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Investments - Additional Information (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Investment [Line Items] |  |
Other investments | $ 469,438 | $ 391,217
Securities fair value | 7,613 | 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 | 0 | 144,672
Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other investments | 233,904 | 177,357
Reported Value Measurement [Member] | Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other investments | 78,998 | $ 27,154
Commitments [Member] | Partnership Interest [Member] |  |
Investment [Line Items] |  |
Other investments | $ 25,000 |

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