Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	0 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Receivables [Abstract]
Minimum number of days to consider the loans as non-accrual			Greater than 90 days
Carrying amount of purchased credit impaired loans	$ 2,037,000	$ 2,151,000	$ 2,037,000	$ 2,605,000	$ 2,151,000
Contractual balance			2,817,000		3,275,000
Loans held for sale			10,231,000	10,429,000	8,803,000
Average recorded investment in impaired loans			20,263,000	26,770,000	24,497,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets			20,377,000	27,553,000	21,716,000
Additional funds advanced in connection with impaired loans			0
Interest income recognized on impaired loans	0	0
Interest income recognized on impaired loans					162,000
Allowance for purchased credit impaired loans			0		0
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
Loans held by subsidiaries subject to blanket liens			1,724,000,000
Letters of credit outstanding, amount			5,000,000
Advances from Federal Home Loan Bank of Dallas			$ 15,983,000	$ 76,092,000	$ 1,010,000