Allowance for Credit Losses - Recorded Investment in Loans and Leases on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in loans and leases, individually evaluated for impairment	$ 760,309	$ 760,452
Recorded investment in loans and leases, collectively evaluated for impairment	86,818,659	89,514,932
Recorded investment in loans and leases, purchased impaired	410,015	578,032
Loans and leases, net of unearned discount	87,988,983	90,853,416
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in loans and leases, individually evaluated for impairment	266,376	268,877
Recorded investment in loans and leases, collectively evaluated for impairment	21,476,254	22,340,529
Recorded investment in loans and leases, purchased impaired	21	641
Loans and leases, net of unearned discount	21,742,651	22,610,047
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in loans and leases, individually evaluated for impairment	226,205	224,630
Recorded investment in loans and leases, collectively evaluated for impairment	33,117,512	33,222,080
Recorded investment in loans and leases, purchased impaired	22,656	59,684
Loans and leases, net of unearned discount	33,366,373	33,506,394
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in loans and leases, individually evaluated for impairment	202,163	201,479
Recorded investment in loans and leases, collectively evaluated for impairment	19,023,843	21,871,726
Recorded investment in loans and leases, purchased impaired	387,338	517,707
Loans and leases, net of unearned discount	19,613,344	22,590,912
Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in loans and leases, individually evaluated for impairment	65,565	65,466
Recorded investment in loans and leases, collectively evaluated for impairment	13,201,050	12,080,597
Loans and leases, net of unearned discount	$ 13,266,615	$ 12,146,063

The balance of financing receivables that were purchased impaired amount.

No definition available.

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ 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 (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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- Definition

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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- Definition

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=82891408&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=84173941&loc=d3e5074-111524

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