Loans and Allowance for Loan Losses - Additional Information (Detail)				3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Security_Loan	Sep. 30, 2014 USD ($) Security_Loan	Sep. 30, 2015 USD ($) Security_Loan	Sep. 30, 2014 USD ($) Security_Loan	Dec. 31, 2014 USD ($)
Receivables [Abstract]
Minimum number of days to consider the loans as non-accrual						Greater than 90 days
Carrying amount of purchased credit impaired loans	$ 2,422,000	$ 2,151,000	$ 2,383,000
Contractual balance	3,213,000	3,275,000	3,525,000	$ 3,213,000	$ 3,525,000	$ 3,213,000	$ 3,525,000	$ 3,275,000
Loans held for sale	21,605,000	8,803,000	11,266,000	21,605,000	11,266,000	21,605,000	11,266,000	8,803,000
Average recorded investment in impaired loans				20,026,000		19,755,000	26,524,000	24,497,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	22,742,000	21,716,000	23,629,000	22,742,000	23,629,000	22,742,000	23,629,000	21,716,000
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								162,000
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 | Security_Loan				0	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	1,752,727,000			1,752,727,000		1,752,727,000
Letters of credit outstanding, amount	5,000,000			5,000,000		5,000,000
Advances from Federal Home Loan Bank of Dallas	$ 170,028,000	$ 1,010,000	$ 1,038,000	$ 170,028,000	$ 1,038,000	$ 170,028,000	$ 1,038,000	$ 1,010,000