Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 5,179,780	$ 5,111,957
Energy:	1,546,918	1,602,598
Commercial mortgages	4,555,171	4,121,966
Construction	1,370,206	1,267,717
Land	290,706	306,755
Total commercial real estate	6,216,083	5,696,438
Home equity loans	363,224	353,924
Home equity lines of credit	350,461	337,168
Other	458,618	427,898
Total consumer real estate	1,172,303	1,118,990
Total real estate	7,388,386	6,815,428
Consumer and other	519,844	569,750
Total loans	$ 14,634,928	$ 14,099,733
Total commercial and industrial percentage of total loans	35.40%	36.30%
Percentage of total energy loans to total loans	10.50%	11.40%
Commercial mortgages percentage of total loans	31.10%	29.20%
Construction percentage of total loans	9.40%	9.00%
Land percentage of total loans	2.00%	2.20%
Total commercial real estate percentage of total loans	42.50%	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	50.50%	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:	$ 1,282,180	$ 1,309,314
Percentage of energy production loans to total loans	8.70%	9.30%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 189,350	$ 168,775
Percentage of energy service loans to total loans	1.30%	1.20%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 75,388	$ 124,509
Percentage of other energy loans to total loans	0.50%	0.90%