Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 134,929	$ 150,226	$ 132,132	$ 155,364
Provision for loan losses	8,001	2,650	25,404	17,846
Charge-offs	(11,619)	(18,596)	(33,492)	(47,326)
Recoveries	5,248	3,298	12,515	11,694
Net charge-offs	(6,371)	(15,298)	(20,977)	(35,632)
Ending balance	136,559	137,578	136,559	137,578
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,714	57,713	48,580	59,614
Provision for loan losses	(3,527)	1,085	9,999	10,407
Charge-offs	(2,705)	(8,491)	(8,782)	(21,896)
Recoveries	1,185	684	2,870	2,866
Net charge-offs	(1,520)	(7,807)	(5,912)	(19,030)
Ending balance	52,667	50,991	52,667	50,991
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	25,818	37,313	29,052	51,528
Provision for loan losses	8,788	(521)	7,478	(9,811)
Charge-offs	(2,000)	(5,400)	(4,000)	(10,939)
Recoveries	740	53	816	667
Net charge-offs	(1,260)	(5,347)	(3,184)	(10,272)
Ending balance	33,346	31,445	33,346	31,445
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	35,914	38,918	38,777	30,948
Provision for loan losses	(607)	250	(2,972)	8,533
Charge-offs	0	0	(617)	(619)
Recoveries	46	36	165	342
Net charge-offs	46	36	(452)	(277)
Ending balance	35,353	39,204	35,353	39,204
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,637	6,336	6,103	5,657
Provision for loan losses	(650)	246	859	1,615
Charge-offs	(557)	(431)	(2,936)	(1,632)
Recoveries	454	43	858	554
Net charge-offs	(103)	(388)	(2,078)	(1,078)
Ending balance	4,884	6,194	4,884	6,194
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,846	9,946	9,620	7,617
Provision for loan losses	3,997	1,590	10,040	7,102
Charge-offs	(6,357)	(4,274)	(17,157)	(12,240)
Recoveries	2,823	2,482	7,806	7,265
Net charge-offs	(3,534)	(1,792)	(9,351)	(4,975)
Ending balance	$ 10,309	$ 9,744	$ 10,309	$ 9,744