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Investments - Additional Information (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Investment [Line Items] |  |
Other investments | $ 428,556 | $ 391,217
Securities fair value | 6,046 | 5,975
Ambac Assurance [Member] |  |
Investment [Line Items] |  |
Unsettled Amounts Secured Notes from LSNI | 0 | 19,405
Ambac UK [Member] |  |
Investment [Line Items] |  |
Asset Backed Securities Fair Value Disclosure | 144,530 | 144,672
Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other investments | 207,704 | 177,357
Reported Value Measurement [Member] | Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other investments | $ 53,135 | $ 27,154

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