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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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