Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended									9 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2012 Point	Sep. 30, 2011	Sep. 30, 2012 Point	Sep. 30, 2011	Dec. 31, 2011 Point	Sep. 30, 2012 Construction	Sep. 30, 2011 Construction	Sep. 30, 2012 Construction	Sep. 30, 2011 Construction	Dec. 31, 2011 Construction	Sep. 30, 2012 Qualitative Factor Economic Condition Point	Dec. 31, 2011 Qualitative Factor Economic Condition Point	Sep. 30, 2012 Qualitative Factor Past Due Loans Point	Dec. 31, 2011 Qualitative Factor Past Due Loans Point	Sep. 30, 2012 Qualitative Factor Other Internal and External Factors Point	Dec. 31, 2011 Qualitative Factor Other Internal and External Factors Point	Sep. 30, 2012 Overall Qualitative Factor	Dec. 31, 2011 Overall Qualitative Factor	Sep. 30, 2012 Installment Loans	Sep. 30, 2012 Bank Overdrafts	Sep. 30, 2012 Partial charge-off loan	Sep. 30, 2012 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		90 days
Period of performance borrower need to demonstrate for a loan be restored to accrual status																						6 months
Loan partial charge-off	$ 259,000	$ 403,000	$ 897,000	$ 1,388,000		$ 0	$ 0	$ 0	$ 0												$ 371,000
Loans charge off period																			90 days	45 days
Value of significant property																						200,000
Loan portfolio risk factor	1.15		1.15		1.15						1.20	1.20	1.20	1.20	1.20	1.20
Increase in estimated allowance for loan losses																	306,000	317,000
Days loans are past due to be nonperforming loans			90 days
TDR loans, commitments to lend additional funds						$ 33,000		$ 33,000		$ 192,000