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Fair Value Measurements - Fair Value Measurements for Investments in Available-for-Sale Fixed Maturity and Equity Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | $ 522,580 | $ 555,136
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 22,326 | 47,815
Obligations of States and Political Subdivisions [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 61,713 | 68,965
Corporate Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 211,076 | 212,708
Mortgage-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 227,465 | 225,648
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 595,337 | 613,692
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 22,326 | 47,815
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Obligations of States and Political Subdivisions [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 61,713 | 68,965
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Corporate Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 211,076 | 212,708
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Mortgage-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 227,465 | 225,648
Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 72,757 | 58,556
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 70,467 | 54,152
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 70,467 | 54,152
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 524,870 | 559,540
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 22,326 | 47,815
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Obligations of States and Political Subdivisions [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 61,713 | 68,965
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Corporate Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 211,076 | 212,708
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Mortgage-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | 227,465 | 225,648
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member] | Total Investments in the Fair Value Hierarchy [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Fair Value Disclosure | $ 2,290 | $ 4,404

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