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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 779,966,000 | $ 829,181,000
Other Assets, Fair Value Disclosure | 3,921,000 | 7,019,000
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | 752,507,000 | 793,289,000
Other Assets, Fair Value Disclosure | 1,778,000 | 6,089,000
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | 27,459,000 | 35,892,000
Other Assets, Fair Value Disclosure | $ 2,143,000 | 930,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Probability Of Default [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Probability of default |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Prepayment Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Prepayment rates |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discount Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Discount rates |
Other Assets [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Valuation Techniques | Discounted Cash Flow |
Other Assets [Member] | Par Value [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Valuation Techniques | Par Value |
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 1,670,000 |
Equity Securities [Member] | Par Value [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Valuation Techniques | Par Value |
Impaired Loans [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 6,820,000 |
Fair Value Measurements, Valuation Techniques | Discounted Cash Flow |
Impaired Loans [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 1,336,000 |
Fair Value Measurements, Valuation Techniques | Gas Reserve study |
Impaired Loans [Member] | Discount Rates [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Discount Rate |
Impaired Loans [Member] | Discount Rates [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Discount rate |
Impaired Loans [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Gas per MMBTU |
Impaired Loans [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Measurements, Significant Assumptions | Oil per BBL/d |
Fair Value, Measurements, Nonrecurring [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 41,758,000 | 46,235,000
Other Real Estate Owned Assets Fair Value | 3,264,000 | 7,566,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | 26,513,000 | 26,245,000
Other Real Estate Owned Assets Fair Value | 3,264,000 | 7,566,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | 15,245,000 | 19,990,000
Other Real Estate Owned Assets Fair Value | $ 0 | $ 0
Maximum [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 4.68% |
Maximum [Member] | Probability Of Default [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 100.00% |
Maximum [Member] | Prepayment Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 72.02% |
Maximum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Weighted Average Inputs Price | $ 3.61 |
Maximum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Weighted Average Inputs Price | $ 57.65 |
Maximum [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discount Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 11.50% |
Weighted Average [Member] | Probability Of Default [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 8.98% |
Weighted Average [Member] | Prepayment Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 4.46% |
Weighted Average [Member] | Impaired Loans [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 10.00% |
Minimum [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 1.90% |
Minimum [Member] | Probability Of Default [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 0.00% |
Minimum [Member] | Prepayment Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 0.00% |
Minimum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Weighted Average Inputs Price | $ 2.87 |
Minimum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Weighted Average Inputs Price | $ 56.05 |
Minimum [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discount Rates [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value Inputs, Discount Rate | 5.00% |
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | $ 23,646,000 |

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