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, 2014	Dec. 31, 2013	Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	$ 5,510,887	$ 4,907,076	$ 4,808,672
Commercial Real Estate	4,020,478	3,467,336	3,370,969
Consumer Real Estate	847,068	808,204	799,827
Consumer and Other	391,498	358,116	350,112
Unearned discounts	(23,147)	(25,032)	(23,126)
Total loans	10,746,784	9,515,700	9,306,454
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	164,506	210,273	173,513
Commercial Real Estate	141,133	136,601	147,302
Consumer Real Estate	624	745	773
Consumer and Other	0	278	311
Unearned discounts	0	0	0
Total loans	306,263	347,897	321,899
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	5,346,381	4,696,803	4,635,159
Commercial Real Estate	3,879,345	3,330,735	3,223,667
Consumer Real Estate	846,444	807,459	799,054
Consumer and Other	391,498	357,838	349,801
Unearned discounts	(23,147)	(25,032)	(23,126)
Total loans	$ 10,440,521	$ 9,167,803	$ 8,984,555