Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 135,859	$ 99,542
Provision for loan losses	28,500	8,162
Charge-offs	(5,778)	(5,495)
Recoveries	3,299	3,499
Net charge-offs	(2,479)	(1,996)
Ending balance	161,880	105,708
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	42,993	44,273
Provision for loan losses	3,223	346
Charge-offs	(1,861)	(2,132)
Recoveries	729	1,280
Net charge-offs	(1,132)	(852)
Ending balance	45,084	43,767
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	54,696	14,919
Provision for loan losses	31,288	6,025
Charge-offs	(1,011)	0
Recoveries	0	2
Net charge-offs	(1,011)	2
Ending balance	84,973	20,946
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	24,313	27,163
Provision for loan losses	(794)	248
Charge-offs	(28)	(478)
Recoveries	96	172
Net charge-offs	68	(306)
Ending balance	23,587	27,105
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,659	5,178
Provision for loan losses	(972)	(243)
Charge-offs	(154)	(80)
Recoveries	253	52
Net charge-offs	99	(28)
Ending balance	3,786	4,907
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,198	8,009
Provision for loan losses	(4,245)	1,786
Charge-offs	(2,724)	(2,805)
Recoveries	2,221	1,993
Net charge-offs	(503)	(812)
Ending balance	$ 4,450	$ 8,983