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Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016
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 |  |
Unfunded commitments related to nonperforming loans | $ 1,500,000 |  | $ 1,500,000 |  | $ 1,800,000
Off balance sheet reserve to nonperforming loans | 100,000 |  | 100,000 |  | 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales | 0 | $ 0 | 0 | $ 0 |
Total gains or losses included in earnings | 0 |  | $ 0 | 0 |
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 | 100,000 | $ 300,000 | $ 200,000 | 4,200,000 |
Nonaccrual Commercial Loans [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Loans Held for sale, Amount |  |  |  |  | $ 0
Loans and Leases Receivable, Gain (Loss) on Sales, Net |  |  | 0 | $ 0 |
Non Accrual Loans [Member] | Steel and Aluminum Servicing Company [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Nonaccrual | 5,000,000 |  | 5,000,000 |  |
Paydown On Loan Receivable |  |  | 2,400,000 |  |
Non Accrual Loans [Member] | Industrial Manufacturer [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Nonaccrual | 2,100,000 |  | 2,100,000 |  |
Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Nonaccrual | 2,800,000 |  | 2,800,000 |  |
2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Nonaccrual | 600,000 |  | 600,000 |  |
2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Nonaccrual | $ 2,200,000 |  | 2,200,000 |  |
Charge-off recognized |  |  | $ 400,000 |  |

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