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Investments - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2014
Investment [Line Items] |  |  |  |  |
Payment of permitted policy claim (percent) |  |  |  |  | 25.00%
Future payment of permitted policy claim (percent) |  |  |  |  | 45.00%
Securities fair value | $ 6,831 |  | $ 6,831 |  | $ 6,790
Fair Value of Securities Placed in Trust | 403,106 |  | 403,106 |  |
Successor [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Unrealized losses on securities | 58,351 |  | 58,351 |  | 20,081
Other investments | (1,424) | $ 700 | 10,702 | $ 5,905 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,181 |  | 71,181 |  | 32,269
Below Investment Grade Securities and Non-Rated Securities [Member] | Successor [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Fair value of below investment grade securities and non-rated securities | 788,174 |  | 788,174 |  | 473,656
Gross unrealized loss | 44,497 |  | 44,497 |  | 16,001
Ambac Wrapped Securities [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 40,877 |  | 40,877 |  |
Gains (losses) on securities held as of reporting date [Member] | Successor [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Other investments | (3,482) | $ 329 | (41) | $ 5,965 |
Ambac UK [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Asset Backed Securities Fair Value Disclosure | 78,925 |  | $ 78,925 |  | 51,395
National Century Financial Enterprises, Inc. [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Cash recoveries | $ 0 |  |  |  | $ 35

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- Definition Percentage of the Segregated Account of Ambac Assurance's permitted policy claims that are to be paid in cash from July 2014 forward as approved by the Rehabilitation Court. + References No definition available. + Details
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