Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Item	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Item	Sep. 30, 2019 USD ($)	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	$ 316,000		$ 316,000		$ 298,500
Number Of COVID-19 Loan Deferments At Period-End | Item	300		300
COVID 19 Loan Deferment	$ 157,200		$ 157,200
Financing Receivable, Threshold Period Past Due	90 days		90 days
Texas Leading Index economic condition indicator	113.1		113.1		127.9
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	$ 670	$ 937	$ 2,300	$ 3,000
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total energy loans to total loans	7.40%		7.40%		11.20%
Percentage of total energy loans to total loans excluding PPP Loans	9.10%		9.10%
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 910,300		$ 910,300
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	62,600		62,600
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 43,900		$ 43,900		$ 45,500