Loans (Loans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,120,522	$ 4,055,225
Total energy	1,758,195	1,773,945
Commercial mortgages	3,285,041	2,999,082
Construction	720,695	624,888
Land	286,991	291,907
Total commercial real estate	4,292,727	3,915,877
Home equity loans	340,528	342,725
Home equity lines of credit	233,525	220,128
Other	306,696	286,198
Total consumer real estate	880,749	849,051
Total real estate	5,173,476	4,764,928
Consumer and Other	434,338	393,437
Total loans	11,486,531	10,987,535
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	1,249,678	1,160,404
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	272,934	319,618
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	$ 235,583	$ 293,923