Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 263,177
Impact of adopting ASC 326	0	$ 39,377
Ending balance	261,258
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	73,843	51,593
Impact of adopting ASC 326		21,263
Credit loss, expense (reversal)	(1,965)	21,950
Charge-offs	(2,189)	(4,369)
Recoveries	1,203	1,715
Net charge-offs	(986)	(2,654)
Ending balance	70,892	92,152
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	39,553	37,382
Impact of adopting ASC 326		(10,453)
Credit loss, expense (reversal)	(5,801)	110,006
Charge-offs	(1,433)	(33,800)
Recoveries	1,153	66
Net charge-offs	(280)	(33,734)
Ending balance	33,472	103,201
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	134,892	31,037
Impact of adopting ASC 326		(13,519)
Credit loss, expense (reversal)	8,922	34,761
Charge-offs	0	(73)
Recoveries	626	113
Net charge-offs	626	40
Ending balance	144,440	52,319
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	7,926	4,113
Impact of adopting ASC 326		2,392
Credit loss, expense (reversal)	(2,722)	1,570
Charge-offs	(284)	(285)
Recoveries	716	380
Net charge-offs	432	95
Ending balance	5,636	8,170
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,963	8,042
Impact of adopting ASC 326		(2,248)
Credit loss, expense (reversal)	1,566	4,638
Charge-offs	(4,060)	(5,005)
Recoveries	2,349	2,612
Net charge-offs	(1,711)	(2,393)
Ending balance	6,818	8,039
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	263,177	132,167
Impact of adopting ASC 326		(2,565)
Credit loss, expense (reversal)	0	172,925
Charge-offs	(7,966)	(43,532)
Recoveries	6,047	4,886
Net charge-offs	(1,919)	(38,646)
Ending balance	$ 261,258	$ 263,881