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Loans (Non-Accrual Loans, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | $ 61,449 | $ 102,303
Financing Receivable, Nonaccrual, No Allowance | 21,911 | 24,692
Commercial Portfolio Segment [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 19,849 | 26,038
Financing Receivable, Nonaccrual, No Allowance | 4,479 | 13,266
Commercial and Industrial Total Energy Loans [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 23,168 | 65,761
Financing Receivable, Nonaccrual, No Allowance | 639 | 3,281
Paycheck Protection Program [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 0 | 0
Financing Receivable, Nonaccrual, No Allowance | 0 | 0
Commercial Real Estate, Buildings, Land and Other [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 15,737 | 8,912
Financing Receivable, Nonaccrual, No Allowance | 14,116 | 6,558
Construction Loans [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 1,684 | 665
Financing Receivable, Nonaccrual, No Allowance | 1,684 | 665
Total consumer real estate loans [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 993 | 922
Financing Receivable, Nonaccrual, No Allowance | 993 | 922
Consumer Loan [Member] |  |
Non Accrual Loans Segregated By Class Of Loans [Line Items] |  |
Non-accrual loans | 18 | 5
Financing Receivable, Nonaccrual, No Allowance | $ 0 | $ 0

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