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Loans - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Item	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration of loans in a single industry in excess of ten percent	0.00%
Maximum percentage loan related to single industry	10.00%
Loans and Leases Receivable, Related Parties	$ 365,600		$ 353,100
Number Of COVID-19 Loan Deferments At Period-End | Item	22
COVID 19 Loan Deferment	$ 11,400
Texas Leading Index economic condition indicator	121.4		118.6
Financing Receivable, Allowance for Credit Loss	$ 261,258		$ 263,177
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 453	$ 1,000
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total energy loans to total loans	6.20%		7.10%
Percentage of total energy loans to total loans excluding PPP Loans	7.50%
Financing Receivable, Allowance for Credit Loss	$ 33,472	$ 103,201	$ 39,553	$ 37,382
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	0		0
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	968,900
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	70,400
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 51,700		$ 48,700

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Maximum Percentage of Loan Related To Single Industry

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Number Of COVID-19 Loan Deferments At Period-End

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Percentage of total energy loans to total loans excluding PPP Loans

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Percentage of total energy loans to total loans

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Texas Leading Index economic condition indicator

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Unfunded balances

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

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

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