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Stock Compensation Plans (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 22, 2015
Weighted average assumptions used in valuing stock options granted for the years ended [Abstract]        
Risk Free Interest Rate (in hundredths) 1.53%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.86%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.87%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate  
Expected Dividend Yield (in hundredths) 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate  
Expected Life in Years 5 years 5 years 5 years  
Expected Price Volatility (in hundredths) 45.93%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate 54.36%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate 53.06%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate  
Dividend Declared in 2015 Annual [Member] | Subsequent Event [Member]        
Dividends Payable [Line Items]        
Stock dividend (in hundredths)       4.00%fnrn_StockDividend
/ us-gaap_DividendsAxis
= fnrn_DividendDeclaredIn2015AnnualMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Dividends date declared Jan. 22, 2015      
Dividend date to be paid Mar. 31, 2015      
Dividends date of record Feb. 27, 2015      
Stock Options [Member]        
Stock options activity [Roll Forward]        
Beginning balance (in shares) 382,904us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Granted (in shares) 19,726us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Exercised (in shares) 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Cancelled (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Expired (in shares) (89,114)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Ending balance (in shares) 313,516us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
382,904us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Weighted average exercise price [Roll Forward]        
Beginning balance (in dollars per share) 11.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Granted (in dollars per share) 6.99us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Exercised (in dollars per share) 0us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Cancelled (in dollars per share) 0us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Expired (in dollars per share) 8.53us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Ending balance (in dollars per share) 11.53us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
11.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Additional disclosures [Abstract]        
Options exercised (in shares) 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Option exercised, Weighted Average Exercise Price (in dollars per share) 0us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Option exercised, Aggregate Intrinsic Value 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options outstanding and expected to vest (in shares) 313,516us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options outstanding and expected to vest, Weighted Average Exercise Price (in dollars per share) 11.53us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options outstanding and expected to vest, Aggregate Intrinsic Value 268us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options outstanding and expected to vest, Weighted Average Remaining Contractual Term 2 years 11 months 12 days      
Stock options vested and currently exercisable (in shares) 263,396us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options vested and currently exercisable, Weighted Average Exercise Price (in dollars per share) 12.60us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options vested and currently exercisable, Aggregate Intrinsic Value 179us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Stock options vested and currently exercisable, Weighted Average Remaining Contractual Term 1 year 11 months 12 days      
Weighted average fair value per share of options granted (in dollars per share) 2.91us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
2.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
2.19us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Exercise price range, Lower range limit (in dollars per share) 3.85us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Exercise price range, upper range limit (in dollars per share) 21.48us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Unrecognized compensation related to non-vested stock options 89us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Unrecognized compensation cost, period for recognition 2 years 6 months      
Recognized compensation cost 48us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
39us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
29us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Restricted Stock [Member]        
Additional disclosures [Abstract]        
Unrecognized compensation cost, period for recognition 2 years 7 months 6 days      
Recognized compensation cost 112us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
73us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
58us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Non-vested restricted stock awards outstanding [Roll Forward]        
Beginning balance (in shares) 64,003us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Granted (in shares) 25,886us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Vested (in shares) (11,636)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Cancelled/Forfeited (in shares) (4,399)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Ending balance (in shares) 73,854us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
64,003us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Non-vested restricted stock awards outstanding, Weighted average grant date fair value [Roll Forward]        
Beginning balance (in dollars per share) 4.68us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Granted (in dollars per share) 6.99us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
5.48us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
4.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Vested (in dollars per share) 3.85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Cancelled (in dollars per share) 4.55us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Ending balance (in dollars per share) 5.63us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
4.68us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Aggregate intrinsic value of restricted stock awards vested 79us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
50us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
59us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Weighted average fair value of awards granted (in dollars per share) 6.99us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
5.48us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
4.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Unrecognized compensation related to restricted stock 209us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
     
Restricted Stock [Member] | Minimum [Member]        
Weighted average assumptions used in valuing stock options granted for the years ended [Abstract]        
Award vesting period 1 year      
Restricted Stock [Member] | Maximum [Member]        
Weighted average assumptions used in valuing stock options granted for the years ended [Abstract]        
Award vesting period 4 years      
2006 Stock Incentive Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of fixed stock option plan 1fnrn_NumberOfFixedStockOptionPlan
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
     
Number of shares authorized (in shares) 982,926us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
     
Plan termination date Mar. 15, 2016      
2006 Stock Incentive Plan [Member] | Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of grants that vest annually (in hundredths) 25.00%fnrn_OptionsGrantedVestingRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
     
2006 Stock Incentive Plan [Member] | Restricted Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of grants that vest after four years (in hundredths) 100.00%fnrn_RestrictedStockAwardVestingRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
     
Period for vesting of restricted stock 4 years      
2006 Stock Incentive Plan [Member] | Non-Employee Director [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Maximum annual number of shares per director (in shares) 3,000fnrn_MaximumAnnualNumberOfSharesPerDirector
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember
     
2006 Stock Incentive Plan [Member] | Employee [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Maximum annual number of shares per individual (in shares) 25,000fnrn_MaximumAnnualNumberOfSharesPerIndividualInShares
/ us-gaap_PlanNameAxis
= fnrn_StockIncentivePlanMember
/ us-gaap_TitleOfIndividualAxis
= fnrn_EmployeeMember