Loans - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Item	Mar. 31, 2016 USD ($)	Feb. 28, 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%
Maximum percentage loan related to single industry	10.00%
Percentage of total energy loans to total loans	11.20%			11.60%
Troubled debt restructurings	$ 11,212	$ 61,347
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	1
Previously Restructured Loans Past Due	$ 747
Minimum Number Of Days To Be Considered Past Due	90 days
Amount charged off of previously restructured loans	$ 2,000
Proceeds from sale of loans	0	0
Texas Leading Index economic condition indicator			123.4	123.1
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	11,212	61,095
Non-Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	11,200
Additional interest that would have been recognized on non-accrual loans, net of tax	851	$ 844
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	$ 38,400