No definition available.

Name:	fnrn_EmployeeStockPurchasePlanAbstract
Namespace Prefix:	fnrn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Employment period required for plan participation, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Number of shares authorized under the Employee Stock Purchase Plan (ESPP).

+ References

No definition available.

+ Details

Name:	fnrn_EmployeeStockPurchasePlanESPPSharesAuthorized
Namespace Prefix:	fnrn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Refers to the percentage of employee compensation that can be withheld to purchase common stock.

+ References

No definition available.

+ Details

Name:	fnrn_PercentageOfEmployeeCompensationThatCanBeWithheldToPurchaseCommonStock
Namespace Prefix:	fnrn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of eligible employees participating in plan.

+ References

No definition available.

+ Details

Name:	fnrn_PercentageOfEmployeeParticipatingInPlan
Namespace Prefix:	fnrn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The participation periods implemented by the Employee Stock Purchase Plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Purchase price of stock as percentage of lower of the fair market value on the last trading day before the date of participation or the fair market value on the last trading day during the participation period.

+ References

No definition available.

+ Details

Name:	fnrn_PurchasePriceOfStockInPercentageForParticipation
Namespace Prefix:	fnrn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of cash contributions during the period made to the Employee Stock Purchase Plan (ESPP).

+ References

No definition available.

+ Details

Name:	fnrn_ShareBasedCompensationEmployeeStockPurchasePlanCashContributions
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage rate used to calculate dividend payments on stock.

+ References

No definition available.

+ Details

Name:	fnrn_StockDividendPercentage
Namespace Prefix:	fnrn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479830/718-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Date the declared dividend will be paid, in YYYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-3

+ Details

Name:	us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the dividend to be paid was declared, in YYYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DividendsPayableDateDeclaredDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the holder must own the stock to be entitled to the dividend, in YYYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Per share weighted-average price paid for shares purchased on open market for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerShareWeightedAveragePriceOfSharesPurchased
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-SubTopic 10
-Topic 505
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-2

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fnrn_AmendedEmployeeStockPurchasePlan2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Stock Compensation Plans, Employee Stock Purchase Plan (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 25, 2024	Dec. 31, 2023	Dec. 31, 2022
Subsequent Event [Member]
Employee Stock Purchase Plan [Abstract]
Stock dividend percentage	5.00%
Dividends, date declared	Jan. 25, 2024
Dividends, date to be paid	Mar. 25, 2024
Dividends, date of record	Feb. 29, 2024
2016 Amended Employee Stock Purchase Plan [Member]
Employee Stock Purchase Plan [Abstract]
Shares authorized for issuance under plan (in shares)		376,856
Purchase price of stock in percentage for participation		85.00%
Percentage of eligible employee participating in plan		38.00%
Cash contributions to ESPP		$ 96
Shares purchased for award (in shares)		15,034
Weighted average purchase price of shares purchased (in dollars per share)		$ 6.36
Compensation cost related to ESPP		$ 26	$ 20
2016 Amended Employee Stock Purchase Plan [Member] \| Minimum [Member]
Employee Stock Purchase Plan [Abstract]
Eligibility employment period required for plan participation		90 days
2016 Amended Employee Stock Purchase Plan [Member] \| Maximum [Member]
Employee Stock Purchase Plan [Abstract]
Plan participation period		27 months
Percentage of employee compensation that can be withheld to purchase common stock		10.00%