Loans (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013 Item	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2013
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%	10.00%
Commercial and industrial loans related to overdraft for correspondent bank customer	$ 6,800,000	$ 102,400,000
Minimum number of days to be considered past due	90 days
Troubled debt restructurings	11,215,000	2,188,000	7,750,000
Average recorded investment in impaired loans			108,900,000
Previously restructured loans past due	1,900,000
Amount charged off related to loans restructured	1,500,000
Number of days past due for payment of interest on loans, minimum	120 days
Number of loans with calculated risk of 10 or higher greater than $5 million	3
Loan amount threshold	5,000,000
Loan value with weighted-average loan-to-value	25,300,000
Weighted-average loan-to-value ratio	80.70%
Texas Leading Index economic condition indicator		123.5		128.8
Maximum origination value of loans which were subject to further refinement of valuation methodology	1,000,000
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of owner-occupied commercial real estate loans	58.00%
Commercial And Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial loans related to overdraft for correspondent bank customer		95,300,000
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	10,100,000	2,200,000
Additional interest that would have been recognized on non-accrual loans, net of tax	2,200,000	2,600,000	3,300,000
Consumer Real Estate Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	0	0	932,000
Average recorded investment in impaired loans			1,800,000
Consumer And Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment in impaired loans			265,000
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]
Troubled debt restructurings	4,937,000	1,478,000	179,000
Number of days past due for payment of interest on loans, minimum	180 days
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment in impaired loans			53,800,000
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment in impaired loans			$ 53,000,000