Loans (Loans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Industrial	$ 4,792,388	$ 4,344,000
Energy	1,499,093	1,386,061
Commercial mortgages	3,887,742	3,481,157
Construction	1,066,696	1,043,261
Land	331,986	311,030
Total commercial real estate	5,286,424	4,835,448
Home equity loans	355,342	345,130
Home equity lines of credit	291,950	264,862
Other	376,002	326,793
Total consumer real estate	1,023,294	936,785
Total real estate	6,309,718	5,772,233
Consumer and Other	544,466	473,098
Total loans	13,145,665	11,975,392
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy	1,182,326	971,767
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy	171,795	221,213
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy	$ 144,972	$ 193,081