Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 161,880	$ 105,708	$ 135,859	$ 99,542
Provision for loan losses	9,189	2,873	37,689	11,035
Charge-offs	(24,900)	(5,087)	(30,678)	(10,582)
Recoveries	3,545	3,113	6,844	6,612
Net charge-offs	(21,355)	(1,974)	(23,834)	(3,970)
Ending balance	149,714	106,607	149,714	106,607
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	45,084	43,767	42,993	44,273
Provision for loan losses	6,460	341	9,683	687
Charge-offs	(4,857)	(2,374)	(6,718)	(4,506)
Recoveries	891	691	1,620	1,971
Net charge-offs	(3,966)	(1,683)	(5,098)	(2,535)
Ending balance	47,578	42,425	47,578	42,425
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	84,973	20,946	54,696	14,919
Provision for loan losses	(1,887)	5,353	29,401	11,378
Charge-offs	(16,749)	0	(17,760)	0
Recoveries	2	1	2	3
Net charge-offs	(16,747)	1	(17,758)	3
Ending balance	66,339	26,300	66,339	26,300
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	23,587	27,105	24,313	27,163
Provision for loan losses	2,993	(2,464)	2,199	(2,216)
Charge-offs	(19)	0	(47)	(478)
Recoveries	502	294	598	466
Net charge-offs	483	294	551	(12)
Ending balance	27,063	24,935	27,063	24,935
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,786	4,907	4,659	5,178
Provision for loan losses	75	(286)	(897)	(529)
Charge-offs	(23)	(21)	(177)	(101)
Recoveries	97	102	350	154
Net charge-offs	74	81	173	53
Ending balance	3,935	4,702	3,935	4,702
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,450	8,983	9,198	8,009
Provision for loan losses	1,548	(71)	(2,697)	1,715
Charge-offs	(3,252)	(2,692)	(5,976)	(5,497)
Recoveries	2,053	2,025	4,274	4,018
Net charge-offs	(1,199)	(667)	(1,702)	(1,479)
Ending balance	$ 4,799	$ 8,245	$ 4,799	$ 8,245