Loans - Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	$ 4,808,672	$ 4,828,612	$ 4,486,301
Commercial Real Estate	3,370,969	3,319,795	3,250,135
Consumer Real Estate	799,827	777,347	768,482
Consumer and Other	350,112	319,745	325,592
Unearned discounts	(23,126)	(21,651)	(19,470)
Total loans	9,306,454	9,223,848	8,811,040
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	173,513	128,544	161,577
Commercial Real Estate	147,302	165,152	170,077
Consumer Real Estate	773	864	894
Consumer and Other	311	400	420
Unearned discounts	0	0	0
Total loans	321,899	294,960	332,968
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	4,635,159	4,700,068	4,324,724
Commercial Real Estate	3,223,667	3,154,643	3,080,058
Consumer Real Estate	799,054	776,483	767,588
Consumer and Other	349,801	319,345	325,172
Unearned discounts	(23,126)	(21,651)	(19,470)
Total loans	$ 8,984,555	$ 8,928,888	$ 8,478,072