LOANS AND ALLOWANCE FOR CREDIT LOSSES, Financing Receivables by Credit Quality Indicator (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Risk Ratings by Loan Class [Abstract]
2024/2023	$ 47,112		$ 167,123
2023/2022	156,463		242,030
2022/2021	237,768		274,834
2021/2020	270,050		81,433
2020/2019	72,356		79,344
Prior	218,262		145,005
Revolving loans amortized cost basis	61,330		79,214
Total	1,063,341		1,068,983
Revolving loans converted to term loans	630		881
Current Period Charge-offs [Abstract]
2024/2023	(13)		(1,885)
2023/2022	(40)		(196)
2022/2021	(75)		(36)
2021/2020	0		0
2020/2019	(13)		(87)
Prior	(2)		(3)
Revolving loans amortized cost basis	0		(742)
Total	(143)	$ (131)	(2,949)
Current Period Recoveries [Abstract]
2024/2023	0		1,825
2023/2022	0		0
2022/2021	0		0
2021/2020	0		0
2020/2019	0		87
Prior	43		149
Revolving loans amortized cost basis	0		742
Total	43	23	2,803
Current Period Net Charge-offs [Abstract]
2024/2023	(13)		(60)
2023/2022	(40)		(196)
2022/2021	(75)		(36)
2021/2020	0		0
2020/2019	(13)		0
Prior	41		146
Revolving loans amortized cost basis	0		0
Net (charge-offs)/recoveries	(100)		(146)
Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	42,185		166,728
2023/2022	156,071		235,519
2022/2021	233,529		258,011
2021/2020	254,053		78,972
2020/2019	69,950		67,637
Prior	207,894		141,266
Revolving loans amortized cost basis	56,711		75,471
Total	1,020,393		1,023,604
Special Mention [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		3,059
2022/2021	4,239		12,974
2021/2020	7,942		235
2020/2019	212		3,177
Prior	1,543		2,337
Revolving loans amortized cost basis	1,790		2,936
Total	15,726		24,718
Substandard [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	4,927		395
2023/2022	392		3,452
2022/2021	0		3,849
2021/2020	8,055		2,226
2020/2019	2,194		8,530
Prior	8,825		1,402
Revolving loans amortized cost basis	2,829		807
Total	27,222		20,661
Doubtful/Loss [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	14,182		19,776
2023/2022	17,078		18,115
2022/2021	17,571		19,515
2021/2020	17,484		6,158
2020/2019	5,705		7,728
Prior	13,048		6,298
Revolving loans amortized cost basis	23,718		29,307
Total	108,786		106,897
Current Period Charge-offs [Abstract]
2024/2023	0		(47)
2023/2022	(40)		(196)
2022/2021	(75)		(36)
2021/2020	0		0
2020/2019	(13)		(87)
Prior	(2)		0
Revolving loans amortized cost basis	0		0
Total	(130)	(127)	(366)
Current Period Recoveries [Abstract]
2024/2023	0		0
2023/2022	0		0
2022/2021	0		0
2021/2020	0		0
2020/2019	0		87
Prior	41		148
Revolving loans amortized cost basis	0		0
Total	41	23	235
Current Period Net Charge-offs [Abstract]
2024/2023	0		(47)
2023/2022	(40)		(196)
2022/2021	(75)		(36)
2021/2020	0		0
2020/2019	(13)		0
Prior	39		148
Revolving loans amortized cost basis	0		0
Net (charge-offs)/recoveries	(89)		(131)
Commercial [Member] \| Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	13,255		19,776
2023/2022	17,078		16,961
2022/2021	15,223		15,833
2021/2020	14,713		5,381
2020/2019	4,972		7,420
Prior	12,759		6,298
Revolving loans amortized cost basis	21,639		26,183
Total	99,639		97,852
Commercial [Member] \| Special Mention [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		1,122
2022/2021	2,348		2,530
2021/2020	2,771		235
2020/2019	212		308
Prior	289		0
Revolving loans amortized cost basis	1,790		2,936
Total	7,410		7,131
Commercial [Member] \| Substandard [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	927		0
2023/2022	0		32
2022/2021	0		1,152
2021/2020	0		542
2020/2019	521		0
Prior	0		0
Revolving loans amortized cost basis	289		188
Total	1,737		1,914
Commercial [Member] \| Doubtful/Loss [Member]
Risk Ratings 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
Commercial Real Estate [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	24,352		116,202
2023/2022	107,614		173,918
2022/2021	174,355		201,067
2021/2020	198,760		51,834
2020/2019	45,847		61,630
Prior	168,356		110,199
Revolving loans amortized cost basis	6,785		6,879
Total	726,069		721,729
Current Period 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	0
Current Period 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	0
Current Period 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)/recoveries	0		0
Commercial Real Estate [Member] \| Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	24,352		115,807
2023/2022	107,222		173,918
2022/2021	174,355		191,907
2021/2020	189,466		50,150
2020/2019	44,174		52,157
Prior	159,555		107,909
Revolving loans amortized cost basis	6,785		6,879
Total	705,909		698,727
Commercial Real Estate [Member] \| Special Mention [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		0
2022/2021	0		7,448
2021/2020	2,175		0
2020/2019	0		2,869
Prior	1,254		1,273
Revolving loans amortized cost basis	0		0
Total	3,429		11,590
Commercial Real Estate [Member] \| Substandard [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		395
2023/2022	392		0
2022/2021	0		1,712
2021/2020	7,119		1,684
2020/2019	1,673		6,604
Prior	7,547		1,017
Revolving loans amortized cost basis	0		0
Total	16,731		11,412
Commercial Real Estate [Member] \| Doubtful/Loss [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	2,484		6,842
2023/2022	7,051		18,922
2022/2021	18,772		24,453
2021/2020	24,328		8,792
2020/2019	6,707		4,435
Prior	15,645		12,501
Revolving loans amortized cost basis	16,878		29,893
Total	91,865		105,838
Current Period 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	0	(2,567)
Current Period 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	0	2,567
Current Period 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)/recoveries	0		0
