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Deferred Compensation Plans - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Cash surrender value of life insurance $ 58,900,000us-gaap_CashSurrenderValueOfLifeInsurance $ 49,400,000us-gaap_CashSurrenderValueOfLifeInsurance  
Accrued deferred compensation 655,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability 722,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability  
Aggregate compensation expense 743,000us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense 601,000us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense 364,000us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense
Supplemental Executive Retirement Plan Liabilities [Member]      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Accrued deferred compensation $ 1,594,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
$ 851,000us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember