Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 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%
Maximum percentage loan related to single industry	10.00%
Percentage of total energy loans to total loans	10.70%		11.40%
Loans and Leases Receivable, Related Parties	$ 180,700		$ 166,400
Restructured loans on non-accrual status at period end	14,229	$ 11,212
Texas Leading Index economic condition indicator			129.6	131.5
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	12,058	11,212
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	2,171	11,212
Additional interest that would have been recognized on non-accrual loans, net of tax	1,500	$ 851
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,200,000
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 44,800