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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, 2020 | Dec. 31, 2019
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Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, liabilities |  | $ 38 |  | $ 34
Minimum [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage | [1] | 0.00% |  | 0.00%
Maximum [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage | [1] | 49.00% |  | 49.00%
Weighted Average [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage | [1] | 25.00% |  | 25.00%
Variable Interest Entity Primary Beneficiary [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, liabilities |  | $ 294 |  | $ 347
Variable Interest Entity Primary Beneficiary [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To Vie Obligations [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value, assets |  | 114 |  | 136
Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] | Recovery Rates And Breach Rates [Member] | Discounted cash flow [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair Value, assets |  | $ 530 |  | $ 486
Variable Interest Entity Primary Beneficiary [Member] | Minimum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 29.00% | [2] | 37.00%
Variable Interest Entity Primary Beneficiary [Member] | Minimum [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To Vie Obligations [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | (28.00%) | [2] | (20.00%)
Variable Interest Entity Primary Beneficiary [Member] | Maximum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 74.00% | [2] | 76.00%
Variable Interest Entity Primary Beneficiary [Member] | Maximum [Member] | Loans Receivable and Other Instruments at Fair Value [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To Vie Obligations [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 110.00% | [2] | 99.00%
Variable Interest Entity Primary Beneficiary [Member] | Weighted Average [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 59.00% |  | 61.00%
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 Adjusted For Financial Guarantees Provided To Vie Obligations [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 20.00% | [2] | 22.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Direct Price Model [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, liabilities |  | $ 8 |  | $ 7
Credit Derivatives [Member] | Minimum [Member] | Nonperformance Risk [Member] | Direct Price Model [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 54.00% |  | 54.00%
Credit Derivatives [Member] | Maximum [Member] | Nonperformance Risk [Member] | Direct Price Model [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 54.00% |  | 54.00%
Credit Derivatives [Member] | Weighted Average [Member] | Nonperformance Risk [Member] | Direct Price Model [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |  |
Range percentage |  | 54.00% |  | 54.00%
[1] | Midpoint of cash flows are used for the weighted average.
[2] | Weighted average represents the total MBIA guarantees as a percentage of total instrument fair value.

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