Loans - Additional Information (Detail) (USD $)	9 Months Ended				9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2014 Item	Sep. 30, 2013	Jul. 31, 2014	Dec. 31, 2013	Sep. 30, 2014 Commercial Real Estate Loans [Member]	Sep. 30, 2014 Commercial and industrial, Energy [Member]	Sep. 30, 2013 Commercial and industrial, Energy [Member]	Sep. 30, 2014 Non-Accrual Loans [Member]	Sep. 30, 2013 Non-Accrual Loans [Member]	Sep. 30, 2014 Non-Accrual Loans [Member]	Sep. 30, 2013 Non-Accrual Loans [Member]	Sep. 30, 2014 Grade 12 [Member]	Sep. 30, 2014 Grade 13 [Member]	Sep. 30, 2014 Commercial and industrial, Other commercial [Member]	Sep. 30, 2013 Commercial and industrial, Other commercial [Member]	Sep. 30, 2014 Commercial and industrial, Other commercial [Member] Commercial Portfolio Segment [Member]	Sep. 30, 2014 Commercial real estate, Buildings, land and other [Member]	Sep. 30, 2013 Commercial real estate, Buildings, land and other [Member]	Sep. 30, 2014 Commercial real estate, Buildings, land and other [Member] Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 6,874,000	$ 13,833,000				$ 0	$ 528,000							$ 3,752,000	$ 5,862,000	$ 2,900,000	$ 3,122,000	$ 7,443,000	$ 3,100,000
Percentage of owner-occupied commercial real estate loans					55.00%
Maximum loan-to-value percentage on home equity loans	80.00%
Maximum percentage loan related to single industry	10.00%
Percentage of loans related to a single industry						14.90%
Minimum number of days to be considered past due	90 days
Troubled Debt Restructurings	6,524,000	12,604,000				0	537,000							3,516,000	5,067,000		3,008,000	7,000,000
Additional interest that would have been recognized on non-accrual loans, net of tax								414,000	568,000	1,100,000	1,800,000
Previously restructured loans past due	0
Charge-off of previously restructures loans														627,000			0
Previously Restructured Loans Moved to OREO																	500,000
Number of days past due for payment of interest on loans, minimum	120 days											120 days		180 days
General percentage of valuation allowance in excess of principal balance, maximum												30.00%
General percentage of valuation allowance in excess of principal balance, minimum													30.00%
Number of loans with calculated risk of 10 or higher greater than $5 million	5
Loan amount threshold	5,000,000
Loan value with weighted-average loan-to-value	33,000,000
Weighted-average loan-to-value ratio	59.80%
Texas Leading Index economic condition indicator			132.4	129.1
Maximum origination value of loans which were subject to further refinement of valuation methodology	$ 1,000,000