Name:	fnrn_AdditionalPeriodForRetirementPaymentForFullYearOfService
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Average bonus for number years used in calculation, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_AverageYearsOfBonusUsedForBenefitCalculation
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Average salary used in calculating benefit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_AverageYearsOfSalaryUsedForBenefitCalculation
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Refers to benefit payable for voluntary terminations prior to age 55.

No definition available.

Name:	fnrn_BenefitPayableForVoluntaryTerminationsPriorToAge55
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Amount of defined annual benefit levels depending on responsibilities at the Bank.

No definition available.

Name:	fnrn_DefinedAnnualBenefitPlanAmount
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Early retirement age for SERP.

No definition available.

Name:	fnrn_EarlyRetirementAgeForSerp
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Expected age of social security benefit commencement.

No definition available.

Name:	fnrn_ExpectedAgeOfSocialSecurityBenefit
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Number of employees participating in SERP plan.

No definition available.

Name:	fnrn_NumberOfEmployeesParticipatingInSerpPlan
Namespace Prefix:	fnrn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Refers to percentage of average compensation credit for each year of service (up to a maximum of 50%) for executive under a supplemental executive retirement plan ("SERP").

No definition available.

Name:	fnrn_PercentageOfAverageCompensationCreditForExecutive
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Period for average yields on treasury securities used in calculation of contributions to profit sharing plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodForAverageYieldsOnTreasurySecuritiesUsedInCalculationOfContributionsToProfitSharingPlan
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period for retirement benefit will be paid after retirement age, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodForBenefitPaymentAfterRetirementAge
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period for monthly retirement payment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodForMonthlyRetirementPayment
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period of full service used to calculate additional retirement payment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodOfFullServiceUsedToCalculateAdditionalRetirementPayment
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Refers to the period of monthly retirement benefit payment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodOfMonthlyRetirementBenefitPayment
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period of service for reduced benefit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_PeriodOfServicesForReducedBenefit
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Minimum age when reduced benefit are available.

No definition available.

Name:	fnrn_ReducedBenefitAge
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Reduction in benefit for each year payments commence prior to age 65.

No definition available.

Name:	fnrn_ReductionInBenefitForRetirementPriorToRetirementAge
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Reduction in benefit made for social security benefit.

No definition available.

Name:	fnrn_ReductionMadeForSocialSecurityBenefit
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Refers to retirement age.

No definition available.

Name:	fnrn_RetirementAge
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The new SERP benefit is reduced for retirement at age 55.

No definition available.

Name:	fnrn_SerpBenefitReductionForAge55Retirement
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Target benefit of average compensation.

No definition available.

Name:	fnrn_TargetBenefitOfAverageCompensation
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Target service period for each executive, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_TargetServicePeriodForExecutive
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Refers to duration for treasury securities, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fnrn_TreasurySecuritiesDuration
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	us-gaap_DefinedBenefitPlanAdditionalInformationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Indicates status of funding for defined benefit plan designed to provide retirement benefits.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2410-114920

Name:	us-gaap_DefinedBenefitPlanFundingStatusExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	instant

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RetirementPlanNameAxis=fnrn_SalaryContinuationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Namespace Prefix:
Data Type:	na
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