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Investments - Gross Unrealized Gains and Losses and Cost or Amortized Cost and Fair Value of Investments Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | $ 473,315 | $ 461,236
Fixed maturity, Gross Unrealized Gains | 2,377 | 7,072
Fixed maturity, Gross Unrealized Losses | (7,673) | (2,714)
Fixed maturity, Fair Value | 468,019 | 465,594
Equity securities, Cost or Amortized Cost | 14,532 | 8,503
Equity securities, Gross Unrealized Gains |  | 779
Equity securities, Fair Value | 14,791 | 9,282
Totals, Cost or Amortized Cost |  | 469,739
Totals, Gross Unrealized Gains |  | 7,851
Totals, Gross Unrealized Losses |  | (2,714)
Totals, Fair Value |  | 474,876
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 229,007 | 244,898
Fixed maturity, Gross Unrealized Gains | 2,334 | 6,819
Fixed maturity, Gross Unrealized Losses | (3,854) | (577)
Fixed maturity, Fair Value | 227,487 | 251,140
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 163,834 | 130,210
Fixed maturity, Gross Unrealized Gains | 43 | 224
Fixed maturity, Gross Unrealized Losses | (929) | (212)
Fixed maturity, Fair Value | 162,948 | 130,222
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 13,256 | 18,813
Fixed maturity, Gross Unrealized Losses | (476) | (799)
Fixed maturity, Fair Value | 12,780 | 18,014
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 67,218 | 67,315
Fixed maturity, Gross Unrealized Gains |  | 29
Fixed maturity, Gross Unrealized Losses | (2,414) | (1,126)
Fixed maturity, Fair Value | $ 64,804 | $ 66,218

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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