Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)		3 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		$ 893,101,000	$ 909,247,000
Other Investments		25,378,000	32,126,000
Other Investments and Securities, at Cost			1,700,000
Assets		938,919,000	957,719,000
Impaired Loans [Member] | Discounted Cash Flow [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		3,187,000
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] | Reserve Study [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		1,044,000
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	[1]
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		2.61
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		47.09
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		3.49
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		$ 53.14
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]
Discount Rate		10.00%
Limited Partnership Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		$ 3,200,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at fair value		0	0
Other Investments		1,670,000	1,670,000
Assets		4,870,000	4,366,000
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Real Estate Owned Assets Fair Value		4,300,000	4,035,000
Assets		33,540,000	34,431,000
Fair Value, 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		$ 14,849,000	$ 15,320,000

[1] 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.