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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, 2019 | Dec. 31, 2018
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 962,731,000 | $ 957,719,000
Other Assets, Fair Value Disclosure | 27,689,000 | 4,465,000
Other investments | 16,761,000 | 32,126,000
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | 955,266,000 | 953,353,000
Other Assets, Fair Value Disclosure | 21,894,000 | 1,769,000
Other investments | 15,091,000 | 30,456,000
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | 7,465,000 | 4,366,000
Other Assets, Fair Value Disclosure | $ 5,795,000 | $ 2,696,000
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 | $ 3,249,000
Equity Securities, FV-NI, Measurement Input [Extensible List] | Discounted Cash Flow | Discounted Cash Flow
Impaired Loans [Member] | Reserve [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Assets, Fair Value Disclosure | $ 884,000 | $ 1,104,000
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 | 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 | $ 32,393,000 | $ 34,431,000
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 | 14,875,000 | 19,111,000
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 | $ 17,518,000 | $ 15,320,000
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 | $ 3.49 | $ 3.35
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 | $ 53.14 | $ 59.55
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 | $ 2.61 | $ 2.81
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 | $ 47.09 | $ 51.59

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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 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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