Loans - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Receivables [Abstract]
Percentage of outstanding principal balances of commercial real estate loans secured by owner-occupied properties	80.00%		80.00%		81.00%
Loans serviced for others			$ 59,000,000		$ 60,200,000
Concentrations of loans related to a single industry	0		0		0
Troubled debt restructurings, loans	8,200,000		8,200,000		8,700,000
Reserves allocated to customers whose loan terms are modified in troubled debt restructurings	823,000		823,000		718,000
Loans classified as troubled debt restructurings modified terms	7,500,000		7,500,000
Loans classified as troubled debt restructurings nonaccrual status	736,000		736,000
Loans restructured, subsequently defaulted	0	0	0	0
Outstanding balance of commercial loans classified under credit risk, minimum amount	275,000		275,000
Loans listed as not rated under risk category, maximum amount	$ 275,000		$ 275,000
Non-performing loans past due days and greater	90 days		90 days