Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Receivables [Abstract]
Loans held for sale	$ 8,803,000	$ 5,163,000
Average recorded investment in impaired loans	24,497,000	31,293,000	24,025,000
Nonaccrual, past due 90 days still accruing, restructured loans and foreclosed assets	21,716,000	31,128,000
Non-accrual loans	20,194,000	27,926,000
Additional funds advanced in connection with impaired loans	0
Interest income recognized on impaired loans	0
Interest income recognized on impaired loans	162,000	685,000	384,000
Allowance for purchased credit impaired loans	0	0
Carrying amount of purchased credit impaired loans	2,151,000	2,707,000
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
Loans held by subsidiaries subject to blanket liens	1,667,472,000
Letters of credit outstanding, amount	5,000,000
Short-term borrowings outstanding	$ 1,010,000