Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2022	Dec. 31, 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	$ 13,665,000	$ 12,838,000
Assets	932,760,000	1,114,587,000
Impaired Loans [Member] | Reserve Study [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	$ 498,000	$ 598,000
Impaired Loans [Member] | Reserve Study [Member] | Discount rates [Member] | Weighted average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Discount Rate	10.00%	10.00%
Impaired Loans [Member] | Reserve Study [Member] | Gas per MCF [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Inputs Monetary	$ 3.00	$ 2.00
Impaired Loans [Member] | Reserve Study [Member] | Gas per MCF [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Inputs Monetary	3.00	2.00
Impaired Loans [Member] | Reserve Study [Member] | Oil per BBL/d [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Inputs Monetary	80.00	50.00
Impaired Loans [Member] | Reserve Study [Member] | Oil per BBL/d [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Inputs Monetary	80.00	50.00
Limited Partnership Investment [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	16,613,000	14,981,000
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	1,170,000	1,170,000
Assets	$ 17,783,000	$ 16,151,000