Stock Compensation Plans, Share-based Compensation Plans (Details)		12 Months Ended
Jan. 24, 2019	Dec. 31, 2018 Plan shares
Subsequent Event [Member]
Dividends [Abstract]
Stock dividend percentage	5.00%
Dividends date declared	Jan. 24, 2019
Dividend date to be paid	Mar. 29, 2019
Dividends payable, date of record	Feb. 28, 2019
2016 Stock Incentive Plan [Member]
Share-based Compensation Plans [Abstract]
Number of fixed stock option plan \| Plan		1
Number of shares authorized (in shares)		735,555
2016 Stock Incentive Plan [Member] \| Non-Employee Director [Member]
Share-based Compensation Plans [Abstract]
Maximum annual number of shares per individual (in shares)		3,000
Maximum annual number of shares of common stock which may be issued upon exercise or settlement of awards (in shares)		100,000
2016 Stock Incentive Plan [Member] \| Employee [Member]
Share-based Compensation Plans [Abstract]
Maximum annual number of shares per individual (in shares)		25,000
2016 Stock Incentive Plan [Member] \| Stock Options [Member]
Share-based Compensation Plans [Abstract]
Percentage of grants that vest annually		25.00%
Award expiration period		10 years
2016 Stock Incentive Plan [Member] \| Restricted Stock [Member]
Share-based Compensation Plans [Abstract]
Percentage of grants that vest after four years		100.00%
Award vesting period		4 years

Refers to maximum annual number of shares of common stock that may be issued per individual.

No definition available.

Name:	fnrn_MaximumAnnualNumberOfSharesOfCommonStockIssuablePerIndividual
Namespace Prefix:	fnrn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to maximum annual number of shares of common stock that may be issued upon exercise or settlement of awards per individual.

+ References

No definition available.

+ Details

Name:	fnrn_MaximumAnnualNumberOfSharesOfCommonStockIssuableUponExerciseOrSettlementOfAwardsPerIndividual
Namespace Prefix:	fnrn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of fixed stock option plan operated by the company.

+ References

No definition available.

+ Details

Name:	fnrn_NumberOfFixedStockOptionPlan
Namespace Prefix:	fnrn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Annual vesting rate of options granted.

+ References

No definition available.

+ Details

Name:	fnrn_OptionsGrantedVestingRate
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The rate of vesting of restricted stock award granted.

+ References

No definition available.

+ Details

Name:	fnrn_RestrictedStockAwardVestingRate
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage rate used to calculate dividend payments on stock.

+ References

No definition available.

+ Details

Name:	fnrn_StockDividendPercentage
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the declared dividend will be paid, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

+ Details

Name:	us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Date the dividend to be paid was declared, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DividendsPayableDateDeclaredDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fnrn_StockIncentivePlan2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_DirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=fnrn_EmployeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: