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Variable Interest Entities - Additional Information (Detail) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 USD ($) Entity | Dec. 31, 2018 USD ($) Entity | Dec. 31, 2017 USD ($) | Feb. 12, 2018 USD ($)
Variable Interest Entities [Line Items] |  |  |  |
Fixed income securities, amortized cost | $ 3,187 | $ 3,451 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 132 | 133 |  |
satisfaction and discharge of segregated account deferred amounts |  |  |  | $ 3,000
satisfaction and discharge of segregated account deferred interest amounts |  |  |  | 857
Available-for-sale Securities | 3,314 |  |  |
Restricted Cash | 55 | 19 |  |
Fair value of special purpose entities | 3 | 5 |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets | 292 |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities | 364 |  |  |
Equity Method Investments | 46 | 40 |  |
Loans, Unpaid principal balance |  | 19 | $ 19 |
Other Investments | 478 | 391 |  |
Deconsolidated VIE Assets | 1,233 |  |  |
Deconsolidated VIE Liabilities | 1,230 |  |  |
Long-term debt | 2,822 | 2,929 |  |
Derivative assets | 75 | 59 |  |
Other assets | 95 | 138 |  |
Assets | 13,320 | 14,589 |  |
Accrued interest payable | 441 | 376 |  |
Derivative liabilities | 90 | 77 |  |
Other liabilities | 93 | 64 |  |
Liabilities | $ 11,783 | 12,956 |  |
Asset-Backed Securities and Utility Obligations [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Weighted average life | 1 year 1 month 6 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 | $ 403 | 393 |  |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | 3,121 | 2,737 |  |
Restricted Cash | 2 | 1 | $ 1 |
Long-term Debt, Fair Value | 4,351 | 5,269 |  |
Long-term debt | 4,554 | 5,269 |  |
Financing Receivable, after Allowance for Credit Loss | 3,108 | 4,288 |  |
Derivative assets | 52 | 66 |  |
Other assets | 3 | 1 |  |
Assets | 6,286 | 7,093 |  |
Accrued interest payable | 1 | 1 |  |
Long-term Debt, at par less amortized discount | 203 | 0 |  |
Derivative liabilities | 1,657 | 1,712 |  |
Liabilities | $ 6,212 | $ 6,981 |  |
Consolidated Entities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of DeConsolidated Variable Interest Entities | 1 | 4 | 1 |
Number of consolidated Variable Interest Entities | Entity | 7 | 7 |  |
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | $ 2,957 | $ 2,737 |  |
Restricted Cash | 1 | 1 |  |
Long-term Debt, Fair Value | 4,351 | 5,269 |  |
Long-term debt | 4,351 | 5,269 |  |
Financing Receivable, after Allowance for Credit Loss | 3,108 | 4,288 |  |
Derivative assets | 52 | 66 |  |
Other assets | 1 | 1 |  |
Assets | 6,119 | 7,093 |  |
Accrued interest payable | 1 | 1 |  |
Long-term Debt, at par less amortized discount | 0 | 0 |  |
Derivative liabilities | 1,657 | 1,712 |  |
Liabilities | $ 6,009 | $ 6,981 |  |
Ambac UK [Member] | Consolidated Entities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of consolidated Variable Interest Entities | Entity | 6 | 7 |  |
Ambac Assurance [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Secured Notes Received from Ambac LSNI |  |  |  | 644
Unsettled Amounts Secured Notes from LSNI | $ 55 | $ 19 |  |
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | 164 | 0 |  |
Restricted Cash | 1 | 0 |  |
Long-term Debt, Fair Value | 0 | 0 |  |
Long-term debt | 203 | 0 |  |
Financing Receivable, after Allowance for Credit Loss | 0 | 0 |  |
Derivative assets | 0 | 0 |  |
Other assets | 2 | 0 |  |
Assets | 167 | 0 |  |
Accrued interest payable | 0 | 0 |  |
Long-term Debt, at par less amortized discount | 203 | 0 |  |
Derivative liabilities | 0 | 0 |  |
Liabilities | $ 203 | $ 0 |  |
Ambac Assurance [Member] | Consolidated Entities [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Number of consolidated Variable Interest Entities | Entity | 1 | 0 |  |
AFG [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Secured Notes Received from Ambac LSNI |  |  |  | $ 125
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | $ 2,957 | $ 2,737 |  |
Corporate Debt Securities [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | 2,957 | 2,737 |  |
Corporate Debt Securities [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | 0 | 0 |  |
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Fixed income securities, amortized cost | 139 |  |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 25 |  |  |
Available-for-sale Securities | 164 | 0 |  |
Municipal Bonds [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | 0 | 0 |  |
Municipal Bonds [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Variable Interest Entities [Line Items] |  |  |  |
Available-for-sale Securities | $ 164 | $ 0 |  |

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