LOANS AND ALLOWANCE FOR CREDIT LOSSES, Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	$ 15,484	$ 14,258	$ 14,792	$ 13,952
Provision for credit losses/ Provision for (reversal of) loan losses	2,600	300	2,600	600
Charge-offs	(2,618)	(302)	(2,749)	(306)
Recoveries	113	19	136	29
Net (charge-offs)/recoveries	(2,505)	(283)	(2,613)	(277)
Ending balance	15,579	14,275	15,579	14,275
Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			800
Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			15,592
Commercial [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	1,750	1,747	1,463	1,604
Provision for credit losses/ Provision for (reversal of) loan losses	(36)	183	(268)	319
Charge-offs	(51)	(297)	(178)	(297)
Recoveries	112	17	135	24
Net (charge-offs)/recoveries	61	(280)	(43)	(273)
Ending balance	1,775	1,650	1,775	1,650
Commercial [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			623
Commercial [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			2,086
Commercial Real Estate [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	9,155	9,380	10,073	8,808
Provision for credit losses/ Provision for (reversal of) loan losses	895	191	441	763
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Ending balance	10,050	9,571	10,050	9,571
Commercial Real Estate [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			(464)
Commercial Real Estate [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			9,609
Agriculture [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	873	1,607	1,757	1,482
Provision for credit losses/ Provision for (reversal of) loan losses	2,633	87	2,420	212
Charge-offs	(2,567)	0	(2,567)	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	(2,567)	0	(2,567)	0
Ending balance	939	1,694	939	1,694
Agriculture [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			(671)
Agriculture [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			1,086
Residential Mortgage [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	1,678	754	880	742
Provision for credit losses/ Provision for (reversal of) loan losses	146	48	113	60
Charge-offs	0	0	(3)	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	(3)	0
Ending balance	1,824	802	1,824	802
Residential Mortgage [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			834
Residential Mortgage [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			1,714
Residential Construction [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	417	135	178	74
Provision for credit losses/ Provision for (reversal of) loan losses	70	16	109	77
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Ending balance	487	151	487	151
Residential Construction [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			200
Residential Construction [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			378
Consumer [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	365	191	173	167
Provision for credit losses/ Provision for (reversal of) loan losses	1	(9)	(7)	16
Charge-offs	0	(5)	(1)	(9)
Recoveries	1	2	1	5
Net (charge-offs)/recoveries	1	(3)	0	(4)
Ending balance	367	179	367	179
Consumer [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			201
Consumer [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			374
Unallocated [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance	1,246	444	268	1,075
Provision for credit losses/ Provision for (reversal of) loan losses	(1,109)	(216)	(208)	(847)
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge-offs)/recoveries	0	0	0	0
Ending balance	$ 137	$ 228	137	$ 228
Unallocated [Member] \| Impact of Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			77
Unallocated [Member] \| Post Adopting ASC 326 [Member] \| ASU 2016-13 [Member]
Activity in ACL on loans by loan class [Roll Forward]
Beginning balance			$ 345

Amount, after recovery, of writeoff of financing receivable, charged against allowance for credit loss.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (4)
-Publisher SEC

+ Details

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

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(4))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)
-Publisher SEC

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)
-Publisher SEC

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (c)
-Publisher SEC

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnrn_AgricultureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: