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Loans (Impaired Loans) (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | $ 128,536 |
Recorded Investment With No Allowance | 21,842 |
Recorded Investment With Allowance | 77,611 |
Total Recorded Investment | 99,453 |
Related Allowance | 28,483 |
Average Recorded Investment | 84,352 | $ 108,127
Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 30,909 |
Recorded Investment With No Allowance | 11,588 |
Recorded Investment With Allowance | 12,772 |
Total Recorded Investment | 24,360 |
Related Allowance | 7,849 |
Average Recorded Investment | 14,913 | 18,246
Commercial and Industrial Total Energy Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 87,103 |
Recorded Investment With No Allowance | 2,764 |
Recorded Investment With Allowance | 62,480 |
Total Recorded Investment | 65,244 |
Related Allowance | 20,246 |
Average Recorded Investment | 53,563 | 75,453
Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 9,252 |
Recorded Investment With No Allowance | 6,255 |
Recorded Investment With Allowance | 2,354 |
Total Recorded Investment | 8,609 |
Related Allowance | 383 |
Average Recorded Investment | 13,690 | 12,799
Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 697 |
Recorded Investment With No Allowance | 665 |
Recorded Investment With Allowance | 0 |
Total Recorded Investment | 665 |
Related Allowance | 0 |
Average Recorded Investment | 354 | 0
Total consumer real estate loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 570 |
Recorded Investment With No Allowance | 570 |
Recorded Investment With Allowance | 0 |
Total Recorded Investment | 570 |
Related Allowance | 0 |
Average Recorded Investment | 547 | 704
Consumer Loan [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Contractual Principal Balance | 5 |
Recorded Investment With No Allowance | 0 |
Recorded Investment With Allowance | 5 |
Total Recorded Investment | 5 |
Related Allowance | 5 |
Average Recorded Investment | $ 1,285 | $ 925

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