Loans - Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,380,487	$ 5,111,957
Energy:	1,481,349	1,602,598
Commercial mortgages	4,291,781	4,121,966
Construction	1,323,040	1,267,717
Land	299,435	306,755
Total commercial real estate	5,914,256	5,696,438
Home equity loans	356,754	353,924
Home equity lines of credit	349,061	337,168
Other	453,083	427,898
Total consumer real estate	1,158,898	1,118,990
Total real estate	7,073,154	6,815,428
Consumer and other	524,159	569,750
Total loans	$ 14,459,149	$ 14,099,733
Total commercial and industrial percentage of total loans	37.20%	36.30%
Percentage of total energy loans to total loans	10.20%	11.40%
Commercial mortgages percentage of total loans	29.70%	29.20%
Construction percentage of total loans	9.20%	9.00%
Land percentage of total loans	2.10%	2.20%
Total commercial real estate percentage of total loans	41.00%	40.40%
Home equity loans percentage of total loans	2.50%	2.50%
Home equity lines of credit percentage of total loans	2.40%	2.40%
Other percentage of total loans	3.10%	3.00%
Total consumer real estate percentage of total loans	8.00%	7.90%
Total real estate percentage of total loans	49.00%	48.30%
Total consumer and other percentage of total loans	3.60%	4.00%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,197,717	$ 1,309,314
Percentage of energy production loans to total loans	8.30%	9.30%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 173,473	$ 168,775
Percentage of energy service loans to total loans	1.20%	1.20%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 110,159	$ 124,509
Percentage of other energy loans to total loans	0.70%	0.90%