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Investments - Major Categories of the Company's Net Investment Income (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
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] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | $ 31,418 | $ 30,383 | $ 29,753
Investment expenses |  |  |  |  |  |  |  |  | (966) | (1,102) | (1,647)
Net investment income | $ 8,056 | $ 7,884 | $ 7,303 | $ 7,209 | $ 7,312 | $ 7,788 | $ 7,471 | $ 6,710 | 30,452 | 29,281 | 28,106
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | 28,762 | 28,961 | 27,837
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | 448 | 201 |
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | $ 2,208 | 1,117 | 348
Other Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  |  | $ 104 | $ 1,568

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- Definition Amount, before investment expense, of income earned from investments in securities, real estate, and alternate investments. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References No definition available. + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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