Loans (Investment in Loans Related to Allowance for Loan Loss by Portfolio Segment Disaggregated Based on Impairment Methodology) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Industrial	$ 5,111,957	$ 4,792,388
Energy	1,602,598	1,499,093
Commercial Real Estate	5,696,438	5,286,424
Consumer Real Estate	1,118,990	1,023,294
Consumer and Other	569,750	544,466
Total loans	14,099,733	13,145,665
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Industrial	7,129	43,760
Energy	46,707	94,242
Commercial Real Estate	14,685	6,394
Consumer Real Estate	293	1,214
Consumer and Other	1,407	0
Total loans	70,221	145,610
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Industrial	5,104,828	4,748,628
Energy	1,555,891	1,404,851
Commercial Real Estate	5,681,753	5,280,030
Consumer Real Estate	1,118,697	1,022,080
Consumer and Other	568,343	544,466
Total loans	$ 14,029,512	$ 13,000,055

Loans Receivable, Gross, Energy

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount before allowance of other consumer installment loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
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Amount before allowance of consumer real estate loans. Includes, but not limited to, construction financing, residential mortgage loans, and home equity lines of credit. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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.

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Amount before allowance of commercial loans issued to businesses to acquire, develop, construct, improve, or refinance land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

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