Agriculture [Member] \| Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	2,484		6,842
2023/2022	7,051		16,985
2022/2021	16,881		20,511
2021/2020	20,433		8,792
2020/2019	6,707		2,509
Prior	15,645		11,437
Revolving loans amortized cost basis	14,940		29,893
Total	84,141		96,969
Agriculture [Member] \| Special Mention [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		1,937
2022/2021	1,891		2,996
2021/2020	2,996		0
2020/2019	0		0
Prior	0		1,064
Revolving loans amortized cost basis	0		0
Total	4,887		5,997
Agriculture [Member] \| Substandard [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		0
2022/2021	0		946
2021/2020	899		0
2020/2019	0		1,926
Prior	0		0
Revolving loans amortized cost basis	1,938		0
Total	2,837		2,872
Agriculture [Member] \| Doubtful/Loss [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	1,312		20,239
2023/2022	20,536		24,906
2022/2021	23,824		26,468
2021/2020	28,012		14,500
2020/2019	13,966		5,481
Prior	20,903		15,734
Revolving loans amortized cost basis	0		0
Total	108,553		107,328
Current Period 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)	(3)
Current Period 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	0
Current Period 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)/recoveries	0		(3)
Residential Mortgage [Member] \| Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	1,312		20,239
2023/2022	20,536		24,906
2022/2021	23,824		26,429
2021/2020	27,975		14,500
2020/2019	13,966		5,481
Prior	19,625		15,349
Revolving loans amortized cost basis	0		0
Total	107,238		106,904
Residential Mortgage [Member] \| Special Mention [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	0		0
2023/2022	0		0
2022/2021	0		39
2021/2020	37		0
2020/2019	0		0
Prior	1,278		385
Revolving loans amortized cost basis	0		0
Total	1,315		424
Residential Mortgage [Member] \| Doubtful/Loss [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	4,575		3,714
2023/2022	3,998		5,411
2022/2021	2,073		3,198
2021/2020	1,335		0
2020/2019	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Total	11,981		12,323
Current Period 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	0
Current Period 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	0
Current Period 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)/recoveries	0		0
Residential Construction [Member] \| Pass [Member]
Risk Ratings by Loan Class [Abstract]
2024/2023	575		3,714
2023/2022	3,998		1,991
2022/2021	2,073		3,198
2021/2020	1,335		0
2020/2019	0		0
Prior	0		0
Revolving loans amortized cost basis	0		0
Total	7,981		8,903
Residential Construction [Member] \| Special Mention [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	4,000		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	4,000		3,420
Residential Construction [Member] \| Doubtful/Loss [Member]
Risk Ratings 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 Ratings by Loan Class [Abstract]
2024/2023	207		350
2023/2022	186		758
2022/2021	1,173		133
2021/2020	131		149
2020/2019	131		70
Prior	310		273
Revolving loans amortized cost basis	13,949		13,135
Total	16,087		14,868
Current Period Charge-offs [Abstract]
2024/2023	(13)		(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	(13)	(1)	(13)
Current Period Recoveries [Abstract]
2024/2023	0		0
2023/2022	0		0
2022/2021	0		0
2021/2020	0		0
2020/2019	0		0
Prior	2		1
Revolving loans amortized cost basis	0		0
Total	2	$ 0	1
Current Period Net Charge-offs [Abstract]
2024/2023	(13)		(13)
2023/2022	0		0
2022/2021	0		0
2021/2020	0		0
2020/2019	0		0
Prior	2		1
Revolving loans amortized cost basis	0		0
Net (charge-offs)/recoveries	(11)		(12)
Consumer [Member] \| Pass [Member]

No definition available.

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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

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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

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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

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Data Type:	xbrli:monetaryItemType
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- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- 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

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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

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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

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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

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- References

No definition available.

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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

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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

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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

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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

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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

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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

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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/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
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