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		Mar. 31, 2023	Dec. 31, 2022
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	$ 115
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		154	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		$ 83	$ 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	[1]	44.00%	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	[1]	13.00%	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	[1]	72.00%	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	[1]	98.00%	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	[1]	68.00%	68.00%
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 Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage	[1]	58.00%	52.00%

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