Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended							3 Months Ended	12 Months Ended	3 Months Ended			12 Months Ended	3 Months Ended
	Mar. 31, 2013 Point	Mar. 31, 2012	Dec. 31, 2012 Point	Mar. 31, 2013 Qualitative Factor Economic Condition Point	Dec. 31, 2012 Qualitative Factor Economic Condition Point	Mar. 31, 2013 Qualitative Factor Past Due Loans Point	Dec. 31, 2012 Qualitative Factor Past Due Loans Point	Mar. 31, 2013 Qualitative Factor Other Internal and External Factors Point	Dec. 31, 2011 Qualitative Factor Other Internal and External Factors Point	Mar. 31, 2013 Overall Qualitative Factor	Dec. 31, 2012 Overall Qualitative Factor	Mar. 31, 2013 Installment Loans	Mar. 31, 2013 Bank Overdrafts	Mar. 31, 2013 Classified Loans	Dec. 31, 2012 Classified Loans	Mar. 31, 2013 Partial charge-off loan	Mar. 31, 2013 Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days principal payments should be past due for recognition of income on a loan be discontinued and previously accrued interest is reversed	90 days
Period of performance borrower need to demonstrate for a loan be restored to accrual status																	6 months
Loan charge-off, amount	$ 317,000	$ 485,000														$ 473,000
Loan charge-off period												90 days	45 days
Value of significant property																	200,000
Loan portfolio risk factor	1.18		1.15	1.20	1.20	1.20	1.20	1.30	1.20
Increase in estimated allowance for loan losses	$ 74,000									$ 483,000	$ 419,000			$ 648,000	$ 664,000
Days loans are past due to be nonperforming loans	90 days		90 days