Loans - Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,184,960	$ 4,120,522
Total energy	1,656,360	1,758,195
Commercial mortgages	3,351,533	3,285,041
Construction	752,916	720,695
Land	282,354	286,991
Total commercial real estate	4,386,803	4,292,727
Home equity loans	341,327	340,528
Home equity lines of credit	236,290	233,525
Other	302,611	306,696
Total consumer real estate	880,228	880,749
Total real estate	5,267,031	5,173,476
Consumer and other	433,684	434,338
Total loans	$ 11,542,035	$ 11,486,531
Total commercial and industrial percentage of total loans	36.30%	35.90%
Percentage of total energy loans to total loans	14.40%	15.30%
Commercial mortgages percentage of total loans	29.00%	28.60%
Construction percentage of total loans	6.50%	6.30%
Land percentage of total loans	2.50%	2.50%
Total commercial real estate percentage of total loans	38.00%	37.40%
Home equity loans percentage of total loans	3.00%	3.00%
Home equity lines of credit percentage of total loans	2.00%	2.00%
Other percentage of total loans	2.60%	2.60%
Total consumer real estate percentage of total loans	7.60%	7.60%
Total real estate percentage of total loans	45.60%	45.00%
Total consumer and other percentage of total loans	3.70%	3.80%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	$ 1,175,149	$ 1,249,678
Percentage of energy production loans to total loans	10.20%	10.90%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	$ 251,147	$ 272,934
Percentage of energy service loans to total loans	2.20%	2.40%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total energy	$ 230,064	$ 235,583
Percentage of other energy loans to total loans	2.00%	2.00%