Entity | 5 | 5 | 6 |  |
Ambac Assurance [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Restricted Cash | $ 9,000,000 | $ 9,000,000 | $ 55,000,000 |  |
Secured Notes Received from Ambac LSNI |  |  |  |  | 644,000,000
Ambac Assurance [Member] | Consolidated Entities [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Number of consolidated Variable Interest Entities | Entity | 1 | 1 | 1 |  |
AFG [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Secured Notes Received from Ambac LSNI |  |  |  |  | $ 125,000,000
Ambac Assurance Corporation [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Long-term debt | $ 2,739,000,000 | $ 2,739,000,000 | $ 2,822,000,000 |  |
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member] |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Long-term debt | $ 247,000,000 | $ 247,000,000 | $ 252,000,000 |  |
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Notes |  |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |  |
Note Holders | 1 |  |  |  |

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