SUBSEQUENT EVENTS (Details) - Subsequent Event [Member] - Columbia State Bank Branches [Member] $ in Millions	Nov. 05, 2022 USD ($) Branches
Purchase Agreement [Abstract]
Number of branches to be acquired \| Branches	3
Purchase price of expected asset acquisition	3.15%
Aggregate deposits to be assumed	$ 128
Aggregate principal balance of the loans to be acquired	$ 4

Purchase price of expected asset acquisition prior to consideration being transferred, as a percentage of the average daily closing balance of certain deposit accounts plus the net book values of certain assets of Columbia and accrued interest and fees with respect to certain loans.

No definition available.

Name:	fnrn_AssetAcquisitionPriceOfAcquisitionPercentageOfCertainDepositAccountsAssetsAndAccruedInterestAndFeesOnCertainLoans
Namespace Prefix:	fnrn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant