Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Aug. 31, 2021	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 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%		0.00%
Maximum percentage loan related to single industry	10.00%		10.00%
Loans and Leases Receivable, Related Parties	$ 318,800		$ 318,800				$ 353,100
Texas Leading Index economic condition indicator					130.4		118.1
Financing Receivable, Allowance for Credit Loss	250,150		250,150				$ 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	$ 474	$ 670	$ 1,400	$ 2,300
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%		6.20%				7.10%
Percentage of total energy loans to total loans excluding PPP Loans	6.50%		6.50%
Financing Receivable, Allowance for Credit Loss	$ 20,449	$ 54,631	$ 20,449	$ 54,631		$ 28,010	$ 39,553	$ 40,817	$ 37,382
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	0		0				0
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,000,000		1,000,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	63,700		63,700
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 39,000		$ 39,000				$ 48,700