Loans - Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	$ 5,965,909	$ 5,767,743	$ 5,623,757
Commercial Real Estate	4,189,698	4,002,097	3,834,855
Consumer Real Estate	867,690	849,051	863,050
Consumer and Other	401,993	393,601	378,156
Unearned discounts	(24,466)	(24,957)	(22,489)
Total loans	11,400,824	10,987,535	10,677,329
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	29,000	32,496	31,652
Commercial Real Estate	13,389	20,499	20,193
Consumer Real Estate	1,149	596	689
Consumer and Other	0	0	252
Unearned discounts	0	0	0
Total loans	43,538	53,591	52,786
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	5,936,909	5,735,247	5,592,105
Commercial Real Estate	4,176,309	3,981,598	3,814,662
Consumer Real Estate	866,541	848,455	862,361
Consumer and Other	401,993	393,601	377,904
Unearned discounts	(24,466)	(24,957)	(22,489)
Total loans	$ 11,357,286	$ 10,933,944	$ 10,624,543