Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		$ 812,877,000	$ 731,358,000
Assets		$ 853,683,000	779,966,000
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]	5.00%
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]	11.50%
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		71.89%
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		7.42%
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		8.73%
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		11,181,000
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		$ 1,290,000
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]	2.81
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]	51.59
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]	3.35
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]	$ 59.55
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		$ 2,292,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		15,802,000	25,316,000
Assets		18,094,000	27,459,000
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		14,132,000	23,646,000
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			23,646,000
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		14,132,000
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		3,278,000	3,264,000
Assets		49,062,000	41,758,000
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		19,238,000	$ 15,245,000
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.