Quarterly Financial Data - Summary of Quarterly Financial Data (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of Income:
Interest income	$ 8,519	$ 8,471	$ 8,304	$ 8,159	$ 8,117	$ 7,844	$ 7,765	$ 7,702	$ 33,453	$ 31,428	$ 33,273
Interest expense	907us-gaap_InterestExpense	895us-gaap_InterestExpense	970us-gaap_InterestExpense	944us-gaap_InterestExpense	1,058us-gaap_InterestExpense	1,129us-gaap_InterestExpense	1,184us-gaap_InterestExpense	1,233us-gaap_InterestExpense	3,716us-gaap_InterestExpense	4,604us-gaap_InterestExpense	6,250us-gaap_InterestExpense
Net Interest Income	7,612us-gaap_InterestIncomeExpenseNet	7,576us-gaap_InterestIncomeExpenseNet	7,334us-gaap_InterestIncomeExpenseNet	7,215us-gaap_InterestIncomeExpenseNet	7,059us-gaap_InterestIncomeExpenseNet	6,715us-gaap_InterestIncomeExpenseNet	6,581us-gaap_InterestIncomeExpenseNet	6,469us-gaap_InterestIncomeExpenseNet	29,737us-gaap_InterestIncomeExpenseNet	26,824us-gaap_InterestIncomeExpenseNet	27,023us-gaap_InterestIncomeExpenseNet
Provision for loan loss	37us-gaap_ProvisionForLoanAndLeaseLosses	282us-gaap_ProvisionForLoanAndLeaseLosses	444us-gaap_ProvisionForLoanAndLeaseLosses	428us-gaap_ProvisionForLoanAndLeaseLosses	276us-gaap_ProvisionForLoanAndLeaseLosses	303us-gaap_ProvisionForLoanAndLeaseLosses	112us-gaap_ProvisionForLoanAndLeaseLosses	167us-gaap_ProvisionForLoanAndLeaseLosses	1,191us-gaap_ProvisionForLoanAndLeaseLosses	858us-gaap_ProvisionForLoanAndLeaseLosses	738us-gaap_ProvisionForLoanAndLeaseLosses
Net interest income after provision for loan loss	7,575	7,294	6,890	6,787	6,783	6,412	6,469	6,302	28,546	25,966	26,285
Other income (expense)	(3,800)us-gaap_OtherNoncashIncomeExpense	(3,818)us-gaap_OtherNoncashIncomeExpense	(3,456)us-gaap_OtherNoncashIncomeExpense	(3,955)us-gaap_OtherNoncashIncomeExpense	(3,632)us-gaap_OtherNoncashIncomeExpense	(3,555)us-gaap_OtherNoncashIncomeExpense	(2,958)us-gaap_OtherNoncashIncomeExpense	(3,218)us-gaap_OtherNoncashIncomeExpense
Income Before Income Taxes	3,775	3,476	3,434	2,832	3,151	2,857	3,511	3,084	13,517	12,603	13,692
Income Taxes	1,114us-gaap_IncomeTaxExpenseBenefit	1,002us-gaap_IncomeTaxExpenseBenefit	881us-gaap_IncomeTaxExpenseBenefit	874us-gaap_IncomeTaxExpenseBenefit	864us-gaap_IncomeTaxExpenseBenefit	791us-gaap_IncomeTaxExpenseBenefit	1,009us-gaap_IncomeTaxExpenseBenefit	932us-gaap_IncomeTaxExpenseBenefit	3,871us-gaap_IncomeTaxExpenseBenefit	3,596us-gaap_IncomeTaxExpenseBenefit	3,904us-gaap_IncomeTaxExpenseBenefit
Net income	$ 2,661us-gaap_NetIncomeLoss	$ 2,474us-gaap_NetIncomeLoss	$ 2,553us-gaap_NetIncomeLoss	$ 1,958us-gaap_NetIncomeLoss	$ 2,287us-gaap_NetIncomeLoss	$ 2,066us-gaap_NetIncomeLoss	$ 2,502us-gaap_NetIncomeLoss	$ 2,152us-gaap_NetIncomeLoss	$ 9,646us-gaap_NetIncomeLoss	$ 9,007us-gaap_NetIncomeLoss	$ 9,788us-gaap_NetIncomeLoss
Earnings per Common Share	$ 0.57us-gaap_EarningsPerShareBasic	$ 0.54us-gaap_EarningsPerShareBasic	$ 0.55us-gaap_EarningsPerShareBasic	$ 0.42us-gaap_EarningsPerShareBasic	$ 0.49us-gaap_EarningsPerShareBasic	$ 0.45us-gaap_EarningsPerShareBasic	$ 0.53us-gaap_EarningsPerShareBasic	$ 0.46us-gaap_EarningsPerShareBasic	$ 2.08us-gaap_EarningsPerShareBasic	$ 1.93us-gaap_EarningsPerShareBasic	$ 2.08us-gaap_EarningsPerShareBasic
Average common shares outstanding	4,627,338	4,621,298	4,626,309	4,637,172	4,660,096	4,682,655	4,679,971	4,683,659	4,595,967	4,645,929	4,665,988

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