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, 2023	Dec. 31, 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	$ 48,979,000	$ 26,414,000
Assets	991,940,000	869,373,000
Impaired Loans [Member] | Reserve Study [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	$ 253,000	$ 363,000
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	$ 3.00
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	3.00
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	80.00
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	80.00
Limited Partnership Investment [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets	26,397,000	17,691,000
Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other Investments	6,182,000	1,170,000
Assets	$ 32,579,000	$ 18,861,000