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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions |  |  | 9 Months Ended
--- | --- | --- | ---
Nov. 02, 2018 USD ($) | Oct. 11, 2018 USD ($) | Sep. 30, 2018 USD ($) segments $ / shares shares
Business Acquisition [Line Items] |  |  |
Number of operating segments | segments |  |  | 3
Warrants outstanding | shares |  |  | 11,850,000
Warrant exercise price | $ / shares |  |  | $ 9.59
Treasury shares issued (in shares) | shares |  |  | 1,300,000
Zohar [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Aggregate claim amount |  |  | $ 919
Puerto Rico [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Claims payments |  |  | $ 276
SubsequentEvent [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Increase in MBIA Inc's liquidity | $ 41 |  |
National [Member] | SubsequentEvent [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Dividends from subsidiaries |  | $ 108 |
MBIA Inc. Senior Notes [Member] | SubsequentEvent [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Long-term debt, interest rate | 5.70% |  |
Long-term debt, maturity year | 2034 |  |
Mbia Inc. Debentures [Member] | SubsequentEvent [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Long-term debt, interest rate | 7.00% |  |
Long-term debt, maturity year | 2025 |  |

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