ACCOUNTING POLICIES (Details) - CARES Act [Member]	9 Months Ended
Sep. 30, 2021
Minimum [Member]
Recently Issued Accounting Pronouncements [Abstract]
Term of payment deferrals	3 months
Maximum [Member]
Recently Issued Accounting Pronouncements [Abstract]
Term of payment deferrals	6 months

Represents term of payment deferrals under loan modifications, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but not limited to, weeks in a year or quarter.

No definition available.

Name:	fnrn_FinancingReceivableLoanModificationTermOfPaymentDeferrals
Namespace Prefix:	fnrn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration