Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) SecurityLoan	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs			$ 3,100,000	$ 2,500,000
Percentage of average loans annualized, charged-offs			0.19%	0.17%
Increase in allowance for loan losses			$ 13,000	$ 1,400,000
Troubled debt restructurings charged off	$ 69,000	$ 41,000	69,000	41,000
Amount of increase (decrease) on total allowance			$ (639,000)
Adjustment in basis points			10.00%
Adjustment factors contributed to total allowance			$ 2,500,000
Interest income on impaired loans	449,000	$ 393,000	921,000	$ 747,000
Balance of troubled debt restructurings	33,000,000		33,000,000		$ 36,200,000
TDRs in nonaccrual status			19,400,000		13,600,000
TDRs in accrual status			13,600,000		22,600,000
Troubled debt restructuring related reserves	6,400,000		$ 6,400,000		$ 4,200,000
Number of term loans modified in troubled debt restructurings | SecurityLoan			18
Usual sustained repayment performance period			6 months
Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	9,100,000
Increase in allowance for loan losses			$ 3,600,000
Troubled debt restructurings charged off	$ 1,900,000		$ 1,900,000
Basis points	0.075%		0.075%
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of term loans modified in troubled debt restructurings | SecurityLoan			12
Number of term loans modified in troubled debt restructurings, extended maturity | SecurityLoan			5
Number of term loans modified in troubled debt restructurings, discharged bankruptcies | SecurityLoan			6
Number of term loans modified in troubled debt restructurings, rate concession | SecurityLoan			1
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of term loans modified in troubled debt restructurings | SecurityLoan			5
Number of term loans modified in troubled debt restructurings, extended maturity | SecurityLoan			1
Number of term loans modified in troubled debt restructurings, discharged bankruptcies | SecurityLoan			3
Number of term loans modified in troubled debt restructurings of forbearance agreements | SecurityLoan			1
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of term loans modified in troubled debt restructurings | SecurityLoan			1
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 18 quarter look-back period.
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impairment loans, charge off period			90 days
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of residential loans in the process of foreclosure | SecurityLoan			32		36
Total loans outstanding, residential loans			$ 3,400,000		$ 4,400,000
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of increase (decrease) on total allowance			1,700,000
Amount contributed to total allowance			$ 458,000
Number of commercial loans | SecurityLoan			15		12
Total loans outstanding,commercial loans	$ 3,400,000		$ 3,400,000		$ 1,100,000
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans			90 days
Basis points	0.03%		0.03%
Minimum [Member] | Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis points	0.50%		0.50%
Maximum [Member] | Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis points	0.92%		0.92%