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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Parenthetical) (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Ambac UK [Member] |  |
Investment [Line Items] |  |
Asset-backed securities fair value | $ 170,280 | $ 118,813

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- Definition Asset-backed securities guaranteed by Ambac UK. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |