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, 2020		Dec. 31, 2019
Cash Flows [Member] | Other Derivatives Liabilities [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, liabilities		$ 37		$ 34
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, liabilities		$ 8		$ 7
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%		54.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%		54.00%
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] | Direct Price Model [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage		54.00%		54.00%
Other Derivatives [Member] | Cash Flows [Member] | Other Derivatives Liabilities [Member] | Minimum [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage	[1]	0.00%		0.00%
Other Derivatives [Member] | Cash Flows [Member] | Other Derivatives Liabilities [Member] | Maximum [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage	[1]	49.00%		49.00%
Other Derivatives [Member] | Cash Flows [Member] | Other Derivatives Liabilities [Member] | Weighted Average [Member] | Discounted cash flow [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Range percentage	[1]	25.00%		25.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [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		$ 98		$ 136
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [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%)
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [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		161.00%	[2]	99.00%
Loans Receivable and Other Instruments at Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [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		19.00%	[2]	22.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [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		$ 506		$ 486
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [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		$ 281		$ 347
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [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		34.00%	[2]	37.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [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 Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [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		60.00%	[2]	61.00%

[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.