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Loans (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 545,235us-gaap_LoansAndLeasesReceivableGrossCarryingAmount $ 515,002us-gaap_LoansAndLeasesReceivableGrossCarryingAmount $ 448,228us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Allowance for loan losses (8,583)us-gaap_LoansAndLeasesReceivableAllowance (9,353)us-gaap_LoansAndLeasesReceivableAllowance  
Net deferred origination fees and costs 1,327us-gaap_OtherDeferredCostsNet 1,201us-gaap_OtherDeferredCostsNet  
Loans, net 537,979us-gaap_LoansAndLeasesReceivableNetReportedAmount 506,850us-gaap_LoansAndLeasesReceivableNetReportedAmount  
Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Delinquent loans revaluation period 3 years    
Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Delinquent loans revaluation period 12 years    
Commercial [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 120,751us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
110,644us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
88,810us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
Loan portfolio by segment (in hundredths) 22.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember
   
Commercial [Member] | Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 3 months    
Commercial [Member] | Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 6 months    
Commercial Real Estate [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 256,955us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
235,296us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
188,426us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Categories of loans 2fnrn_CommercialRealEstateLoansCategories
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Loan portfolio by segment (in hundredths) 47.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Commercial Real Estate [Member] | Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 3 months    
Commercial Real Estate [Member] | Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 6 months    
Agriculture [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 61,144us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnrn_AgricultureMember
51,730us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnrn_AgricultureMember
52,747us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnrn_AgricultureMember
Loan portfolio by segment (in hundredths) 11.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnrn_AgricultureMember
   
Agriculture [Member] | Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 3 months    
Agriculture [Member] | Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 6 months    
Residential Mortgage [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 50,511us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
52,809us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
51,266us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Loan portfolio by segment (in hundredths) 9.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
   
Residential Construction [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 5,963us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
10,444us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
7,586us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
Loan portfolio by segment (in hundredths) 1.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
   
Residential Construction [Member] | Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 3 months    
Residential Construction [Member] | Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
General frequency of collateral valuation 6 months    
Consumer [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans 49,911us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
$ 54,079us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
$ 59,393us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Loan portfolio by segment (in hundredths) 10.00%fnrn_LoanPortfolioBySegment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember