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Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  | $ 893,101,000 | $ 909,247,000
Other Investments |  | 25,378,000 | 32,126,000
Other Investments and Securities, at Cost |  |  | 1,700,000
Assets |  | 938,919,000 | 957,719,000
Impaired Loans [Member] | Discounted Cash Flow [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets |  | 3,187,000 |
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 |  | 1,044,000 |
Impaired Loans [Member] | Reserve Study [Member] | NGL per gallon [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value Inputs Monetary | [1] |  |
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 |  | $ 3,200,000 |
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 |  | 4,870,000 | 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 |  | 4,300,000 | 4,035,000
Assets |  | 33,540,000 | 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 |  | $ 14,849,000 | $ 15,320,000
[1] | The remainder of impaired loans valued using Level 3 inputs are not included in this disclosure as the values of those loans are based on bankruptcy agreement documentation.

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