| **Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Thousands**            | 3 Months Ended | 9 Months Ended |          |
|-----------------------------------------------------------------------------------------------------------------|----------------|----------------|----------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Non Accrual Status Of Loans After Number Of Days Past Due                                                       |           |           | 90 days  |   |   |
| Default Status Of TDRs After Number Of Days Past Due                                                            |           |           | 90 days  |   |   |
| Charge-offs                                                                                                     | $ 2,401        | $ 2,880        | $ 14,462 | $ 14,629 |   |
| Unfunded commitments related to nonperforming loans                                                             | 500            |           | 500      |   | $ 0 |
| Off balance sheet reserve to nonperforming loans                                                                | 0              |           | 0        |   | 0 |
| Loans held for sale                                                                                             | 0              |           | 0        |   |   |
| Impaired Loans Sold                                                                                             |           |           | 2,400    |   |   |
| Gain on impaired loans sold                                                                                     |           |           | 400      |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales                  | 0              | 0              | 0        | 3,112 |   |
| Total gains or losses included in earnings                                                                      | 0              | 0              | 105      | 77 |   |
| Nonaccrual                                                                                                      | 28,803         |           | 28,803   |   | 42,667 |
| Recorded investment                                                                                             | 40,827         |           | $ 40,827 |   | $ 55,251 |
| Loans to individuals [Member]                                                                                   |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Non Accrual Status Of Loans After Number Of Days Past Due                                                       |           |           | 150 days |   |   |
| Impaired Loans [Member]                                                                                         |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Loans with modifications to rate and payment due to reamortization                                              | 0              | 100            | $ 400    | 500 |   |
| Loans Held For Sale [Member]                                                                                    |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales                  |           | 0              |     | 3,112 |   |
| Total gains or losses included in earnings                                                                      |           | $ 0            |     | $ 77 |   |
| Non Accrual Loans [Member] | Industrial Manufacturer [Member]                                                   |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Nonaccrual                                                                                                      | 6,100          |           | 6,100    |   |   |
| Non Accrual Loans [Member] | Local Energy Company [Member] [Member]                                             |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Nonaccrual                                                                                                      | 5,700          |           | 5,700    |   |   |
| 2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Nonaccrual                                                                                                      | 2,300          |           | 2,300    |   |   |
| Charge-off recognized                                                                                           |           |           | 3,300    |   |   |
| 2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |           |           |     |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |           |           |     |   |   |
| Nonaccrual                                                                                                      | $ 2,900        |           | $ 2,900  |   |   |

| X                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Charge-off recognized
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| X                                                                                                                                                                                                                                                                                 |
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| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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| X                                                                                                                                                                                                                         |
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| X                                                                                                                                                                                                                                         |
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| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans with modifications to rate and payment due to reamortization.
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| X                                                                                                                                                                                                                                                                                          |
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| - Definition
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| X                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Off balance sheet reserve to nonperforming loans.
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| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unfunded commitments related to nonperforming loans.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
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No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
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| X                                                                                                                                                 |
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| X                                                                                                                                                               |
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| X                                                                                                                                                    |
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| X                                                                                                                                                   |
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| X                                                                                                                                                       |
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| X                                                                                                                                                         |
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| X                                                                                                                                                               |
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| X                                                                                                                                                               |
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