Loans and Allowance for Credit Losses, Financing Receivables by Credit Quality Indicator (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Risk Rating by Loan Class [Abstract]
2024/2023	$ 119,025	$ 167,123
2023/2022	157,623	242,030
2022/2021	235,166	274,834
2021/2020	234,125	81,433
2020/2019	65,220	79,344
Prior	179,773	145,005
Revolving loans amortized cost basis	71,663	79,214
Total	1,062,595	1,068,983
Revolving loans converted to term loans	3,121	881
Year-to-date Charge-offs [Abstract]
2024/2023	(75)	(1,885)
2023/2022	(508)	(196)
2022/2021	(224)	(36)
2021/2020	(5)	0
2020/2019	(163)	(87)
Prior	(9)	(3)
Revolving loans amortized cost basis	0	(742)
Total	(984)	(2,949)
Year-to-date Recoveries [Abstract]
2024/2023	10	1,825
2023/2022	0	0
2022/2021	4	0
2021/2020	0	0
2020/2019	0	87
Prior	59	149
Revolving loans amortized cost basis	0	742
Total	73	2,803
Year-to-date Net Charge-offs [Abstract]
2024/2023	(65)	(60)
2023/2022	(508)	(196)
2022/2021	(220)	(36)
2021/2020	(5)	0
2020/2019	(163)	0
Prior	50	146
Revolving loans amortized cost basis	0	0
Net Charge-offs	(911)	(146)
Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	115,810	166,728
2023/2022	157,242	235,519
2022/2021	230,408	258,011
2021/2020	217,073	78,972
2020/2019	62,385	67,637
Prior	159,316	141,266
Revolving loans amortized cost basis	67,292	75,471
Total	1,009,526	1,023,604
Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	2,909	0
2023/2022	0	3,059
2022/2021	4,758	12,974
2021/2020	14,194	235
2020/2019	726	3,177
Prior	5,737	2,337
Revolving loans amortized cost basis	1,890	2,936
Total	30,214	24,718
Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	306	395
2023/2022	381	3,452
2022/2021	0	3,849
2021/2020	2,858	2,226
2020/2019	2,109	8,530
Prior	14,720	1,402
Revolving loans amortized cost basis	2,481	807
Total	22,855	20,661
Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Commercial [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	36,292	19,776
2023/2022	14,319	18,115
2022/2021	12,759	19,515
2021/2020	13,487	6,158
2020/2019	3,913	7,728
Prior	8,030	6,298
Revolving loans amortized cost basis	29,121	29,307
Total	117,921	106,897
Year-to-date Charge-offs [Abstract]
2024/2023	(47)	(47)
2023/2022	(508)	(196)
2022/2021	(224)	(36)
2021/2020	(5)	0
2020/2019	(163)	(87)
Prior	(9)	0
Revolving loans amortized cost basis	0	0
Total	(956)	(366)
Year-to-date Recoveries [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	4	0
2021/2020	0	0
2020/2019	0	87
Prior	56	148
Revolving loans amortized cost basis	0	0
Total	60	235
Year-to-date Net Charge-offs [Abstract]
2024/2023	(47)	(47)
2023/2022	(508)	(196)
2022/2021	(220)	(36)
2021/2020	(5)	0
2020/2019	(163)	0
Prior	47	148
Revolving loans amortized cost basis	0	0
Net Charge-offs	(896)	(131)
Commercial [Member] \| Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	36,065	19,776
2023/2022	14,319	16,961
2022/2021	11,885	15,833
2021/2020	11,894	5,381
2020/2019	3,442	7,420
Prior	8,030	6,298
Revolving loans amortized cost basis	27,272	26,183
Total	112,907	97,852
Commercial [Member] \| Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	1,122
2022/2021	874	2,530
2021/2020	1,561	235
2020/2019	0	308
Prior	0	0
Revolving loans amortized cost basis	1,590	2,936
Total	4,025	7,131
Commercial [Member] \| Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	227	0
2023/2022	0	32
2022/2021	0	1,152
2021/2020	32	542
2020/2019	471	0
Prior	0	0
Revolving loans amortized cost basis	259	188
Total	989	1,914
Commercial [Member] \| Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Commercial Real Estate [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	71,187	116,202
2023/2022	114,318	173,918
2022/2021	178,142	201,067
2021/2020	169,692	51,834
2020/2019	41,551	61,630
Prior	142,231	110,199
Revolving loans amortized cost basis	6,529	6,879
Total	723,650	721,729
Year-to-date Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Year-to-date Recoveries [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Year-to-date Net Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Net Charge-offs	0	0
Commercial Real Estate [Member] \| Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	68,278	115,807
2023/2022	113,937	173,918
2022/2021	178,142	191,907
2021/2020	160,484	50,150
2020/2019	39,913	52,157
Prior	121,862	107,909
Revolving loans amortized cost basis	6,529	6,879
Total	689,145	698,727
Commercial Real Estate [Member] \| Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	2,909	0
2023/2022	0	0
2022/2021	0	7,448
2021/2020	7,156	0
2020/2019	0	2,869
Prior	5,737	1,273
Revolving loans amortized cost basis	0	0
Total	15,802	11,590
Commercial Real Estate [Member] \| Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	395
2023/2022	381	0
2022/2021	0	1,712
2021/2020	2,052	1,684
2020/2019	1,638	6,604
Prior	14,632	1,017
Revolving loans amortized cost basis	0	0
Total	18,703	11,412
Commercial Real Estate [Member] \| Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Agriculture [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	4,857	6,842
2023/2022	6,562	18,922
2022/2021	18,730	24,453
2021/2020	23,462	8,792
2020/2019	6,401	4,435
Prior	10,252	12,501
Revolving loans amortized cost basis	22,300	29,893
Total	92,564	105,838
Year-to-date Charge-offs [Abstract]
2024/2023	0	(1,825)
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	(742)
Total	0	(2,567)
Year-to-date Recoveries [Abstract]
2024/2023	0	1,825
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	742
Total	0	2,567
Year-to-date Net Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Net Charge-offs	0	0
Agriculture [Member] \| Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	4,857	6,842
2023/2022	6,562	16,985
2022/2021	14,846	20,511
2021/2020	17,245	8,792
2020/2019	5,675	2,509
Prior	10,252	11,437
Revolving loans amortized cost basis	20,420	29,893
Total	79,857	96,969
Agriculture [Member] \| Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	1,937
2022/2021	3,884	2,996
2021/2020	5,477	0
2020/2019	726	0
Prior	0	1,064
Revolving loans amortized cost basis	300	0
Total	10,387	5,997
Agriculture [Member] \| Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	946
2021/2020	740	0
2020/2019	0	1,926
Prior	0	0
Revolving loans amortized cost basis	1,580	0
Total	2,320	2,872
Agriculture [Member] \| Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Residential Mortgage [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	4,952	20,239
2023/2022	20,162	24,906
2022/2021	22,408	26,468
2021/2020	26,157	14,500
2020/2019	13,233	5,481
Prior	18,974	15,734
Revolving loans amortized cost basis	0	0
Total	105,886	107,328
Year-to-date Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	(3)
Revolving loans amortized cost basis	0	0
Total	0	(3)
Year-to-date Recoveries [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Year-to-date Net Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	(3)
Revolving loans amortized cost basis	0	0
Net Charge-offs	0	(3)
Residential Mortgage [Member] \| Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	4,873	20,239
2023/2022	20,162	24,906
2022/2021	22,408	26,429
2021/2020	26,123	14,500
2020/2019	13,233	5,481
Prior	18,886	15,349
Revolving loans amortized cost basis	0	0
Total	105,685	106,904
Residential Mortgage [Member] \| Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Residential Mortgage [Member] \| Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	79	0
2023/2022	0	0
2022/2021	0	39
2021/2020	34	0
2020/2019	0	0
Prior	88	385
Revolving loans amortized cost basis	0	0
Total	201	424
Residential Mortgage [Member] \| Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Residential Construction [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	1,525	3,714
2023/2022	2,117	5,411
2022/2021	1,998	3,198
2021/2020	1,218	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	6,858	12,323
Year-to-date Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Year-to-date Recoveries [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Year-to-date Net Charge-offs [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Net Charge-offs	0	0
Residential Construction [Member] \| Pass [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	1,525	3,714
2023/2022	2,117	1,991
2022/2021	1,998	3,198
2021/2020	1,218	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	6,858	8,903
Residential Construction [Member] \| Special Mention [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Residential Construction [Member] \| Substandard [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	3,420
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	3,420
Residential Construction [Member] \| Doubtful/Loss [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	0	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	0	0
Consumer [Member]
Risk Rating by Loan Class [Abstract]
2024/2023	212	350
2023/2022	145	758
2022/2021	1,129	133
2021/2020	109	149
2020/2019	122	70
Prior	286	273
Revolving loans amortized cost basis	13,713	13,135
Total	15,716	14,868
Year-to-date Charge-offs [Abstract]
2024/2023	(28)	(13)
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	0	0
Revolving loans amortized cost basis	0	0
Total	(28)	(13)
Year-to-date Recoveries [Abstract]
2024/2023	10	0
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	3	1
Revolving loans amortized cost basis	0	0
Total	13	1
Year-to-date Net Charge-offs [Abstract]
2024/2023	(18)	(13)
2023/2022	0	0
2022/2021	0	0
2021/2020	0	0
2020/2019	0	0
Prior	3	1
Revolving loans amortized cost basis	0	0
Net Charge-offs	(15)	(12)

