FAIR VALUE MEASUREMENTS, Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Loans receivable [Abstract]
Mortgage servicing rights	$ 2,010	$ 2,094
Carrying Amount [Member] \| Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	186,909	149,211
Carrying Amount [Member] \| Level 2 [Member]
Financial assets [Abstract]
Certificates of deposit	18,185	19,710
Loans receivable [Abstract]
Interest receivable	6,667	6,810
Financial liabilities [Abstract]
Interest payable	1,770	1,567
Carrying Amount [Member] \| Level 3 [Member]
Financial assets [Abstract]
Stock in Federal Home Loan Bank and other equity securities	10,518	10,518
Loans receivable [Abstract]
Net loans	1,047,296	1,052,465
Mortgage servicing rights	1,434	1,482
Financial liabilities [Abstract]
Time deposits	156,115	135,696
Fair Value [Member] \| Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	186,909	149,211
Fair Value [Member] \| Level 2 [Member]
Financial assets [Abstract]
Certificates of deposit	18,032	19,570
Loans receivable [Abstract]
Interest receivable	6,667	6,810
Financial liabilities [Abstract]
Interest payable	1,770	1,567
Fair Value [Member] \| Level 3 [Member]
Financial assets [Abstract]
Stock in Federal Home Loan Bank and other equity securities	10,518	10,518
Loans receivable [Abstract]
Net loans	956,356	958,077
Mortgage servicing rights	2,010	2,094
Financial liabilities [Abstract]
Time deposits	$ 155,594	$ 135,540

Refers to fair value of certificate of deposit assets.

No definition available.

Name:	fnrn_CertificateOfDepositsFairValueDisclosure
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Amounts due from interest bearing liabilities, which may be included in Interest Payable and Other Liabilities as categorized on the statement of financial position.

+ References

No definition available.

+ Details

Name:	fnrn_InterestPayableFairValueDisclosure
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Amounts due from interest earning assets, which may be included in Interest Receivable and Other Assets as categorized on the statement of financial position.

+ References

No definition available.

+ Details

Name:	fnrn_InterestReceivableFairValueDisclosure
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock and other equity securities.

+ References

No definition available.

+ Details

Name:	fnrn_InvestmentInFederalHomeLoanBankStockAndOtherEquitySecuritiesFairValueDisclosure
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

Name:	us-gaap_DepositsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Publisher FASB
-URI https://asc.fasb.org//310-10/tableOfContent

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481260/860-50-45-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481260/860-50-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481260/860-50-45-2

+ Details

Name:	us-gaap_ServicingAssetAtFairValueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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