Loans (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Day	Sep. 30, 2011	Jul. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum loan-to-value percentage on home equity loans			80.00%
Maximum percentage loan related to single industry			10.00%
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 646,000	$ 823,000	$ 1,900,000	$ 2,600,000
Troubled debt restructuring	445,000	9,100,000	445,000	9,100,000
Number of days past due for payment of interest on loans, minimum			120
Number of loans with calculated risk of 10 or higher greater than $5 million			3
Loan amount threshold	5,000,000		5,000,000
Loan value with weighted-average loan-to-value			31,800,000
Weighted-average loan-to-value ratio			68.00%
Texas Leading Index economic condition indicator		118.3		118.3	122.2	120.3
Maximum origination value of loans which were subject to further refinement of valuation methodology			1,000,000
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
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring		7,500,000		7,500,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring		969,000		969,000
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring		469,000		469,000
Commercial And Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring		$ 191,000		$ 191,000
Grade Twelve [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 Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of owner-occupied commercial real estate loans			56.20%