Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 136,350	$ 149,885	$ 132,132	$ 155,364
Provision for loan losses	6,400	8,251	17,403	15,196
Charge-offs	(11,650)	(11,932)	(21,873)	(28,730)
Recoveries	3,829	4,022	7,267	8,396
Net charge-offs	(7,821)	(7,910)	(14,606)	(20,334)
Ending balance	134,929	150,226	134,929	150,226
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	58,571	57,733	48,580	59,614
Provision for loan losses	1,597	3,528	13,526	9,322
Charge-offs	(3,389)	(4,153)	(6,077)	(13,405)
Recoveries	935	605	1,685	2,182
Net charge-offs	(2,454)	(3,548)	(4,392)	(11,223)
Ending balance	57,714	57,713	57,714	57,713
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	25,343	39,039	29,052	51,528
Provision for loan losses	2,446	350	(1,310)	(9,290)
Charge-offs	(2,000)	(2,689)	(2,000)	(5,539)
Recoveries	29	613	76	614
Net charge-offs	(1,971)	(2,076)	(1,924)	(4,925)
Ending balance	25,818	37,313	25,818	37,313
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	36,455	38,474	38,777	30,948
Provision for loan losses	(13)	840	(2,365)	8,283
Charge-offs	(557)	(614)	(617)	(619)
Recoveries	29	218	119	306
Net charge-offs	(528)	(396)	(498)	(313)
Ending balance	35,914	38,918	35,914	38,918
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,661	6,349	6,103	5,657
Provision for loan losses	262	151	1,509	1,369
Charge-offs	(601)	(482)	(2,379)	(1,201)
Recoveries	315	318	404	511
Net charge-offs	(286)	(164)	(1,975)	(690)
Ending balance	5,637	6,336	5,637	6,336
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	10,320	8,290	9,620	7,617
Provision for loan losses	2,108	3,382	6,043	5,512
Charge-offs	(5,103)	(3,994)	(10,800)	(7,966)
Recoveries	2,521	2,268	4,983	4,783
Net charge-offs	(2,582)	(1,726)	(5,817)	(3,183)
Ending balance	$ 9,846	$ 9,946	$ 9,846	$ 9,946

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of other addition (reduction) in allowance reserve for credit loss.

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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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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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

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