Loans and Allowance for Loan Losses - Additional Information (Detail)				3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2015 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2015 USD ($)
Receivables [Abstract]
Minimum number of days to consider the loans as non-accrual						Greater than 90 days
Carrying amount of purchased credit impaired loans	$ 1,654,000	$ 2,178,000	$ 1,123,000
Contractual balance	2,362,000	2,936,000	1,648,000	$ 2,362,000	$ 1,648,000	$ 2,362,000	$ 1,648,000	$ 2,936,000
Loans held for sale	25,733,000	33,543,000	25,544,000	25,733,000	25,544,000	25,733,000	25,544,000	33,543,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	40,387,000	29,768,000	18,135,000	40,387,000	18,135,000	40,387,000	18,135,000	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 purchased credit impaired loans	0	0	0	$ 0	$ 0	$ 0	$ 0	0
Default for purposes of this disclosure is a troubled debt restructured loan						90 days
Number of loans modified | SecurityLoan				1	0	1	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	2,029,811,000			2,029,811,000		2,029,811,000
Advances from Federal Home Loan Bank of Dallas	$ 210,000,000	$ 299,020,000	$ 265,036,000	$ 210,000,000	$ 265,036,000	$ 210,000,000	$ 265,036,000	$ 299,020,000