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Loans (Weighted Average Risk Grades for All Commercial Loans b Class) (Detail) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) Grade | Dec. 31, 2016 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans | $ 4,792,388 | $ 4,344,000
Loans | 5,286,424 | 4,835,448
Loans | $ 1,066,696 | $ 1,043,261
Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 6.41 | 6.35
Loans | $ 4,792,388 | $ 4,344,000
Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 6.97 | 7.95
Loans | $ 1,499,093 | $ 1,386,061
Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 7.00 | 6.95
Loans | $ 4,219,728 | $ 3,792,187
Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 7.23 | 7.01
Loans | $ 1,066,696 | $ 1,043,261
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 6.06 | 6.01
Loans | $ 4,378,839 | $ 3,989,722
Risk Grade One To Eight [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 6.01 | 6.34
Loans | $ 1,199,207 | $ 854,688
Risk Grade One To Eight [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 6.75 | 6.67
Loans | $ 3,868,659 | $ 3,463,064
Risk Grade One To Eight [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 7.11 | 6.97
Loans | $ 1,019,635 | $ 1,023,194
Risk Grade Nine [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 9.00 | 9.00
Loans | $ 170,285 | $ 106,988
Risk Grade Nine [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 9.00 | 9.00
Loans | $ 50,427 | $ 78,524
Risk Grade Nine [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 9.00 | 9.00
Loans | $ 151,487 | $ 109,110
Risk Grade Nine [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 9.00 | 9.00
Loans | $ 18,042 | $ 15,829
Risk Grade Ten [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 10.00 | 10.00
Loans | $ 99,260 | $ 115,420
Risk Grade Ten [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 10.00 | 10.00
Loans | $ 64,282 | $ 150,872
Risk Grade Ten [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 10.00 | 10.00
Loans | $ 129,391 | $ 145,067
Risk Grade Ten [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 10.00 | 10.00
Loans | $ 23,393 | $ 2,889
Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 11.00 | 11.00
Loans | $ 97,818 | $ 100,245
Risk Grade Eleven [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 11.00 | 11.00
Loans | $ 90,875 | $ 244,406
Risk Grade Eleven [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 11.00 | 11.00
Loans | $ 62,602 | $ 66,396
Risk Grade Eleven [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 11.00 | 11.00
Loans | $ 5,626 | $ 1,349
Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 12.00 | 12.00
Loans | $ 38,633 | $ 25,939
Risk Grade Twelve [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 12.00 | 12.00
Loans | $ 81,035 | $ 53,821
Risk Grade Twelve [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 12.00 | 12.00
Loans | $ 7,589 | $ 8,550
Risk Grade Twelve [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 12.00 | 12.00
Loans | $ 0 | $ 0
Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 13.00 | 13.00
Loans | $ 7,553 | $ 5,686
Risk Grade Thirteen [Member] | Commercial and Industrial, Energy [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 13.00 | 13.00
Loans | $ 13,267 | $ 3,750
Risk Grade Thirteen [Member] | Commercial Real Estate, Buildings, Land and Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 13.00 | 13.00
Loans | $ 0 | $ 0
Risk Grade Thirteen [Member] | Construction Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 13.00 | 13.00
Loans | $ 0 | $ 0
Minimum [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 1.00 | 1.00
Maximum [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Weighted Average Risk Grade | Grade | 8.00 | 8.00

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- Definition Amount before allowance of loans issued to commercial and industrial institutions. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes real estate loans and loans covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)(1)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5066-111524 + Details
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