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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 | Jun. 30, 2021 | Dec. 31, 2020
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Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value, liabilities |  |  | $ 49
Minimum [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range dollars |  |  | 49
Maximum [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range dollars |  |  | 49
Weighted Average [Member] | Cash Flows [Member] | Discounted cash flow [Member] | Other Derivatives Liabilities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range dollars |  |  | 49
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value, liabilities |  | $ 290 | 303
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, assets | [1] | $ 129 | 120
Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] | Recovery Rates And Breach Rates [Member] | Discounted cash flow [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, assets | [2] |  | $ 604
Variable Interest Entity Primary Beneficiary [Member] | Minimum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To VIE Obligations [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | 31.00% | 30.00%
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | (34.00%) | (28.00%)
Variable Interest Entity Primary Beneficiary [Member] | Maximum [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To VIE Obligations [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | 72.00% | 73.00%
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | 69.00% | 109.00%
Variable Interest Entity Primary Beneficiary [Member] | Weighted Average [Member] | Variable Interest Entity Notes [Member] | Impact Of Financial Guarantee [Member] | Market Prices Adjusted For Financial Guarantees Provided To VIE Obligations [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | 58.00% | 57.00%
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Range percentage | [3] | 21.00% | 22.00%
[1] | Negative percentage represents financial guarantee policies in a receivable position.
[2] | Recovery value reflects an estimate of the amount to be awarded to the Company as part of litigation seeking to enforce its contractual rights.
[3] | Weighted average represents the total MBIA guarantees as a percentage of total instrument fair value.

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