Loans and Allowance for Credit Losses, Impaired Loans (Details) $ in Thousands	12 Months Ended
Dec. 31, 2022 USD ($)
Impaired loans [Abstract]
Unpaid contractual principal balance	$ 11,527
Recorded investment [Abstract]
Recorded investment with no allowance	8,176
Recorded investment with allowance	563
Total recorded investment	8,739
Related allowance	77
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	9,895
Interest income recognized	75
Interest Income on Impaired Loans Recognized using Cash Basis Method of Accounting [Abstract]
Interest income recognized	0
Commercial [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	0
Recorded investment [Abstract]
Recorded investment with no allowance	0
Recorded investment with allowance	0
Total recorded investment	0
Related allowance	0
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	40
Interest income recognized	3
Commercial Real Estate [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	0
Recorded investment [Abstract]
Recorded investment with no allowance	0
Recorded investment with allowance	0
Total recorded investment	0
Related allowance	0
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	384
Interest income recognized	32
Agriculture [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	10,032
Recorded investment [Abstract]
Recorded investment with no allowance	7,416
Recorded investment with allowance	0
Total recorded investment	7,416
Related allowance	0
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	8,047
Interest income recognized	0
Residential Mortgage [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	673
Recorded investment [Abstract]
Recorded investment with no allowance	123
Recorded investment with allowance	499
Total recorded investment	622
Related allowance	75
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	639
Interest income recognized	19
Residential Construction [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	0
Recorded investment [Abstract]
Recorded investment with no allowance	0
Recorded investment with allowance	0
Total recorded investment	0
Related allowance	0
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	48
Interest income recognized	0
Consumer [Member]
Impaired loans [Abstract]
Unpaid contractual principal balance	822
Recorded investment [Abstract]
Recorded investment with no allowance	637
Recorded investment with allowance	64
Total recorded investment	701
Related allowance	2
Interest income on impaired loans recognized using an accrual method of accounting [Abstract]
Average recorded investment	737
Interest income recognized	$ 21

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-15

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-17

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-15

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-15

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-14A

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-15

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-15

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- 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: