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Loans (Unallocated Portion of Allowance for Loan Losses) (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | $ 155,364 | $ 153,045 | $ 135,859 | $ 99,542
Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 75,481 | 91,663 |  |
Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 20,820 | 9,186 |  |
General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 23,283 | 16,843 |  |
Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 35,780 | 35,353 |  |
Commercial Portfolio Segment [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 59,614 | 52,915 | 42,993 | 44,273
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 26,401 | 33,251 |  |
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 7,553 | 5,436 |  |
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 9,112 | 6,708 |  |
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 16,548 | 7,520 |  |
Commercial And Industrial Energy Financing Receivable [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 51,528 | 60,653 | 54,696 | 14,919
Commercial And Industrial Energy Financing Receivable [Member] | Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 22,073 | 34,626 |  |
Commercial And Industrial Energy Financing Receivable [Member] | Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 13,267 | 3,750 |  |
Commercial And Industrial Energy Financing Receivable [Member] | General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 7,964 | 3,769 |  |
Commercial And Industrial Energy Financing Receivable [Member] | Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 8,224 | 18,508 |  |
Commercial Real Estate Portfolio Segment [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 30,948 | 30,213 | 24,313 | 27,163
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 18,931 | 16,976 |  |
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 0 | 0 |  |
Commercial Real Estate Portfolio Segment [Member] | General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 4,165 | 5,004 |  |
Commercial Real Estate Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 7,852 | 8,233 |  |
Residential Portfolio Segment [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 5,657 | 4,238 | 4,659 | 5,178
Residential Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 2,473 | 2,225 |  |
Residential Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 0 | 0 |  |
Residential Portfolio Segment [Member] | General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 2,133 | 1,506 |  |
Residential Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 1,051 | 507 |  |
Consumer Portfolio Segment [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 7,617 | 5,026 | $ 9,198 | $ 8,009
Consumer Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 5,603 | 4,585 |  |
Consumer Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | 0 | 0 |  |
Consumer Portfolio Segment [Member] | General Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | (91) | (144) |  |
Consumer Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |  |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |  |  |
Portion of allowance for loan losses | $ 2,105 | $ 585 |  |

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- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 3: 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(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544 + Details
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