Fair Values of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	$ 392,270	$ 343,018
Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	31	28
Other Investments [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	12,217	11,748
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	392,270	343,018
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	380,022	331,242
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	12,217	11,748
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	31	28
Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	171,419	157,374
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	201,304	165,370
Fair Value, Measurements, Recurring [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	7,299	8,498
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	31	28
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	31	28
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	380,022	331,242
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	380,022	331,242
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	171,419	157,374
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	201,304	165,370
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	7,299	8,498
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	12,217	11,748
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	$ 12,217	$ 11,748