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Loans (Age Analysis of Past Due Loans, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | $ 181,209 |
Current Loans | 12,964,456 |
Total loans | 13,145,665 | $ 11,975,392
Accruing Loans 90 or More Days Past Due | 14,432 |
Commercial Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 53,587 |
Current Loans | 4,738,801 |
Total loans | 4,792,388 |
Accruing Loans 90 or More Days Past Due | 5,589 |
Commercial and Industrial, Energy [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 71,314 |
Current Loans | 1,427,779 |
Total loans | 1,499,093 |
Accruing Loans 90 or More Days Past Due | 100 |
Commercial Real Estate, Buildings, Land and Other [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 34,196 |
Current Loans | 4,185,532 |
Total loans | 4,219,728 |
Accruing Loans 90 or More Days Past Due | 2,615 |
Construction Loans [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 4,522 |
Current Loans | 1,062,174 |
Total loans | 1,066,696 |
Accruing Loans 90 or More Days Past Due | 2,331 |
Residential Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 12,134 |
Current Loans | 1,011,160 |
Total loans | 1,023,294 |
Accruing Loans 90 or More Days Past Due | 3,138 |
Consumer Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 5,456 |
Current Loans | 539,010 |
Total loans | 544,466 |
Accruing Loans 90 or More Days Past Due | 659 |
Financing Receivables, 30 to 89 Days Past Due [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 107,672 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 41,169 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial and Industrial, Energy [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 22,100 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 29,714 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 2,191 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 7,707 |
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 4,791 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 73,537 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 12,418 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and Industrial, Energy [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 49,214 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 4,482 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Construction Loans [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 2,331 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | 4,427 |
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |
Total Past Due Loans | $ 665 |

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