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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015
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Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | $ 482,826 | $ 392,270
Equity Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 32 | 31
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 470,652 | 380,053
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 470,620 | 380,022
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Equity Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 32 | 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 | 196,780 | 171,419
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 199,212 | 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 | 11,280 | 7,299
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member] | Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 63,348 |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 63,380 | 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 | 63,348 |
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 | 32 | 31
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member] | Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 63,348 |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Available-for-Sale, Fair Value | 407,272 | 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 | 407,272 | 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 | 196,780 | 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 | 199,212 | 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 | $ 11,280 | $ 7,299

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