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ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Accumulated Other Comprehensive Income [Line Items]    
Balance $ 66us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ (1,073)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Current period other comprehensive income/ (loss) 342us-gaap_OtherComprehensiveIncomeLossNetOfTax 516us-gaap_OtherComprehensiveIncomeLossNetOfTax
Balance 408us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (557)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Unrealized Gains/(Losses) on Securities [Member]    
Accumulated Other Comprehensive Income [Line Items]    
Balance 703us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
(639)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Current period other comprehensive income/ (loss) 375us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
516us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Balance 1,078us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
(123)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Officers' Retirement Plan [Member]    
Accumulated Other Comprehensive Income [Line Items]    
Balance (678)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
(480)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Current period other comprehensive income/ (loss) 0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Balance (678)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
(480)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Directors' Retirement Plan [Member]    
Accumulated Other Comprehensive Income [Line Items]    
Balance 41us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember
46us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember
Current period other comprehensive income/ (loss) (33)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember
Balance $ 8us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember
$ 46us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= fnrn_DirectorsRetirementPlanMember