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Loans - Allowance for Credit Losses By Loan Class Accordance With Methodology Used Prior to CECL (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | $ 132,167
Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 69,087
Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | $ 14,944 | 28,483
General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 19,732
Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 14,865
Commercial Portfolio Segment [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 51,593
Commercial Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 29,015
Commercial Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | 5,256 | 7,849
Commercial Portfolio Segment [Member] | General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 9,840
Commercial Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 4,889
Commercial and Industrial Total Energy Loans [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 37,382
Commercial and Industrial Total Energy Loans [Member] | Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 7,873
Commercial and Industrial Total Energy Loans [Member] | Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | 8,357 | 20,246
Commercial and Industrial Total Energy Loans [Member] | General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 5,196
Commercial and Industrial Total Energy Loans [Member] | Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 4,067
Commercial Real Estate Portfolio Segment [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 31,037
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 21,947
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | 1,313 | 383
Commercial Real Estate Portfolio Segment [Member] | General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 4,201
Commercial Real Estate Portfolio Segment [Member] | Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 4,506
Total consumer real estate loans [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 4,113
Total consumer real estate loans [Member] | Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 2,690
Total consumer real estate loans [Member] | Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | 0 | 0
Total consumer real estate loans [Member] | General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 904
Total consumer real estate loans [Member] | Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 519
Consumer Loan [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 8,042
Consumer Loan [Member] | Historical Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | 7,562
Consumer Loan [Member] | Specific Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance | $ 18 | 5
Consumer Loan [Member] | General Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | (409)
Consumer Loan [Member] | Macroeconomic Valuation Allowances [Member] |  |
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items] |  |
Loans and Leases Receivable, Allowance |  | $ 884

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=121614798&loc=d3e15032-111544 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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