Loans - Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,043,272	$ 4,792,388
Energy:	1,528,028	1,499,093
Commercial mortgages	4,097,255	3,887,742
Construction	1,106,999	1,066,696
Land	305,585	331,986
Total commercial real estate	5,509,839	5,286,424
Home equity loans	352,243	355,342
Home equity lines of credit	321,795	291,950
Other	400,661	376,002
Total consumer real estate	1,074,699	1,023,294
Total real estate	6,584,538	6,309,718
Consumer and other	555,924	544,466
Total loans	$ 13,711,762	$ 13,145,665
Total commercial and industrial percentage of total loans	36.80%	36.40%
Percentage of total energy loans to total loans	11.10%	11.40%
Commercial mortgages percentage of total loans	29.90%	29.60%
Construction percentage of total loans	8.10%	8.10%
Land percentage of total loans	2.20%	2.50%
Total commercial real estate percentage of total loans	40.20%	40.20%
Home equity loans percentage of total loans	2.60%	2.70%
Home equity lines of credit percentage of total loans	2.30%	2.20%
Other percentage of total loans	3.00%	2.90%
Total consumer real estate percentage of total loans	7.90%	7.80%
Total real estate percentage of total loans	48.10%	48.00%
Total consumer and other percentage of total loans	4.00%	4.20%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,211,261	$ 1,182,326
Percentage of energy production loans to total loans	8.80%	9.00%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 163,013	$ 171,795
Percentage of energy service loans to total loans	1.20%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 153,754	$ 144,972
Percentage of other energy loans to total loans	1.10%	1.10%