Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Apr. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized subsequent to classification as impaired			$ 0		$ 0
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%
Percentage of total energy loans to total loans	11.10%		11.10%		11.40%
Loans and Leases Receivable, Related Parties	$ 213,200		$ 213,200		$ 166,400
Restructured loans on non-accrual status at period end			843	$ 13,101
Texas Leading Index economic condition indicator					129.4	129.7
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			0	12,458
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			843	11,405
Additional interest that would have been recognized on non-accrual loans, net of tax	1,400	$ 798	2,900	$ 1,600
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,100,000		1,100,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 47,600		$ 47,600