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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance			$ 263,177
Impact of adopting ASC 326	$ 0	$ 0	0	$ 39,377
Ending balance	250,150		250,150
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	65,271	98,536	73,843	51,593
Impact of adopting ASC 326				21,263
Credit loss, expense (reversal)	(1,020)	(18,547)	(8,886)	10,737
Charge-offs	(1,611)	(8,605)	(4,485)	(14,815)
Recoveries	1,126	1,121	3,294	3,727
Net charge-offs	(485)	(7,484)	(1,191)	(11,088)
Ending balance	63,766	72,505	63,766	72,505
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	28,010	40,817	39,553	37,382
Impact of adopting ASC 326				(10,453)
Credit loss, expense (reversal)	(7,720)	13,814	(19,048)	96,478
Charge-offs	0	0	(1,433)	(68,842)
Recoveries	159	0	1,377	66
Net charge-offs	159	0	(56)	(68,776)
Ending balance	20,449	54,631	20,449	54,631
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	147,993	93,425	134,892	31,037
Impact of adopting ASC 326				(13,519)
Credit loss, expense (reversal)	4,007	25,368	16,583	104,716
Charge-offs	0	(242)	(137)	(3,826)
Recoveries	54	42	716	185
Net charge-offs	54	(200)	579	(3,641)
Ending balance	152,054	118,593	152,054	118,593
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,154	8,998	7,926	4,113
Impact of adopting ASC 326				2,392
Credit loss, expense (reversal)	(982)	1,794	(3,093)	3,716
Charge-offs	0	(1,088)	(672)	(1,508)
Recoveries	857	573	1,868	1,564
Net charge-offs	857	(515)	1,196	56
Ending balance	6,029	10,277	6,029	10,277
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	7,860	8,285	6,963	8,042
Impact of adopting ASC 326				(2,248)
Credit loss, expense (reversal)	2,692	1,161	7,042	8,096
Charge-offs	(5,073)	(4,219)	(13,015)	(13,402)
Recoveries	2,373	2,242	6,862	6,981
Net charge-offs	(2,700)	(1,977)	(6,153)	(6,421)
Ending balance	7,852	7,469	7,852	7,469
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	255,288	250,061	263,177	132,167
Impact of adopting ASC 326				(2,565)
Credit loss, expense (reversal)	(3,023)	23,590	(7,402)	223,743
Charge-offs	(6,684)	(14,154)	(19,742)	(102,393)
Recoveries	4,569	3,978	14,117	12,523
Net charge-offs	(2,115)	(10,176)	(5,625)	(89,870)
Ending balance	$ 250,150	$ 263,475	$ 250,150	$ 263,475