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Loans (Investment in Loans Related to Allowance for Loan Loss by Portfolio Segment Disaggregated Based on Impairment Methodology) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | $ 17,481,309 | $ 14,750,332
Portion of allowance for loan losses |  | 132,167
Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 4,955,341 | 5,187,466
Portion of allowance for loan losses |  | 51,593
Commercial and Industrial Total Energy Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 1,235,198 | 1,652,882
Portion of allowance for loan losses |  | 37,382
Paycheck Protection Program [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 2,433,849 | 0
Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 5,796,653 | 4,883,580
Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 1,223,814 | 1,312,659
Total consumer real estate loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 1,330,774 | 1,194,413
Portion of allowance for loan losses |  | 4,113
Consumer Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 505,680 | 519,332
Portion of allowance for loan losses |  | 8,042
Specific Valuation Allowances [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 15,279 | 28,483
Specific Valuation Allowances [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 5,321 | 7,849
Specific Valuation Allowances [Member] | Commercial and Industrial Total Energy Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 9,427 | 20,246
Specific Valuation Allowances [Member] | Paycheck Protection Program [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 0 | 0
Specific Valuation Allowances [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 513 | 383
Specific Valuation Allowances [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 0 | 0
Specific Valuation Allowances [Member] | Total consumer real estate loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 0 | 0
Specific Valuation Allowances [Member] | Consumer Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Portion of allowance for loan losses | 18 | 5
Loans Individually Evaluated for Impairment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 80,495 | 99,453
Loans Individually Evaluated for Impairment [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 21,287 | 24,360
Loans Individually Evaluated for Impairment [Member] | Commercial and Industrial Total Energy Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 22,888 | 65,244
Loans Individually Evaluated for Impairment [Member] | Paycheck Protection Program [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 0 | 0
Loans Individually Evaluated for Impairment [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 34,057 | 8,609
Loans Individually Evaluated for Impairment [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 1,684 | 665
Loans Individually Evaluated for Impairment [Member] | Total consumer real estate loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | 561 | 570
Loans Individually Evaluated for Impairment [Member] | Consumer Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Total loans | $ 18 | $ 5

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