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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 |
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at fair value | $ 812,383,000 | $ 909,247,000
Other Investments | 11,561,000 | 32,126,000
Other Investments and Securities, at Cost | 1,700,000 | 1,700,000
Assets | 875,934,000 | 957,719,000
Impaired Loans [Member] | Discounted Cash Flow [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 2,539,000 |
Impaired Loans [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0 |
Impaired Loans [Member] | Discounted Cash Flow [Member] | Maximum [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0 |
Impaired Loans [Member] | Reserve Study [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 932,000 |
Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Gas per MCF [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value Inputs Monetary | 2.61 |
Impaired Loans [Member] | Reserve Study [Member] | Minimum [Member] | Oil per BBL/d [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value Inputs Monetary | 47.09 |
Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Gas per MCF [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value Inputs Monetary | 3.49 |
Impaired Loans [Member] | Reserve Study [Member] | Maximum [Member] | Oil per BBL/d [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value Inputs Monetary | $ 53.14 |
Impaired Loans [Member] | Reserve Study [Member] | Weighted average [Member] | Discount rates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Discount Rate | 10.00% |
Limited Partnership Investment [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | $ 4,131,000 |
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at fair value | 0 | 0
Other Investments | 1,670,000 | 1,670,000
Assets | 5,801,000 | 4,366,000
Fair Value, Nonrecurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other Real Estate Owned Assets Fair Value | 1,981,000 | 4,035,000
Assets | 35,760,000 | 34,431,000
Fair Value, Nonrecurring [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other Real Estate Owned Assets Fair Value | 0 | 0
Assets | $ 13,372,000 | $ 15,320,000

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