Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,677,923	$ 4,344,000
Energy:	1,387,060	1,386,061
Commercial mortgages	3,714,172	3,481,157
Construction	1,082,229	1,043,261
Land	307,701	311,030
Total commercial real estate	5,104,102	4,835,448
Home equity loans	357,542	345,130
Home equity lines of credit	288,981	264,862
Other	367,948	326,793
Total consumer real estate	1,014,471	936,785
Total real estate	6,118,573	5,772,233
Consumer and other	522,748	473,098
Total loans	$ 12,706,304	$ 11,975,392
Total commercial and industrial percentage of total loans	36.80%	36.30%
Percentage of total energy loans to total loans	10.90%	11.60%
Commercial mortgages percentage of total loans	29.20%	29.10%
Construction percentage of total loans	8.50%	8.70%
Land percentage of total loans	2.40%	2.60%
Total commercial real estate percentage of total loans	40.10%	40.40%
Home equity loans percentage of total loans	2.80%	2.90%
Home equity lines of credit percentage of total loans	2.30%	2.20%
Other percentage of total loans	2.90%	2.70%
Total consumer real estate percentage of total loans	8.00%	7.80%
Total real estate percentage of total loans	48.10%	48.20%
Total consumer and other percentage of total loans	4.20%	3.90%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,094,927	$ 971,767
Percentage of energy production loans to total loans	8.60%	8.10%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 159,893	$ 221,213
Percentage of energy service loans to total loans	1.30%	1.80%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 132,240	$ 193,081
Percentage of other energy loans to total loans	1.00%	1.70%