Loans - Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,164,914	$ 4,120,522
Energy:	1,384,907	1,758,195
Total commercial real estate	4,638,902	4,292,727
Total consumer real estate	929,257	880,749
Consumer and other	463,430	434,338
Total loans	11,581,410	11,486,531
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	28,868	22,860
Energy:	51,378	21,139
Total commercial real estate	8,613	32,994
Total consumer real estate	662	485
Consumer and other	41	0
Total loans	89,562	77,478
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	4,136,046	4,097,662
Energy:	1,333,529	1,737,056
Total commercial real estate	4,630,289	4,259,733
Total consumer real estate	928,595	880,264
Consumer and other	463,389	434,338
Total loans	$ 11,491,848	$ 11,409,053

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.

+ References

No definition available.

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: 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)(4),(b))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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.

+ References

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5074-111524

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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.

+ References

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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.

+ 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))
-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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: 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=66008989&loc=d3e5066-111524

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