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Quarterly Results for 2018 and 2017 - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized gains (losses) of equity securities | $ (8,000) |  |  |  |  |  |  |  | $ (17,169) | $ 0 | $ 0
Net income (loss) | $ (6,468) | $ 37,380 | $ 46,084 | $ 40,055 | $ 36,396 | $ 9,964 | $ 29,376 | $ 31,199 | $ 117,051 | $ 106,935 | $ 99,410

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