Fair Value Measurements - Fair Value Measurements for Investments in Available-for-Sale Fixed Maturity and Equity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	$ 519,979	$ 538,946
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	43,360	44,049
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	97,934	132,117
Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	119,025	105,739
Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	259,660	257,041
Assets and Liabilities on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	573,581	589,391
Assets and Liabilities on Recurring Basis [Member] | Investments Measured at Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	14,849	13,709
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	558,732	575,682
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	43,360	44,049
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	97,934	132,117
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	119,025	105,739
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	259,660	257,041
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	38,753	36,736
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Assets and Liabilities on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	38,753	36,736
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	38,753	36,736
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	38,753	36,736
Significant Other Observable Inputs (Level 2) [Member] | Assets and Liabilities on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Fair Value Disclosure	519,979	538,946
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	519,979	538,946
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	43,360	44,049
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	97,934	132,117
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	119,025	105,739
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	$ 259,660	$ 257,041