Fair Values of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	$ 492,460	$ 392,270
Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	33	31
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	479,130	380,053
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	479,097	380,022
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	33	31
Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	232,293	171,419
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	182,515	201,304
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	9,342	7,299
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	54,947
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	54,980	31
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	54,947
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	33	31
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	54,947
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	424,150	380,022
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	424,150	380,022
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	232,293	171,419
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	182,515	201,304
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	$ 9,342	$ 7,299