Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) Location	Dec. 31, 2019 USD ($)
Lessee, Operating Lease, Description [Abstract]
Number of branch and administrative locations leased by the Bank | Location	10
Lease right-of-use assets	$ 5,913	$ 6,962
Lease liabilities	$ 6,453	$ 7,483
Operating Lease, Liability, Statement of Financial Position [Extensible List]	fnrn:InterestPayableAndOtherLiabilities	fnrn:InterestPayableAndOtherLiabilities
Lease expenses	$ 1,275	$ 1,049
Minimum [Member]
Lessee, Operating Lease, Description [Abstract]
Renewal lease term	3 years
Maximum [Member]
Lessee, Operating Lease, Description [Abstract]
Renewal lease term	10 years