No definition available.

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

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated more than five years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossRevolvingRecovery
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after recovery, of revolving write-off of financing receivable, charged against allowance for credit loss.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossRevolvingWriteOffAfterRecovery
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated four years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossYearFiveOriginatedFourYearsBeforeCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated three years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossYearFourOriginatedThreeYearsBeforeCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated in current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossYearOneOriginatedCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated two years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery, originated prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableAllowanceForCreditLossYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearYearToDateRecoveries
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The net write-offs amount of financing receivable originated more than five years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net write-offs amount of financing receivable, originated four years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net write-offs amount of financing receivable, originated three years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net write-offs amount of financing receivable originated in current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableYearOneOriginatedCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net write-offs amount of financing receivable, originated two years prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net write-offs amount of financing receivable originated in fiscal year prior to current fiscal year.

+ References

No definition available.

+ Details

Name:	fnrn_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearNetWriteOffs
Namespace Prefix:	fnrn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph a
-Subparagraph (4)
-Publisher SEC

Reference 2: 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 3: 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

+ Details

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

- References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

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
-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

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

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
-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 4: 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

+ Details

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

- Definition

Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ 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

+ Details

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

- Definition

Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated more than five years before current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

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

- Definition

Amortized cost of revolving financing receivable converted to term loan.

+ 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 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6A

+ Details

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

- Definition

Amount of writeoff of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated four years before current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated three years before current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated two years before current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of writeoff of financing receivable originated in fiscal year before current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amortized cost, before allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-13

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-7A

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 29
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-29

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph a
-Subparagraph (1)
-Publisher SEC

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph a
-Subparagraph (2)
-Publisher SEC

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1404
-Paragraph a
-Publisher SEC

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-80

Reference 8: 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 9: http://www.xbrl.org/2009/role/commonPracticeRef
-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 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-14

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fnrn_DoubtfulOrUnlikelyToBeCollectedFinancingReceivableMember
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: