Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	$ 12,719,000	$ 12,227,000
Assets	1,206,844,000	938,309,000
Impaired Loans [Member] | Reserve Study [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	$ 657,000	$ 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	0
Limited Partnership Investment [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	14,613,000	6,619,000
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	1,670,000	1,670,000
Assets	$ 16,283,000	$ 8,289,000