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Fair Value Measurement, Key Methods and Assumptions Used (Details)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Discount rate (in hundredths) 10.06%us-gaap_FairValueInputsDiscountRate 10.05%us-gaap_FairValueInputsDiscountRate 11.05%us-gaap_FairValueInputsDiscountRate
Constant prepayment rate (in hundredths) 12.12%us-gaap_FairValueInputsPrepaymentRate 9.09%us-gaap_FairValueInputsPrepaymentRate 25.58%us-gaap_FairValueInputsPrepaymentRate
Impaired Loans [Member] | Collateral, Market, Income, Enterprise, Liquidation and Discounted Cash Flows [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Assumption Inputs External appraised values, management assumptions regarding market trends or other relevant factors; selling costs ranging 6% to 7%. External appraised values, management assumptions regarding market trends or other relevant factors; selling costs ranging 6% to 7%.  
Impaired Loans [Member] | Collateral, Market, Income, Enterprise, Liquidation and Discounted Cash Flows [Member] | Minimum [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Selling costs (in hundredths) 6.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_ImpairedLoansMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMarketIncomeEnterpriseLiquidationAndDiscountedCashFlowsMember		 6.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_ImpairedLoansMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMarketIncomeEnterpriseLiquidationAndDiscountedCashFlowsMember		  
Impaired Loans [Member] | Collateral, Market, Income, Enterprise, Liquidation and Discounted Cash Flows [Member] | Maximum [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Selling costs (in hundredths) 7.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_ImpairedLoansMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMarketIncomeEnterpriseLiquidationAndDiscountedCashFlowsMember		 7.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_ImpairedLoansMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMarketIncomeEnterpriseLiquidationAndDiscountedCashFlowsMember		  
Other Real Estate Owned [Member] | Collateral [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Assumption Inputs External appraised values, management assumptions regarding market trends or other relevant factors; selling costs ranging 6% to 11%. External appraised values, management assumptions regarding market trends or other relevant factors; selling costs ranging 6% to 11%.  
Other Real Estate Owned [Member] | Collateral [Member] | Minimum [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Selling costs (in hundredths) 6.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMember		 6.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMember		  
Other Real Estate Owned [Member] | Collateral [Member] | Maximum [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Selling costs (in hundredths) 11.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMember		 11.00%fnrn_FairValueInputsSellingCosts
/ us-gaap_FairValueByAssetClassAxis
= fnrn_OtherRealEstateOwnedMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= fnrn_CollateralMember		  
Loan Servicing Rights [Member] | Discounted Cash Flows [Member]      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Assumption Inputs Present value of expected future cash flows is estimated using a discount rate factor of 10.06% and 10.05% as of December 31, 2014 and December 31, 2013, respectively. A constant prepayment rate of 12.12% and 9.09% as of December 31, 2014 and December 31, 2013, respectively, was utilized. Present value of expected future cash flows is estimated using a discount rate factor of 10.06% and 10.05% as of December 31, 2014 and December 31, 2013, respectively. A constant prepayment rate of 12.12% and 9.09% as of December 31, 2014 and December 31, 2013, respectively, was utilized.  
Discount rate (in hundredths) 10.06%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fnrn_LoanServicingRightsMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_IncomeApproachValuationTechniqueMember		 10.05%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fnrn_LoanServicingRightsMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_IncomeApproachValuationTechniqueMember		  
Constant prepayment rate (in hundredths) 12.12%us-gaap_FairValueInputsPrepaymentRate
/ us-gaap_FairValueByAssetClassAxis
= fnrn_LoanServicingRightsMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_IncomeApproachValuationTechniqueMember		 9.09%us-gaap_FairValueInputsPrepaymentRate
/ us-gaap_FairValueByAssetClassAxis
= fnrn_LoanServicingRightsMember
/ us-gaap_ValuationTechniqueAxis
= us-gaap_IncomeApproachValuationTechniqueMember