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Capital Adequacy and Restriction on Dividends (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Tier 1 Leverage Capital (to Average Assets), Capital $ 91,799us-gaap_TierOneLeverageCapital $ 85,316us-gaap_TierOneLeverageCapital
Tier 1 Leverage Capital (to Average Assets), Ratio (in hundredths) 9.40%us-gaap_TierOneLeverageCapitalToAverageAssets 9.60%us-gaap_TierOneLeverageCapitalToAverageAssets
Tier 1 Leverage Capital (to Average Assets), Adequately Capitalized Ratio (in hundredths) 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets  
Tier 1 Capital (to Risk-Weighted Assets), Capital 91,799us-gaap_TierOneRiskBasedCapital 85,316us-gaap_TierOneRiskBasedCapital
Tier 1 Capital ( to Risk Weighted Assets), Ratio (in hundredths) 15.70%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 15.30%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Tier 1 Capital (to Risk-Weighted Assets), Adequacy Capitalized Ratio (in hundredths) 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets  
Total Risk-Based Capital (to Risk-Weighted Assets), Capital 99,137us-gaap_Capital 92,308us-gaap_Capital
Total Risk-Based Capital (to Risk-Weighted Assets), Ratio (in hundredths) 17.00%us-gaap_CapitalToRiskWeightedAssets 16.60%us-gaap_CapitalToRiskWeightedAssets
Total Risk-Based Capital (to Risk-Weighted Assets), Adequately Capitalized Ratio (in hundredths) 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets  
Conservation buffer (in hundredths) 2.50%fnrn_ConservationBuffer  
Tier 1 leverage capital well capitalized modified (in hundredths) 5.00%fnrn_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssetsModified  
Common equity Tier 1 capital ratio well capitalized modified (in hundredths) 6.50%fnrn_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssetsModified  
Tier 1 capital ratio well capitalized modified (in hundredths) 8.00%fnrn_TierOneRiskBasedCapitalModified  
Total capital ratio well capitalized modified (in hundredths) 10.00%fnrn_CapitalRequiredToBeWellCapitalizedModified  
Tier 1 leverage capital adequately capitalized modified (in hundredths) 4.00%fnrn_TierOneLeverageCapitalRequiredForCapitalAdequacyModified  
Common equity Tier 1 capital ratio adequately capitalized modified (in hundredths) 4.50%fnrn_TierOneRiskBasedCapitalRequiredForCapitalAdequacyModified  
Tier 1 capital ratio adequately capitalized modified (in hundredths) 6.00%fnrn_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsModified  
Total capital ratio adequately capitalized modified (in hundredths) 8.00%fnrn_CapitalRequiredForCapitalAdequacyModified  
Minimum [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
First - minimum Tier I Capital ratio CAMELS (in hundredths) 3.00%fnrn_TierOneRiskBasedCapitalToRiskWeightedAssetsCamel
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Second - minimum Total Risk-Based Capital to risk-weighted assets ratio (in hundredths) 8.00%fnrn_RiskBasedCapitalToRiskWeightedAssetsTotalMinimum
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Maximum [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
First - minimum Tier I Capital ratio CAMELS (in hundredths) 5.00%fnrn_TierOneRiskBasedCapitalToRiskWeightedAssetsCamel
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
The Bank [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Tier 1 Leverage Capital (to Average Assets), Capital 87,903us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
81,478us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Tier 1 Leverage Capital (to Average Assets), Ratio (in hundredths) 9.00%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
9.10%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Tier 1 Leverage Capital (to Average Assets), Adequately Capitalized Ratio (in hundredths) 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
 
Tier 1 Leverage Capital (to Average Assets), Well Capitalized Ratio (in hundredths) 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
 
Tier 1 Capital (to Risk-Weighted Assets), Capital 87,903us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
81,478us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Tier 1 Capital ( to Risk Weighted Assets), Ratio (in hundredths) 15.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
14.70%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Tier 1 Capital (to Risk-Weighted Assets), Adequacy Capitalized Ratio (in hundredths) 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
 
Tier 1 Capital (to Risk Weighted Assets), Well Capitalized Ratio (in hundredths) 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
 
Total Risk-Based Capital (to Risk-Weighted Assets), Capital $ 95,241us-gaap_Capital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
$ 88,470us-gaap_Capital
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Total Risk-Based Capital (to Risk-Weighted Assets), Ratio (in hundredths) 16.30%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
15.90%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Total Risk-Based Capital (to Risk-Weighted Assets), Adequately Capitalized Ratio (in hundredths) 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
 
Total Risk-Based Capital (to Risk-Weighted Assets), Well Capitalized Ratio (in hundredths) 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember