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Loans (Net (Charge-Offs)/Recoveries, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | $ (33,141) | $ (34,487) | $ (15,528)
Commercial Portfolio Segment [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | (17,453) | (12,259) | (6,535)
Commercial and Industrial, Energy [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | (10,009) | (18,588) | (5,997)
Commercial Real Estate, Buildings, Land and Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | 735 | 813 | 314
Construction Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | 11 | 23 | 18
Residential Portfolio Segment [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | (506) | (257) | (91)
Consumer Portfolio Segment [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Net charge-offs | $ (5,919) | $ (4,219) | $ (3,237)

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