Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	255,288		255,288
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	70,892	92,152	73,843	51,593
Impact of adopting ASC 326				21,263
Credit loss, expense (reversal)	(5,901)	7,334	(7,866)	29,284
Charge-offs	(685)	(1,841)	(2,874)	(6,210)
Recoveries	965	891	2,168	2,606
Net charge-offs	280	(950)	(706)	(3,604)
Ending balance	65,271	98,536	65,271	98,536
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	33,472	103,201	39,553	37,382
Impact of adopting ASC 326				(10,453)
Credit loss, expense (reversal)	(5,527)	(27,342)	(11,328)	82,664
Charge-offs	0	(35,042)	(1,433)	(68,842)
Recoveries	65	0	1,218	66
Net charge-offs	65	(35,042)	(215)	(68,776)
Ending balance	28,010	40,817	28,010	40,817
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	144,440	52,319	134,892	31,037
Impact of adopting ASC 326				(13,519)
Credit loss, expense (reversal)	3,654	44,587	12,576	79,348
Charge-offs	(137)	(3,511)	(137)	(3,584)
Recoveries	36	30	662	143
Net charge-offs	(101)	(3,481)	525	(3,441)
Ending balance	147,993	93,425	147,993	93,425
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	5,636	8,170	7,926	4,113
Impact of adopting ASC 326				2,392
Credit loss, expense (reversal)	611	352	(2,111)	1,922
Charge-offs	(388)	(135)	(672)	(420)
Recoveries	295	611	1,011	991
Net charge-offs	(93)	476	339	571
Ending balance	6,154	8,998	6,154	8,998
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,818	8,039	6,963	8,042
Impact of adopting ASC 326				(2,248)
Credit loss, expense (reversal)	2,784	2,297	4,350	6,935
Charge-offs	(3,882)	(4,178)	(7,942)	(9,183)
Recoveries	2,140	2,127	4,489	4,739
Net charge-offs	(1,742)	(2,051)	(3,453)	(4,444)
Ending balance	7,860	8,285	7,860	8,285
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	261,258	263,881	263,177	132,167
Impact of adopting ASC 326				(2,565)
Credit loss, expense (reversal)	(4,379)	27,228	(4,379)	200,153
Charge-offs	(5,092)	(44,707)	(13,058)	(88,239)
Recoveries	3,501	3,659	9,548	8,545
Net charge-offs	(1,591)	(41,048)	(3,510)	(79,694)
Ending balance	$ 255,288	$ 250,061	$ 255,288	$ 250,061