Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Contract	Dec. 31, 2011	Dec. 31, 2010
Summary Of Allowance For Loan Losses On Impaired Loans [Line Items]
Loans held for sale	$ 11,457,000	$ 10,629,000
Average recorded investment in impaired loans	24,025,000	22,348,000	17,242,000
Nonaccrual loans still accruing, past due 90 days, restructured loans and foreclosed assets	25,462,000	29,535,000
Non-accrual loans	21,800,000	19,975,000
Interest income recognized on impaired loans	384,000	1,137,000	425,000
Number of loans	8
Default for purposes of this disclosure is a troubled debt restructured loan	90 days
Loans held by subsidiaries subject to blanket liens	223,678,000
Letters of credit outstanding, amount	$ 87,700,000