Loans and the Allowance for Credit Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Contract	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Deferred loan fees and costs	$ 2,400,000	$ 2,700,000
Aggregate amount of residential real estate, home equity and consumer loans classified as substandard	15,200,000	14,400,000
Internally assigned loan grades to residential real estate, home equity and consumer loans	2,200,000	2,000,000
Non-accrual status period	90 days
Number of restructured contracts greater than $1 million	0
Accruing and non accrual TDR permitted interest-only payment period	3 months
Percentage of TDRs defaulted during the period that were restructured within the last twelve months	0.20%
Other real estate owned	4,920,000	4,689,000
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Other real estate owned	600,000	400,000
Foreclosure proceedings in process on residential real estate loans	$ 5,600,000