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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, 2018 | Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  | $ 889,056,000 | $ 731,358,000
Assets |  | 933,939,000 | 779,966,000
Equity Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  | 1,670,000 | 1,670,000
Impaired Loans [Member] | Discounted Cash Flow [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets |  | 7,883,000 |
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 |  | 1,184,000 |
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 | [1] | 2.81 |
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 | [1] | 51.59 |
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 | [1] | 3.35 |
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 | [1] | $ 59.55 |
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 |  | $ 2,521,000 |
Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  | 1,670,000 | 25,316,000
Assets |  | 4,191,000 | 27,459,000
Level 3 [Member] | Equity Securities [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  |  | 1,670,000
Assets |  | 1,670,000 |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  |  | 23,646,000
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Securities available for sale, at fair value |  |  | 23,646,000
Fair Value, Measurements, Nonrecurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Other Real Estate Owned Assets Fair Value |  | 4,164,000 | 3,264,000
Assets |  | 38,650,000 | 41,758,000
Fair Value, Measurements, 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 |  | $ 19,546,000 | $ 15,245,000
[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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