Fair Value Measurement, Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Loans receivable [Abstract]
Mortgage servicing rights	$ 1,242	$ 1,631
Carrying Amount [Member] \| Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	267,177	111,493
Carrying Amount [Member] \| Level 2 [Member]
Financial assets [Abstract]
Certificates of deposit	16,923	14,700
Loans receivable [Abstract]
Loans held-for-sale	9,190	4,130
Interest receivable	5,099	4,295
Financial liabilities [Abstract]
Interest payable	59	92
Carrying Amount [Member] \| Level 3 [Member]
Financial assets [Abstract]
Other equity securities	6,480	6,574
Loans receivable [Abstract]
Net loans	875,830	768,873
Mortgage servicing rights	1,242	1,481
Financial liabilities [Abstract]
Deposits	1,478,162	1,138,632
Fair Value [Member] \| Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	267,177	111,493
Fair Value [Member] \| Level 2 [Member]
Financial assets [Abstract]
Certificates of deposit	17,455	14,984
Loans receivable [Abstract]
Loans held-for-sale	9,522	4,213
Interest receivable	5,099	4,295
Financial liabilities [Abstract]
Interest payable	59	92
Fair Value [Member] \| Level 3 [Member]
Financial assets [Abstract]
Other equity securities	6,480	6,574
Loans receivable [Abstract]
Net loans	830,448	723,507
Mortgage servicing rights	1,242	1,631
Financial liabilities [Abstract]
Deposits	$ 1,457,051	$ 1,035,644

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

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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

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

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

- Details

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