Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			$ 132,167
Impact of adopting ASC 326	$ 0	$ 0	39,377	$ 0
Ending balance	263,475		263,475
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	98,536	57,714	51,593	48,580
Impact of adopting ASC 326			21,263
Credit loss expense	(18,547)	(3,527)	10,737	9,999
Charge-offs	(8,605)	(2,705)	(14,815)	(8,782)
Recoveries	1,121	1,185	3,727	2,870
Net charge-offs	(7,484)	(1,520)	(11,088)	(5,912)
Ending balance	72,505	52,667	72,505	52,667
Commercial and Industrial Total Energy Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	40,817	25,818	37,382	29,052
Impact of adopting ASC 326			(10,453)
Credit loss expense	13,814	8,788	96,478	7,478
Charge-offs	0	(2,000)	(68,842)	(4,000)
Recoveries	0	740	66	816
Net charge-offs	0	(1,260)	(68,776)	(3,184)
Ending balance	54,631	33,346	54,631	33,346
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	93,425	35,914	31,037	38,777
Impact of adopting ASC 326			(13,519)
Credit loss expense	25,368	(607)	104,716	(2,972)
Charge-offs	(242)	0	(3,826)	(617)
Recoveries	42	46	185	165
Net charge-offs	(200)	46	(3,641)	(452)
Ending balance	118,593	35,353	118,593	35,353
Total consumer real estate loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,998	5,637	4,113	6,103
Impact of adopting ASC 326			2,392
Credit loss expense	1,794	(650)	3,716	859
Charge-offs	(1,088)	(557)	(1,508)	(2,936)
Recoveries	573	454	1,564	858
Net charge-offs	(515)	(103)	56	(2,078)
Ending balance	10,277	4,884	10,277	4,884
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,285	9,846	8,042	9,620
Impact of adopting ASC 326			(2,248)
Credit loss expense	1,161	3,997	8,096	10,040
Charge-offs	(4,219)	(6,357)	(13,402)	(17,157)
Recoveries	2,242	2,823	6,981	7,806
Net charge-offs	(1,977)	(3,534)	(6,421)	(9,351)
Ending balance	7,469	10,309	7,469	10,309
Loans Receivable [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	250,061	134,929	132,167	132,132
Impact of adopting ASC 326			(2,565)
Credit loss expense	23,590	8,001	223,743	25,404
Charge-offs	(14,154)	(11,619)	(102,393)	(33,492)
Recoveries	3,978	5,248	12,523	12,515
Net charge-offs	(10,176)	(6,371)	(89,870)	(20,977)
Ending balance	$ 263,475	$ 136,559	$ 263,475	$ 136,559