Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		$ 778,644,000	$ 778,612,000
Assets		$ 833,517,000	829,181,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Minimum [Member] | Discount rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate	[1]	5.25%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Maximum [Member] | Discount rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate	[1]	12.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Minimum [Member] | Probability of default [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Minimum [Member] | Prepayment rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Maximum [Member] | Probability of default [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		100.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Maximum [Member] | Prepayment rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		72.15%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Probability of default [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		9.71%
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Prepayment rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		3.41%
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		$ 1,670,000	1,670,000
Impaired Loans [Member] | Discounted Cash Flow [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		7,122,000
Impaired Loans [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		0
Impaired Loans [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		$ 0
Impaired Loans [Member] | Discounted Cash Flow [Member] | Weighted average [Member] | Discount rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate
Impaired Loans [Member] | Reserve Study [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		$ 2,146,000
Impaired Loans [Member] | Reserve Study [Member] | NGL per gallon [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Monetary	[2]
Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Gas per MCF [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Monetary	[2]	2.87
Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Oil per BBL/d [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Monetary	[2]	56.05
Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Gas per MCF [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Monetary	[2]	3.61
Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Oil per BBL/d [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Monetary	[2]	$ 57.65
Impaired Loans [Member] | Reserve Study [Member] | Weighted average [Member] | Discount rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average rate		10.00%
Limited Partnership Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		$ 1,562,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		36,299,000	34,962,000
Assets		37,861,000	35,892,000
Level 3 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		1,670,000	1,670,000
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		34,629,000	33,292,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value			33,292,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Level 3 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		34,629,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Real Estate Owned Assets Fair Value		6,083,000	7,566,000
Assets		42,704,000	46,235,000
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Real Estate Owned Assets Fair Value		0	0
Assets		16,743,000	$ 19,990,000
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Other Real Estate Owned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Real Estate Owned Assets Fair Value		$ 0

[1] ncorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.

[2] The remainder of impaired loans valued using Level 3 inputs are not included in this disclosure as the values of those loans are based on bankruptcy agreement documentation.