Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) Item	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Item	Jun. 30, 2019 USD ($)	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	$ 314,200		$ 314,200		$ 298,500
Number Of COVID-19 Loan Deferments At Period-End | Item	3,100		3,100
COVID 19 Loan Deferment	$ 2,100,000		$ 2,100,000
Financing Receivable, Threshold Period Past Due	90 days		90 days
Texas Leading Index economic condition indicator	106.7		106.7		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	$ 570	$ 1,100	$ 1,600	$ 2,100
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total energy loans to total loans	7.90%		7.90%		11.20%
Percentage of total energy loans to total loans excluding PPP Loans	9.60%		9.60%
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 982,600		$ 982,600
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	63,000		63,000
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 39,900		$ 39,900		$ 45,500