Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	$ 479,690	$ 492,460
Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	2,044	33
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	470,734	479,130
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	468,690	479,097
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	2,044	33
Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	237,717	232,293
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	161,031	182,515
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	9,183	9,342
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	60,759	54,947
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	62,803	54,980
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	60,759	54,947
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | Equity Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	2,044	33
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Treasury Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	60,759	54,947
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	407,931	424,150
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	407,931	424,150
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	237,717	232,293
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	161,031	182,515
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] | Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Available-for-Sale, Fair Value	$ 9,183	$ 9,342