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Fair Value Of Financial Instruments (Gains And Losses On Fair Value Option Included In The Company's Consolidated Statements Of Operations) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange |  | $ 5 | $ (11) | $ 18 | $ (55)
Other net realized gains (losses) |  | 1 | (1) | 0 | 36
Non Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange |  | 5 | (11) | 18 | (55)
Other net realized gains (losses) |  | 1 | (1) | 0 | 36
Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange |  | 12 | 21 | 29 | 2
Other net realized gains (losses) |  | (33) | 0 | (126) | 28
Variable Interest Entity [Member] | Loan Repurchase Commitments [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | 0 | (1) | 9 | 3
Investments Carried At Fair Value [Member] | Non Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [2] | 1 | 2 | (4) | 8
Fixed Maturity Securities Held At Fair Value - VIE [Member] | Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | (7) | (2) | (19) | (16)
Residential Mortgage Loans [Member] | Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | (3) | (55) | (79) | (157)
Corporate Loans [Member] | Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | 0 | (2) | 11 | 4
Other Assets [Member] | Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | 1 | 0 | 1 | 0
Medium Term Notes [Member] | Non Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [2] | 7 | (4) | 14 | (26)
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | [1] | 23 | 70 | 106 | 160
Other Liabilities [Member] | Non Variable Interest Entity [Member] |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |
Other net realized gains (losses) | [3] | $ 0 | $ (1) | $ (2) | $ (1)
[1] | Reported within Net gains (losses) of financial instruments at fair value and foreign exchange-VIE on MBIA's consolidated statements of operations.
[2] | Reported within Net gains (losses) of financial instruments at fair value and foreign exchange on MBIA's consolidated statements of operations.
[3] | Reported within "Other net realized gains (losses)" on MBIA's consolidated statements of operations.

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