Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 132,167
Impact of adopting ASC 326	39,377	$ 0
Ending balance	263,881
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	51,593	48,580
Impact of adopting ASC 326	21,263
Credit loss expense	21,950	11,929
Charge-offs	(4,369)	(2,688)
Recoveries	1,715	750
Net charge-offs	(2,654)	(1,938)
Ending balance	92,152	58,571
Commercial and Industrial Total Energy Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	37,382	29,052
Impact of adopting ASC 326	(10,453)
Credit loss expense	110,006	(3,756)
Charge-offs	(33,800)	0
Recoveries	66	47
Net charge-offs	(33,734)	47
Ending balance	103,201	25,343
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31,037	38,777
Impact of adopting ASC 326	(13,519)
Credit loss expense	34,761	(2,352)
Charge-offs	(73)	(60)
Recoveries	113	90
Net charge-offs	40	30
Ending balance	52,319	36,455
Total consumer real estate loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,113	6,103
Impact of adopting ASC 326	2,392
Credit loss expense	1,570	1,247
Charge-offs	(285)	(1,778)
Recoveries	380	89
Net charge-offs	95	(1,689)
Ending balance	8,170	5,661
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,042	9,620
Impact of adopting ASC 326	(2,248)
Credit loss expense	4,638	3,935
Charge-offs	(5,005)	(5,697)
Recoveries	2,612	2,462
Net charge-offs	(2,393)	(3,235)
Ending balance	8,039	10,320
Loans Receivable [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	132,167	132,132
Impact of adopting ASC 326	(2,565)
Credit loss expense	172,925	11,003
Charge-offs	(43,532)	(10,223)
Recoveries	4,886	3,438
Net charge-offs	(38,646)	(6,785)
Ending balance	$ 263,881	$ 136,350

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount, after recoveries, of loans and leases that have been written-off.

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- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

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Amount of expense related to estimated loss from loan and lease transactions.

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