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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 | Jun. 30, 2018 | Dec. 31, 2017
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Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | $ 443,358 | $ 461,236
Fixed maturity, Gross Unrealized Gains | 3,428 | 7,072
Fixed maturity, Gross Unrealized Losses | (6,024) | (2,714)
Fixed maturity, Fair Value | 440,762 | 465,594
Equity securities, Cost or Amortized Cost | 13,590 | 8,503
Equity securities, Gross Unrealized Gains |  | 779
Equity securities, Gross Unrealized Losses |  |
Equity securities, Fair Value | 13,590 | 9,282
Totals, Cost or Amortized Cost | 456,948 | 469,739
Totals, Gross Unrealized Gains | 3,428 | 7,851
Totals, Gross Unrealized Losses | (6,024) | (2,714)
Totals, Fair Value | 454,352 | 474,876
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 217,427 | 244,898
Fixed maturity, Gross Unrealized Gains | 3,368 | 6,819
Fixed maturity, Gross Unrealized Losses | (2,495) | (577)
Fixed maturity, Fair Value | 218,300 | 251,140
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 144,950 | 130,210
Fixed maturity, Gross Unrealized Gains | 60 | 224
Fixed maturity, Gross Unrealized Losses | (1,051) | (212)
Fixed maturity, Fair Value | 143,959 | 130,222
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 13,730 | 18,813
Fixed maturity, Gross Unrealized Losses | (368) | (799)
Fixed maturity, Fair Value | 13,362 | 18,014
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Fixed maturity, Cost or Amortized Cost | 67,251 | 67,315
Fixed maturity, Gross Unrealized Gains |  | 29
Fixed maturity, Gross Unrealized Losses | (2,110) | (1,126)
Fixed maturity, Fair Value | $ 65,141 | $ 66,218

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