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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

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