Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) Item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Item	Jun. 30, 2016 USD ($)	May 31, 2017	Dec. 31, 2016
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%
Percentage of total energy loans to total loans	11.30%		11.30%			11.60%
Troubled debt restructurings			$ 13,101	$ 22,980
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	0		0
Minimum Number Of Days To Be Considered Past Due			90 days
Amount charged off of previously restructured loans			$ 10,000	9,500
Proceeds from sale of loans			0	30,470
Texas Leading Index economic condition indicator					124.7	123.1
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings			12,458	20,795
Non-Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings			11,400
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 798	$ 936	1,649	$ 1,780
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	$ 42,900		$ 42,900