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Financial Instruments with Off-Balance Sheet Risk (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks $ 174,348us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability $ 178,261us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Undisbursed Loan Commitments [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks 171,019us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember		 172,563us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommitmentsToExtendCreditMember
Standby Letters of Credit [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks 2,099us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember		 2,489us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_StandbyLettersOfCreditMember
Commitments to Sell Loans [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks 1,230us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_LoanOriginationCommitmentsMember		 3,209us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_LoanOriginationCommitmentsMember
Performance Standby Letters of Credit [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks 2,099us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_PerformanceGuaranteeMember		  
Derivative [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet credit risks 0us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_FinancialInstrumentAxis
= us-gaap_DerivativeMember		  
Reserve for Off-balance Sheet Activity [Member]    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Reserve for unfunded commitments $ 793fnrn_ReserveForOffBalanceSheetActivities
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ReserveForOffBalanceSheetActivitiesMember