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Summary of Significant Accounting Policies (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
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2008
Dec. 31, 2002
Summary of Significant Accounting Policies [Abstract]                
Contractually past due period for delinquent loans 90 days              
Other Real Estate Owned [Abstract]                
Other real estate owned $ 736us-gaap_OtherRealEstateAndForeclosedAssets $ 0us-gaap_OtherRealEstateAndForeclosedAssets            
Gain or Loss on Sale of Loans and Servicing Rights [Abstract]                
Constant prepayment rate (in hundredths) 12.12%us-gaap_FairValueInputsPrepaymentRate 9.09%us-gaap_FairValueInputsPrepaymentRate 25.58%us-gaap_FairValueInputsPrepaymentRate          
Discount rate (in hundredths) 10.06%us-gaap_FairValueInputsDiscountRate 10.05%us-gaap_FairValueInputsDiscountRate 11.05%us-gaap_FairValueInputsDiscountRate          
Weighted average life (years) 6 years 3 months 7 years 0 months 18 days 3 years 4 months 28 days          
Income Taxes [Abstract]                
Equity investment in a partnership         1,000us-gaap_EquityMethodInvestmentAggregateCost 1,000us-gaap_EquityMethodInvestmentAggregateCost 1,000us-gaap_EquityMethodInvestmentAggregateCost 2,355us-gaap_EquityMethodInvestmentAggregateCost
Period for earning tax credit 10 years              
Recapture period of tax credits 15 years              
Advertising Costs [Abstract]                
Advertising costs $ 344us-gaap_AdvertisingExpense $ 374us-gaap_AdvertisingExpense $ 373us-gaap_AdvertisingExpense          
Dividend Declared in 2014 Annual [Member]                
Stock Dividend [Abstract]                
Dividends declared date Jan. 23, 2014              
Stock dividend (in hundredths) 3.00%fnrn_StockDividend
/ us-gaap_DividendsAxis
= us-gaap_DividendDeclaredMember
             
Stock dividend (in shares) 284,871us-gaap_CommonStockDividendsShares
/ us-gaap_DividendsAxis
= us-gaap_DividendDeclaredMember
             
Dividends date paid/ to be paid Mar. 31, 2014              
Dividends date of record Feb. 28, 2014              
Dividend Declared in 2015 Annual [Member] | Subsequent Event [Member]                
Stock Dividend [Abstract]                
Dividends declared date Jan. 22, 2015              
Stock dividend (in hundredths)       4.00%fnrn_StockDividend
/ us-gaap_DividendsAxis
= fnrn_DividendDeclaredIn2015AnnualMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
       
Stock dividend (in shares) 392,578us-gaap_CommonStockDividendsShares
/ us-gaap_DividendsAxis
= fnrn_DividendDeclaredIn2015AnnualMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
             
Dividends date paid/ to be paid Mar. 31, 2015              
Dividends date of record Feb. 27, 2015              
Buildings and Improvements [Member] | Minimum [Member]                
Premises and Equipment [Line Items]                
Useful Life 15 years              
Buildings and Improvements [Member] | Maximum [Member]                
Premises and Equipment [Line Items]                
Useful Life 50 years              
Furniture and Equipment [Member] | Minimum [Member]                
Premises and Equipment [Line Items]                
Useful Life 3 years              
Furniture and Equipment [Member] | Maximum [Member]                
Premises and Equipment [Line Items]                
Useful Life 10 years