Loans - Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 4,253,822	$ 4,120,522
Energy:	1,503,608	1,758,195
Commercial mortgages	3,380,524	3,285,041
Construction	804,341	720,695
Land	290,752	286,991
Total commercial real estate	4,475,617	4,292,727
Home equity loans	340,196	340,528
Home equity lines of credit	248,812	233,525
Other	312,142	306,696
Total consumer real estate	901,150	880,749
Total real estate	5,376,767	5,173,476
Consumer and other	449,985	434,338
Total loans	$ 11,584,182	$ 11,486,531
Total commercial and industrial percentage of total loans	36.70%	35.90%
Percentage of total energy loans to total loans	13.00%	15.30%
Commercial mortgages percentage of total loans	29.20%	28.60%
Construction percentage of total loans	6.90%	6.30%
Land percentage of total loans	2.50%	2.50%
Total commercial real estate percentage of total loans	38.60%	37.40%
Home equity loans percentage of total loans	3.00%	3.00%
Home equity lines of credit percentage of total loans	2.10%	2.00%
Other percentage of total loans	2.70%	2.60%
Total consumer real estate percentage of total loans	7.80%	7.60%
Total real estate percentage of total loans	46.40%	45.00%
Total consumer and other percentage of total loans	3.90%	3.80%
Total loans percentage	100.00%	100.00%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 1,059,524	$ 1,249,678
Percentage of energy production loans to total loans	9.10%	10.90%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 226,637	$ 272,934
Percentage of energy service loans to total loans	2.00%	2.40%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Energy:	$ 217,447	$ 235,583
Percentage of other energy loans to total loans	1.90%	2.00%