Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 22,610,560	$ 21,640,797	$ 20,743,332
Covered loans	0	0	50,119
Total loans	$ 22,610,560	$ 21,640,797	$ 20,793,451
Total loans, net of unearned income, excluding covered loans	100.00%	100.00%	100.00%
Covered loans	0.00%	0.00%	0.00%
Total loans	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 7,289,060	$ 6,787,677	$ 6,406,289
Total loans, net of unearned income, excluding covered loans	32.00%	31.00%	31.00%
Commercial real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 6,575,084	$ 6,580,618	$ 6,402,494
Total loans, net of unearned income, excluding covered loans	29.00%	30.00%	31.00%
Home equity
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 593,500	$ 663,045	$ 689,483
Total loans, net of unearned income, excluding covered loans	3.00%	3.00%	3.00%
Residential real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 895,470	$ 832,120	$ 762,810
Total loans, net of unearned income, excluding covered loans	4.00%	4.00%	3.00%
Premium finance receivables—commercial \| Commercial insurance loans
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 2,833,452	$ 2,634,565	$ 2,648,386
Total loans, net of unearned income, excluding covered loans	12.00%	12.00%	13.00%
Premium finance receivables—commercial \| Life insurance
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 4,302,288	$ 4,035,059	$ 3,719,043
Total loans, net of unearned income, excluding covered loans	19.00%	19.00%	18.00%
Consumer and other
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 121,706	$ 107,713	$ 114,827
Total loans, net of unearned income, excluding covered loans	1.00%	1.00%	1.00%

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.

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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