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Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | $ 27,764,000 | $ 59,339,000 | $ 4,432,000
Gross realized investment gains |  |  |  |  |  |  |  |  | 486,000 | 891,000 | 177,000
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (5,000) | (1,728,000) | (162,000)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 481,000 | (837,000) | 15,000
Impairments |  |  |  |  |  |  |  |  | 0 |  | (2,653,000)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (1,128,000) | 343,000 | 144,000
Net realized gains (losses) on investments | $ 114,000 | $ (192,000) | $ (388,000) | $ (181,000) | $ (1,468,000) | $ 181,000 | $ 545,000 | $ 248,000 | (647,000) | (494,000) | (2,494,000)
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 14,591,000 | 54,730,000 | 4,432,000
Gross realized investment gains |  |  |  |  |  |  |  |  | 485,000 | 823,000 | 177,000
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (5,000) | (1,728,000) | (162,000)
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 480,000 | (905,000) | 15,000
Impairments |  |  |  |  |  |  |  |  |  |  | (2,653,000)
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (520,000) | 274,000 | 102,000
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | (40,000) | (631,000) | (2,536,000)
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 1,000 |  |
Gross realized investment gains |  |  |  |  |  |  |  |  | 1,000 |  |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 1,000 |  |
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | 1,000 |  |
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 13,172,000 | 4,609,000 |
Gross realized investment gains |  |  |  |  |  |  |  |  |  | 68,000 |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  |  | 68,000 |
Other, including gains (losses) on calls and redemptions |  |  |  |  |  |  |  |  | (608,000) | 69,000 | 42,000
Net realized gains (losses) on investments |  |  |  |  |  |  |  |  | $ (608,000) | $ 137,000 | $ 42,000

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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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