Subsequent Events (Details) - Columbia State Bank Branches [Member] $ in Millions	Jan. 20, 2023 USD ($)	Nov. 05, 2022 Branches
Purchase Agreement [Abstract]
Number of branches acquired \| Branches		3
Subsequent Event [Member]
Purchase Agreement [Abstract]
Purchase price of assets acquisition	3.15%
Aggregate deposits assumed	$ 116
Aggregate principal balance of loans acquired	$ 4

Purchase price of expected asset acquisition, 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