Loans - Investment in Loans Related to Allowance for Loan Losses by Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 11,542,035	$ 11,486,531
Commercial and industrial	4,184,960	4,120,522
Total energy	1,656,360	1,758,195
Total commercial real estate	4,386,803	4,292,727
Total consumer real estate	880,228	880,749
Consumer and other	433,684	434,338
Loans Individually Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	171,820	77,478
Commercial and industrial	24,757	22,860
Total energy	114,091	21,139
Total commercial real estate	32,515	32,994
Total consumer real estate	457	485
Consumer and other	0	0
Loans Collectively Evaluated for Impairment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	11,370,215	11,409,053
Commercial and industrial	4,160,203	4,097,662
Total energy	1,542,269	1,737,056
Total commercial real estate	4,354,288	4,259,733
Total consumer real estate	879,771	880,264
Consumer and other	$ 433,684	$ 434,338