Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	6 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	$ 11,527,000		$ 12,227,000
Assets	1,168,287,000		938,309,000
Impaired Loans [Member] | Reserve Study [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	$ 712,000		798,000
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	$ 2.00	$ 1.46
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	2.00	1.48
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	50.00	36.00
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	50.00	$ 36.00
Impaired Loans [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	5,160,000
Limited Partnership Investment [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	7,448,000		6,619,000
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	1,670,000		1,670,000
Assets	$ 9,118,000		$ 8,289,000