LOANS, Allowance for Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Allowance for credit losses [Roll Forward]
Beginning balance	$ 8,583	$ 9,353	$ 9,353
Provision for (reversal of) loan losses	350	600	1,800
Charge-offs	(67)	(1,364)	(2,821)
Recoveries	28	57	251
Net charge-offs	(39)	(1,307)	(2,570)
Ending balance	8,894	8,646	8,583
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	850	940	860
Loans collectively evaluated for impairment	8,044	7,706	7,723
Ending balance	8,894	8,646	8,583
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	10,500	17,222	10,704
Loans collectively evaluated for impairment	533,999	495,349	534,531
Ending Balance	544,499	512,571	545,235
Commercial [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	3,581	3,199	3,199
Provision for (reversal of) loan losses	(268)	1,133	2,612
Charge-offs	0	(1,062)	(2,288)
Recoveries	12	12	58
Net charge-offs	12	(1,050)	(2,230)
Ending balance	3,325	3,282	3,581
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	31	54	39
Loans collectively evaluated for impairment	3,294	3,228	3,542
Ending balance	3,325	3,282	3,581
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	2,578	4,502	2,678
Loans collectively evaluated for impairment	113,355	108,276	118,073
Ending Balance	115,933	112,778	120,751
Commercial Real Estate [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	1,825	2,290	2,290
Provision for (reversal of) loan losses	660	(140)	(396)
Charge-offs	0	0	(69)
Recoveries	0	0	0
Net charge-offs	0	0	(69)
Ending balance	2,485	2,150	1,825
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	45	56	45
Loans collectively evaluated for impairment	2,440	2,094	1,780
Ending balance	2,485	2,150	1,825
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	960	3,692	976
Loans collectively evaluated for impairment	263,920	240,253	255,979
Ending Balance	264,880	243,945	256,955
Agriculture [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	580	557	557
Provision for (reversal of) loan losses	66	(118)	23
Charge-offs	0	0	0
Recoveries	0	0	0
Net charge-offs	0	0	0
Ending balance	646	439	580
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	646	439	580
Ending balance	646	439	580
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	0	1,029	0
Loans collectively evaluated for impairment	54,719	39,897	61,144
Ending Balance	54,719	40,926	61,144
Residential Mortgage [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	1,181	1,216	1,216
Provision for (reversal of) loan losses	(134)	(43)	36
Charge-offs	0	0	(71)
Recoveries	0	0	0
Net charge-offs	0	0	(71)
Ending balance	1,047	1,173	1,181
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	638	693	646
Loans collectively evaluated for impairment	409	480	535
Ending balance	1,047	1,173	1,181
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	4,595	5,394	4,647
Loans collectively evaluated for impairment	45,101	47,361	45,864
Ending Balance	49,696	52,755	50,511
Residential Construction [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	161	441	441
Provision for (reversal of) loan losses	33	(185)	(366)
Charge-offs	0	0	0
Recoveries	1	2	86
Net charge-offs	1	2	86
Ending balance	195	258	161
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	105	118	107
Loans collectively evaluated for impairment	90	140	54
Ending balance	195	258	161
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	885	931	897
Loans collectively evaluated for impairment	9,211	8,456	5,066
Ending Balance	10,096	9,387	5,963
Consumer [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	886	1,023	1,023
Provision for (reversal of) loan losses	(8)	203	149
Charge-offs	(67)	(302)	(393)
Recoveries	15	43	107
Net charge-offs	(52)	(259)	(286)
Ending balance	826	967	886
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	31	19	23
Loans collectively evaluated for impairment	795	948	863
Ending balance	826	967	886
Loans individually and collectively evaluated for impairment [Abstract]
Loans individually evaluated for impairment	1,482	1,674	1,506
Loans collectively evaluated for impairment	47,693	51,106	48,405
Ending Balance	49,175	52,780	49,911
Unallocated [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	369	627	627
Provision for (reversal of) loan losses	1	(250)	(258)
Charge-offs	0	0	0
Recoveries	0	0	0
Net charge-offs	0	0	0
Ending balance	370	377	369
Allowance for loan loss allocations for impairments [Abstract]
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	370	377	369
Ending balance	$ 370	$ 377	$ 369

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

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

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The increase (decrease) in the allowance for credit losses related to financing receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Details

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

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

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

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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