Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) Item	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Item	Jun. 30, 2014 USD ($)	May. 31, 2015	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item	1		1
Maximum loan-to-value percentage on home equity loans			80.00%
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%
Minimum number of days to be considered past due			90 days
Troubled debt restructurings			$ 584	$ 6,741
Previously restructured loans past due			279
Financing receivable, modifications, pre-modification recorded investment			$ 709	6,874
Number of days past due for payment of interest on loans, minimum			120 days
Texas Leading Index economic condition indicator					127.1		129.4
Maximum origination value of loans which were subject to further refinement of valuation methodology			$ 1,000
Loans and Leases Receivable, Allowance	$ 106,607	$ 98,286	$ 106,607	98,286		$ 105,708	$ 99,542	$ 95,156	$ 92,438
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of owner-occupied commercial real estate loans			52.00%
Loans and Leases Receivable, Allowance	24,935	22,046	$ 24,935	22,046		27,105	27,163	21,020	22,590
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans related to a single industry			15.80%
Troubled debt restructurings			$ 0	0
Financing receivable, modifications, pre-modification recorded investment			0	0
Non-Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional interest that would have been recognized on non-accrual loans, net of tax	364	355	760	713
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivables, impaired, troubled debt restructuring, write-down			88
Financing receivable, modifications, pre-modification recorded investment			$ 304
Number of days past due for payment of interest on loans, minimum			180 days
Loans and Leases Receivable, Allowance	68,725	$ 55,672	$ 68,725	55,672		64,713	59,192	$ 57,054	$ 52,790
Grade 12 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days past due for payment of interest on loans, minimum			120 days
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, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings			$ 0	3,069
Financing receivable, modifications, pre-modification recorded investment			0	$ 3,122
Previously Restructured Loans Moved to OREO			500
Charge-off of previously restructures loans			0
Credit and Collateral Exceptions [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Allowance	$ 0		$ 0			$ 3,100	2,164
Credit and Collateral Exceptions [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Allowance							681
Credit and Collateral Exceptions [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Allowance							$ 1,483