Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2022	Jun. 30, 2021	Dec. 31, 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	$ 13,142,000		$ 12,838,000
Assets	983,177,000		1,114,587,000
Impaired Loans [Member] | Reserve Study [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	$ 538,000		$ 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	15,999,000		$ 14,981,000
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	1,170,000		1,170,000
Assets	$ 17,169,000		$ 16,151,000