Loans and Allowance for Loan Losses - Additional Information (Detail)				3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Minimum number of days to consider the loans as non-accrual						Greater than 90 days
Loans individually evaluated for impairment	$ 33,712,000	$ 28,601,000	$ 21,788,000	$ 33,712,000	$ 21,788,000	$ 33,712,000	$ 21,788,000	$ 28,601,000
Contractual balance	47,473,000	35,030,000	29,966,000	47,473,000	29,966,000	47,473,000	29,966,000	35,030,000
Loans held for sale	31,591,000	33,543,000	21,605,000	31,591,000	21,605,000	31,591,000	21,605,000	33,543,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	34,938,000	29,768,000	22,742,000	34,938,000	22,742,000	34,938,000	22,742,000	29,768,000
Additional funds advanced in connection with impaired loans	0			0		0
Interest income recognized on impaired loans	0	0	0
Interest income recognized on impaired loans				0	0	0	0	922,000
Allowance for loan losses	45,298,000	41,877,000	40,420,000	$ 45,298,000	$ 40,420,000	$ 45,298,000	$ 40,420,000	41,877,000	$ 45,060,000	$ 38,999,000	$ 36,824,000
Default for purposes of this disclosure is a troubled debt restructured loan						90 days
Number of loans modified | SecurityLoan				2	0	3	1
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0			$ 0		$ 0
Loans held by subsidiaries subject to blanket liens	2,037,821,000			2,037,821,000		2,037,821,000
Advances from Federal Home Loan Bank of Dallas	160,000,000	299,020,000	170,028,000	160,000,000	$ 170,028,000	160,000,000	$ 170,028,000	299,020,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	1,853,000	2,178,000	2,422,000	1,853,000	2,422,000	1,853,000	2,422,000	2,178,000
Contractual balance	2,528,000	2,936,000	3,213,000	2,528,000	3,213,000	2,528,000	3,213,000	2,936,000
Allowance for loan losses	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0