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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ in Millions |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Oct. 10, 2017 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) segments | Sep. 30, 2016 USD ($) | Nov. 01, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |
Number of operating segments | segments |  |  |  | 3 |  |
Income (loss) before income taxes |  | $ (273) | $ 55 | $ (603) | $ (101) |
Proceeds from Senior Lenders |  |  |  | 325 |  |
Valuation allowance taken in current period |  |  |  | 1,200 |  |
Zohar II [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Claims payments |  |  |  | 770 |  |
Puerto Rico [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Claims payments |  |  |  | 242 |  |
Mbia Inc [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Proceeds from subordinated financing |  |  |  | 38 |  |
Mbia Corp [Member] | Zohar II [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Claims payments |  |  |  | 60 |  |
National [Member] | SubsequentEvent [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Dividends from subsidiaries | $ 118 |  |  |  |  |
Assured Guaranty Corp [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Payment for sale of subsidiary |  |  |  | 23 |  |
Assured Guaranty Corp [Member] | Zohar II [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Outstanding bonds |  | $ 347 |  | 347 |  |
MBIA UK [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Gain recorded on adjustment of carrying value to fair value less costs to sell |  |  |  | $ 5 |  |
Income (loss) before income taxes |  |  | $ 9 |  | $ 32 |
Sale effective date |  |  |  | Jan. 10, 2017 |  |
MBIA Inc Senior Notes [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Long-term debt, interest rate |  | 5.70% |  | 5.70% |  |
Long-term debt, maturity year |  |  |  | 2034 |  |
MBIA Inc Senior Notes [Member] | National [Member] | SubsequentEvent [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Senior notes at par value owned by National |  |  |  |  |  | $ 130
MBIA Corp Surplus Notes [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Long-term debt, interest rate |  | 14.00% |  | 14.00% |  |

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