Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	$ 435,080	$ 342,897
Transfers between Levels [Abstract]
Transfers from Level 1 to Level 2	0	0
Transfers from Level 2 to Level 1	0	0
Assets Recorded at Fair Value on a Non-recurring Basis [Abstract]
Mortgage servicing rights	1,242	1,631
Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	435,080	342,897
Liabilities Recorded at Fair Value [Abstract]
Liabilities	0	0
Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	38,891	43,255
Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	396,189	299,642
Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Non-recurring [Member]
Assets Recorded at Fair Value on a Non-recurring Basis [Abstract]
Impaired loans	28	170
Mortgage servicing rights	1,242
Total assets at fair value	1,270	170
Liabilities Recorded at Fair Value [Abstract]
Liabilities	0	0
Non-recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Non-recurring Basis [Abstract]
Impaired loans	0	0
Mortgage servicing rights	0
Total assets at fair value	0	0
Non-recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Non-recurring Basis [Abstract]
Impaired loans	0	0
Mortgage servicing rights	0
Total assets at fair value	0	0
Non-recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Non-recurring Basis [Abstract]
Impaired loans	28	170
Mortgage servicing rights	1,242
Total assets at fair value	1,270	170
U.S. Treasury Securities [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	38,891	43,255
U.S. Treasury Securities [Member] \| Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	38,891	43,255
U.S. Treasury Securities [Member] \| Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	38,891	43,255
U.S. Treasury Securities [Member] \| Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
U.S. Treasury Securities [Member] \| Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Securities of U.S. Government Agencies and Corporations [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	106,558	53,912
Securities of U.S. Government Agencies and Corporations [Member] \| Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	106,558	53,912
Securities of U.S. Government Agencies and Corporations [Member] \| Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Securities of U.S. Government Agencies and Corporations [Member] \| Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	106,558	53,912
Securities of U.S. Government Agencies and Corporations [Member] \| Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Obligations of States and Political Subdivisions [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	32,882	27,031
Obligations of States and Political Subdivisions [Member] \| Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	32,882	27,031
Obligations of States and Political Subdivisions [Member] \| Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Obligations of States and Political Subdivisions [Member] \| Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	32,882	27,031
Obligations of States and Political Subdivisions [Member] \| Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Collateralized Mortgage Obligations [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	73,460	79,420
Collateralized Mortgage Obligations [Member] \| Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	73,460	79,420
Collateralized Mortgage Obligations [Member] \| Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Collateralized Mortgage Obligations [Member] \| Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	73,460	79,420
Collateralized Mortgage Obligations [Member] \| Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Mortgage-backed Securities [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	183,289	139,279
Mortgage-backed Securities [Member] \| Recurring [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	183,289	139,279
Mortgage-backed Securities [Member] \| Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	0	0
Mortgage-backed Securities [Member] \| Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	183,289	139,279
Mortgage-backed Securities [Member] \| Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Assets Recorded at Fair Value on a Recurring Basis [Abstract]
Total investments at fair value	$ 0	$ 0

No definition available.

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

- Definition

Impaired loans measured at fair value on a nonrecurring basis.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosureAbstract
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

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: