Loans - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
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	11.00%		11.00%		11.40%
Loans and Leases Receivable, Related Parties	$ 238,600		$ 238,600		$ 166,400
Restructured loans on non-accrual status at period end			$ 0	$ 58,898
Texas Leading Index economic condition indicator	129.4		129.4		129.3
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			$ 0	55,023
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end			0	54,082
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 1,300	$ 783	4,200	$ 2,400
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	$ 46,700		$ 46,700