Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Item	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Item	Sep. 30, 2016 USD ($)	Aug. 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	10.90%		10.90%			11.60%
Troubled debt restructurings			$ 58,898	$ 34,099
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	1		1
Previously Restructured Loans Past Due	$ 43,137		$ 43,137
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					124.6	123.1
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings			55,023	31,918
Non-Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings			54,100
Additional interest that would have been recognized on non-accrual loans, net of tax	783	$ 647	2,432	$ 2,427
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	$ 40,900		$ 40,900