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Background and Business Description - Additional Information (Detail) | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 USD ($) Segment $ / shares shares | Sep. 30, 2014 USD ($) | Dec. 31, 2014 $ / shares shares | Apr. 30, 2013 $ / shares shares
Background And Basis Of Presentation [Line Items] |  |  |  |
Entity Incorporation, Date of Incorporation | Apr. 29, 1991 |  |  |
Number of reportable segments | Segment | 2 |  |  |
Investment Warrants Expiration Date |  |  | Apr. 30, 2023 |
Payment of permitted policy claim (percent) |  |  | 25.00% |
Common stock, shares authorized | 130,000,000 |  | 130,000,000 |
Common Stock, Par or Stated Value Per Share | $ / shares | $ 0.01 |  | $ 0.01 | $ 0.01
Preferred stock, shares authorized | 20,000,000 |  | 20,000,000 |
Preferred Stock, Par or Stated Value Per Share | $ / shares | $ 0.01 |  | $ 0.01 |
Common stock, shares issued | 45,036,587 |  | 45,005,932 |
Warrants Distributed In Bankruptcy |  |  |  | 5,047,138
Warrants issued exercise price | $ / shares |  |  |  | $ 16.67
Ambac Assurance [Member] |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |
Net par exposure for policies allocated to the Segregated Account | $ | $ 16,595,339,000 |  |  |
Minimum surplus amount | $ | 100,000,000 |  |  |
Statutory Accounting Practices, Statutory Capital and Surplus, Balance | $ | 350,559,000 |  |  |
Successor [Member] |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |
Proceeds from the sale of Junior Surplus Notes of the Segregated Account | $ | $ 0 | $ 224,262,000 |  |
Common stock, shares issued |  |  |  | 45,000,000
Warrants Distributed In Bankruptcy |  |  |  | 5,047,138

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