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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
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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

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