Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)			12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 26,898,000	$ 33,543,000	$ 26,898,000	$ 33,543,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	29,000,000	29,768,000	29,000,000	29,768,000
Non accrual loans	27,371,000	28,601,000	27,371,000	28,601,000
Additional funds advanced in connection with impaired loans	0		0
Interest income recognized on impaired loans	0	0
Interest income recognized on impaired loans			829,000	922,000	$ 162,000
Allowance for loan losses	45,779,000	41,877,000	45,779,000	41,877,000	$ 36,824,000
Loans individually evaluated for impairment	27,371,000	28,601,000	$ 27,371,000	28,601,000
Default for purposes of this disclosure is a troubled debt restructured loan			90 days
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0		$ 0
Loans held by subsidiaries subject to blanket liens	2,081,615,000		2,081,615,000
Advances from Federal Home Loan Bank of Dallas	75,000,000	299,020,000	75,000,000	299,020,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	1,256,000	2,178,000	1,256,000	2,178,000
Allowance for loan losses	0	0	0	0
Loans individually evaluated for impairment	$ 1,256,000	$ 2,178,000	$ 1,256,000	$ 2,178,000