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Investments - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2018 | Jun. 30, 2018 | Dec. 31, 2017
Investment [Line Items] |  |  |  |
Other Investments |  | $ 391,217 |  | $ 431,630
Cash recoveries | $ 995,000 |  |  |
Securities fair value |  | 5,975 |  | 5,974
Fair value of securities held by bankruptcy remote trust |  | 0 |  | 346,212
Market Value of Secured Note Collateral |  | 209,983 |  |
Real Estate [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 16,123 |  | 33,154
Hedge Funds, Multi-strategy [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 0 |  | 53,054
Interest Rate Contract [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 177,357 |  | 136,603
Illiquid Investments [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 84,297 |  | 67,787
Insurance Linked [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 29,318 |  | 22,666
Equity [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 43,954 |  | 53,675
Equity investments in pooled funds [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  | 351,049 |  | 366,939
Reported Value Measurement [Member] | Interest Rate Contract [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Other Investments |  |  | $ 27,154 | 2,823
Investment Portfolio [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties |  | 102,904 |  | $ 120,645
Ambac Assurance [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Unsettled Amounts Secured Notes from LSNI |  | $ 19,405 |  |

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