Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 SecurityLoan	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	$ 5,400,000	$ 9,900,000
Percentage of average loans annualized, charged-offs	0.18%	0.34%
Calculated probability of default rate	Probability of default is calculated based on the historical rate of migration to impaired status during the last 16 quarters.
Special adjustment factor in reserves	1,200,000
Reserve for construction loans, percent	1.25%
Adjustment in basis points	0.075%
Basis points	0.05%
Interest income on impaired loans	1,800,000	922,000
Balance of troubled debt restructurings	36,200,000	27,600,000
TDRs in nonaccrual status	13,600,000	15,300,000
TDRs in accrual status	22,600,000	12,300,000
Troubled debt restructuring related reserves	4,200,000	4,100,000
Number of term loans modified in troubled debt restructurings, commercial loans	7
Number of term loans modified in troubled debt restructurings, residential and consumer loans	18
Number of term loans modified in troubled debt restructurings extensions of maturities	8
Number of term loans modified in troubled debt restructurings reduction in interest rates	7
Number of term loans modified in troubled debt restructurings reduction in interest rates with extension of maturities	4
Number of term loans modified in troubled debt restructurings of bankruptcies	4
Number of term loans modified in troubled debt restructurings of forbearance agreements	2
Usual sustained repayment performance period	6 months
Increase in allowance for loan losses	2,200,000	82,000
Troubled debt restructurings charged off	$ 54,000	$ 381,000
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans	90 days
Economic relative average period for further reduction factor	3 years
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Economic relative average period for further reduction factor	5 years
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impairment loans, charge off period	90 days
Number of term loans modified in troubled debt restructurings	25
Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Calculated pooled reserves for retail loans	Pooled reserves for retail loans are calculated based solely on the previous four year average net loss rate.