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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 938,309,000 | $ 962,731,000
Other Assets, Fair Value Disclosure | 60,981,000 | 27,689,000
Other investments | 12,227,000 | 16,761,000
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | 930,020,000 | 955,266,000
Other Assets, Fair Value Disclosure | 54,362,000 | 21,894,000
Other investments | 10,557,000 | 15,091,000
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | 8,289,000 | 7,465,000
Other Assets, Fair Value Disclosure | $ 6,619,000 | $ 5,795,000
Other Assets [Member] | Par Value [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] | Par Value | Par Value
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Other investments | $ 1,670,000 | $ 1,670,000
Equity Securities [Member] | Par Value [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] | Carrying Value | Carrying Value
Impaired Loans [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure |  | $ 2,239,000
Equity Securities, FV-NI, Measurement Input [Extensible List] |  | Discounted Cash Flow
Impaired Loans [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 798,000 | $ 884,000
Equity Securities, FV-NI, Measurement Input [Extensible List] | Gas Reserve study | Gas Reserve study
Impaired Loans [Member] | Discount Rates [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] |  | Discount Rate
Impaired Loans [Member] | Discount Rates [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] | Discount rate | Discount rate
Impaired Loans [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] | Gas per MMBTU | Gas per MMBTU
Impaired Loans [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Equity Securities, FV-NI, Measurement Input [Extensible List] | Oil per BBL/d | Oil per BBL/d
Fair Value, Nonrecurring [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 50,466,000 | $ 32,393,000
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | 36,862,000 | 14,875,000
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 13,604,000 | $ 17,518,000
Maximum [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Input, Discount Rate | 9.50% | 9.50%
Maximum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Weighted Average Inputs Price | $ 1.48 | $ 3.49
Maximum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Weighted Average Inputs Price | $ 36.00 | $ 53.14
Weighted Average [Member] | Impaired Loans [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Input, Discount Rate | 10.00% | 10.00%
Minimum [Member] | Discounted Cash Flow [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Input, Discount Rate | 3.84% | 1.90%
Minimum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Weighted Average Inputs Price | $ 1.46 | $ 2.61
Minimum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value Weighted Average Inputs Price | $ 36.00 | $ 47.09

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Description of the valuation processes in place for fair value measurements categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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