Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 829,181,000	$ 966,548,000
Other Assets, Fair Value Disclosure	7,019,000	11,273,000
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	35,892,000	37,828,000
Other Assets, Fair Value Disclosure	930,000	0
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 33,292,000
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	$ 9,303,000
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	$ 2,281,000
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 MCF
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	$ 46,235,000	53,846,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 19,990,000	$ 12,828,000
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	10000.00%
Maximum [Member] | Prepayment Rates [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	7266.00%
Maximum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Weighted Average Inputs Price	$ 3.38
Maximum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Weighted Average Inputs Price	$ 56.16
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	1200.00%
Weighted Average [Member] | Probability Of Default [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	1067.00%
Weighted Average [Member] | Prepayment Rates [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	415.00%
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	$ 1.63
Minimum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Weighted Average Inputs Price	$ 40.97
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	525.00%