Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 153,056	$ 161,880	$ 153,045	$ 135,859
Provision for loan losses	8,426	9,189	16,378	37,689
Charge-offs	(15,535)	(24,900)	(26,899)	(30,678)
Recoveries	3,611	3,545	7,034	6,844
Net charge-offs	(11,924)	(21,355)	(19,865)	(23,834)
Ending balance	149,558	149,714	149,558	149,714
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	45,583	45,084	52,915	42,993
Provision for loan losses	8,184	6,460	3,581	9,683
Charge-offs	(5,579)	(4,857)	(9,106)	(6,718)
Recoveries	718	891	1,516	1,620
Net charge-offs	(4,861)	(3,966)	(7,590)	(5,098)
Ending balance	48,906	47,578	48,906	47,578
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	61,793	84,973	60,653	54,696
Provision for loan losses	(1,280)	(1,887)	4,085	29,401
Charge-offs	(6,317)	(16,749)	(10,595)	(17,760)
Recoveries	81	2	134	2
Net charge-offs	(6,236)	(16,747)	(10,461)	(17,758)
Ending balance	54,277	66,339	54,277	66,339
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	34,009	23,587	30,213	24,313
Provision for loan losses	(1,470)	2,993	2,281	2,199
Charge-offs	(14)	(19)	(14)	(47)
Recoveries	477	502	522	598
Net charge-offs	463	483	508	551
Ending balance	33,002	27,063	33,002	27,063
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,823	3,786	4,238	4,659
Provision for loan losses	601	75	1,090	(897)
Charge-offs	(2)	(23)	(13)	(177)
Recoveries	113	97	220	350
Net charge-offs	111	74	207	173
Ending balance	5,535	3,935	5,535	3,935
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,848	4,450	5,026	9,198
Provision for loan losses	2,391	1,548	5,341	(2,697)
Charge-offs	(3,623)	(3,252)	(7,171)	(5,976)
Recoveries	2,222	2,053	4,642	4,274
Net charge-offs	(1,401)	(1,199)	(2,529)	(1,702)
Ending balance	$ 7,838	$ 4,799	$ 7,838	$ 4,799