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, 2021	Dec. 31, 2020
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] | Market Prices of VIE Assets Adjusted for Financial Guarantees Provided or Market Prices of Similar Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, liabilities		$ 292
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			303
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, assets		$ 77
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			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			$ 604
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage		32.00%
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	[1]		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 of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage	[1]	23.00%
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	[1]		(28.00%)
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage		74.00%
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	[1]		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 of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage	[1]	72.00%
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	[1]		109.00%
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 and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage	[1]	60.00%
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	[1]		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 of Similar Liabilities Adjusted for Financial Guarantees Provided to VIE Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range percentage	[1]	55.00%
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	[1]		22.00%

[1] Weighted average represents the total MBIA guarantees as a percentage of total instrument fair value.