Loans (Narrative) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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%	10.00%
Commercial and industrial loans related to overdraft for correspondent bank customer	$ 7,300,000	$ 6,300,000
Minimum number of days to be considered past due	90 days
Restructured loans on non-accrual status at period end	$ 58,484,000	44,863,000	$ 536,000
Interest income recognized subsequent to classification as impaired	$ 0	$ 0	0
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator	128.7	123.1
Maximum origination value of loans which were subject to further refinement of valuation methodology	$ 1,000,000
Sensitivity stress test price shock	75.00%
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans related to a single industry	11.40%	11.60%
Restructured loans on non-accrual status at period end	$ 54,330,000	$ 43,841,000	0
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	53,622,000	44,863,000	536,000
Additional interest that would have been recognized on non-accrual loans, net of tax	3,700,000	3,100,000	1,600,000
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	$ 3,766,000	1,022,000	536,000
Grade 12 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General percentage of valuation allowance in excess of principal balance, maximum	30.00%
Grade 13 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General percentage of valuation allowance in excess of principal balance, minimum	30.00%
Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days past due for payment of interest on loans, minimum	180 days
Commercial Real Estate Buildings Land And Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans on non-accrual status at period end	$ 0	$ 0	$ 0
Commitments to Extend Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,100,000,000
Standby Letters of Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 46,700,000