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Fair Value of Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Parenthetical) (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Net change in unrealized gains (losses) on investments, net of tax | $ (82) | $ 1,013 | $ 1,081

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