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Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Marketable Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | $ 72,395 | $ 55,090 | $ 15,028
Gross realized investment gains |  |  |  |  |  |  |  |  | 1,017 | 176 | 239
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (18) | (68) | (1,354)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 999 | 108 | (1,115)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | 133 | (188) | (421)
Net realized gains (losses) on investments | $ (332) | $ 309 | $ 163 | $ 992 | $ (53) | $ (4) | $ (82) | $ 59 | 1,132 | (80) | (1,536)
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 25,436 | 33,113 | 15,025
Gross realized investment gains |  |  |  |  |  |  |  |  | 711 | 140 | 238
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (14) | (68) | (1,354)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 697 | 72 | (1,116)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (43) | (196) | (144)
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | 654 | (124) | (1,260)
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  |  |  | 3
Gross realized investment gains |  |  |  |  |  |  |  |  |  |  | 1
Net realized investment gains (losses) |  |  |  |  |  |  |  |  |  |  | 1
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  |  |  | 1
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 46,959 | 21,977 |
Gross realized investment gains |  |  |  |  |  |  |  |  | 306 | 36 |
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (4) |  |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 302 | 36 |
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | 176 | 8 | (277)
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | $ 478 | $ 44 | $ (277)

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