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Fair Value of Financial Instruments (Quantitative Information Regarding The Significant Unobservable Inputs For Certain Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
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Discounted Cash Flow [Member] | Equity Investments [Member] | EBITDA Multiples Discount Rate Hard Asset Values Type Certificate Values [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Fair Value, assets |  | $ 115 | [1] | $ 115 | [2]
Variable Interest Entity Primary Beneficiary [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Of VIE Assets Adjusted For Financial Guarantees Provided Or Market Prices Of Similar Liabilities [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Fair value, liabilities |  | 78 |  | 172 |
Variable Interest Entity Primary Beneficiary [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Fair Value, assets |  | $ 32 |  | $ 78 |
Variable Interest Entity Primary Beneficiary [Member] | Minimum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Of VIE Assets Adjusted For Financial Guarantees Provided Or Market Prices Of Similar Liabilities [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage |  | 74.00% |  | 34.00% |
Variable Interest Entity Primary Beneficiary [Member] | Minimum [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage |  | 32.00% |  | 12.00% |
Variable Interest Entity Primary Beneficiary [Member] | Maximum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Of VIE Assets Adjusted For Financial Guarantees Provided Or Market Prices Of Similar Liabilities [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage |  | 74.00% |  | 82.00% |
Variable Interest Entity Primary Beneficiary [Member] | Maximum [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage |  | 32.00% |  | 88.00% |
Variable Interest Entity Primary Beneficiary [Member] | Weighted Average [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Of VIE Assets Adjusted For Financial Guarantees Provided Or Market Prices Of Similar Liabilities [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage |  | 74.00% | [3] | 68.00% | [4]
Variable Interest Entity Primary Beneficiary [Member] | Weighted Average [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member] |  |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Range percentage | [3] | 32.00% |  | 52.00% |
[1] | Range for EBITDA multiples, discount rate, hard asset values and type certificate values reflects their potential variability.
[2] | Range for EBITDA multiples, discount rate, hard asset values and type certificate values reflects their potential variability.
[3] | Weighted average represents the total MBIA guarantees as a percentage of total instrument fair value.
[4] | Weighted average represents the total MBIA guarantees as a percentage of total instrument fair value.

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