Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs | $			$ 1,300,000	$ 3,100,000
Percentage of average loans annualized, charged-offs			0.07%	0.19%
Interest income on impaired loans | $	$ 180,000	$ 449,000	$ 336,000	$ 921,000
Balance of troubled debt restructurings | $	22,000,000		22,000,000		$ 24,600,000
TDRs in nonaccrual status | $			7,900,000		11,000,000
TDRs in accrual status | $			14,100,000		13,600,000
Troubled debt restructuring related reserves | $	1,500,000		$ 1,500,000		$ 2,100,000
Number of term loans modified in troubled debt restructurings | SecurityLoan			14
Usual sustained repayment performance period			6 months
Increase in allowance for loan losses | $			$ 0	13,000
Troubled debt restructurings charged off | $			$ 80,000	$ 69,000
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of commercial loans | SecurityLoan			9		3
Total loans outstanding,commercial loans | $	$ 1,900,000		$ 1,900,000		$ 675,000
Number of term loans modified in troubled debt restructurings, discharged bankruptcies | SecurityLoan			1
Number of restructured loans extension of maturities | SecurityLoan			1
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of residential loans in the process of foreclosure | SecurityLoan			32		32
Total loans outstanding, residential loans | $			$ 5,200,000		$ 5,000,000
Number of term loans modified in troubled debt restructurings, discharged bankruptcies | SecurityLoan			2
Number of term loans modified in troubled debt restructurings of forbearance agreements | SecurityLoan			3
Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of term loans modified in troubled debt restructurings, rate concession | SecurityLoan			8
Number of term loans modified in troubled debt restructurings, discharged bankruptcies | SecurityLoan			1
Number of term loans modified in troubled debt restructurings | SecurityLoan			9
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans			90 days
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impairment loans, charge off period			90 days
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 average net loss rates over the same 22 quarter look-back period.