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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, 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
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | $ 15,028 | $ 27,764 | $ 59,339
Gross realized investment gains |  |  |  |  |  |  |  |  | 239 | 486 | 891
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (1,354) | (5) | (1,728)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | (1,115) | 481 | (837)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (421) | (1,128) | 343
Net realized gains (losses) on investments | $ (65) | $ (329) | $ (1,111) | $ (31) | $ 114 | $ (192) | $ (388) | $ (181) | (1,536) | (647) | (494)
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 15,025 | 14,591 | 54,730
Gross realized investment gains |  |  |  |  |  |  |  |  | 238 | 485 | 823
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (1,354) | (5) | (1,728)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | (1,116) | 480 | (905)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (144) | (520) | 274
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | (1,260) | (40) | (631)
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 3 | 1 |
Gross realized investment gains |  |  |  |  |  |  |  |  | 1 | 1 |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 1 | 1 |
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | 1 | 1 |
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  |  | 13,172 | 4,609
Gross realized investment gains |  |  |  |  |  |  |  |  |  |  | 68
Net realized investment gains (losses) |  |  |  |  |  |  |  |  |  |  | 68
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (277) | (608) | 69
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | $ (277) | $ (608) | $ 137

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