Loans - Loans (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 mortgages	3,146,004	3,080,202
Construction	761,834	629,988
Land	281,860	291,907
Total commercial real estate	4,189,698	4,002,097	3,834,855
Home equity loans	336,468	342,725
Home equity lines of credit	226,877	220,128
Other	304,345	286,198
Total consumer real estate	867,690	849,051	863,050
Total real estate	5,057,388	4,851,148
Consumer installment	395,054	385,479
Other	6,939	8,122
Total 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
Total commercial and industrial percentage of total loans	52.30%	52.50%
Commercial mortgages percentage of total loans	27.60%	28.00%
Construction percentage of total loans	6.70%	5.70%
Land percentage of total loans	2.50%	2.70%
Total commercial real estate percentage of total loans	36.80%	36.40%
Home equity loans percentage of total loans	3.00%	3.10%
Home equity lines of credit percentage of total loans	2.00%	2.00%
Other percentage of total loans	2.60%	2.60%
Total consumer real estate percentage of total loans	7.60%	7.70%
Total real estate percentage of total loans	44.40%	44.10%
Consumer installment percentage of total loans	3.40%	3.50%
Other percentage of total loans	0.10%	0.10%
Total consumer and other percentage of total loans	3.50%	3.60%
Unearned discounts percentage of total loans	(0.20%)	(0.20%)
Total loans percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	$ 5,621,818	$ 5,429,206
Total commercial and industrial percentage of total loans	49.30%	49.40%
Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial:	$ 344,091	$ 338,537
Total commercial and industrial percentage of total loans	3.00%	3.10%