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MORTGAGE OPERATIONS (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Key assumptions used in measuring fair value of MSR [Abstract]      
Constant prepayment rate (in hundredths) 13.20%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed   12.12%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Discount rate (in hundredths) 10.06%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate   10.06%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
Weighted average life (years) 5 years 11 months 5 days   6 years 3 months
Mortgages held-for-sale $ 4,185,000us-gaap_LoansReceivableHeldForSaleNet   $ 491,000us-gaap_LoansReceivableHeldForSaleNet
Real estate mortgage loans serviced 235,239,000fnrn_MortgageLoansOnRealEstateServicedForOthers   238,974,000fnrn_MortgageLoansOnRealEstateServicedForOthers
Mortgage servicing rights assets [Roll Forward]      
Beginning balance 1,862,000us-gaap_ServicingAssetAtAmortizedValue    
Additions 56,000us-gaap_ServicingAssetAtAmortizedValueAdditions    
Reductions (94,000)us-gaap_ServicingAssetAtAmortizedValueDisposals    
Ending balance 1,824,000us-gaap_ServicingAssetAtAmortizedValue   1,862,000us-gaap_ServicingAssetAtAmortizedValue
Valuation allowance [Roll Forward]      
Beginning balance 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance    
Additions 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsProvisions    
Reductions 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries    
Ending balance 0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance   0us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance
Mortgage servicing rights, net of valuation allowance [Roll Forward]      
Beginning balance 1,862,000fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowanceBalanceBeginningBalance    
Additions 56,000fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowanceAdditions    
Reductions (94,000)fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowanceReductions    
Ending balance 1,824,000fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowanceBalanceEndingBalance    
Fair market value of mortgage servicing rights asset 1,960,000us-gaap_ServicingAssetAtFairValueAmount   2,068,000us-gaap_ServicingAssetAtFairValueAmount
Contractually specified servicing fee $ 151,000us-gaap_ContractuallySpecifiedServicingFeesAmount $ 153,000us-gaap_ContractuallySpecifiedServicingFeesAmount