Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Location	Dec. 31, 2018 USD ($)
Lessee, Operating Lease, Description [Abstract]
Lease right-of-use assets	$ 6,962	$ 0
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]		us-gaap:InterestReceivableAndOtherAssets
Lease liabilities	$ 7,483
Operating Lease, Liability, Statement of Financial Position [Extensible List]	fnrn:InterestPayableAndOtherLiabilities	fnrn:InterestPayableAndOtherLiabilities
Number of branch and administrative locations leased by the Bank | Location	10
Lease expenses	$ 1,049
Short term leases	115
Reduction in lease expense for deferred gain on sale-leaseback	$ 82
Minimum [Member]
Lessee, Operating Lease, Description [Abstract]
Renewal lease term	3 years
Maximum [Member]
Lessee, Operating Lease, Description [Abstract]
Renewal lease term	10 years
Adjustments for New Accounting Pronouncement [Member]
Lessee, Operating Lease, Description [Abstract]
Lease right-of-use assets		$ 4,417
Lease liabilities		4,812
ASU 2016-02 [Member]
Lessee, Operating Lease, Description [Abstract]
Lease right-of-use assets	$ 6,962	$ 4,417
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:InterestReceivableAndOtherAssets	us-gaap:InterestReceivableAndOtherAssets
Lease liabilities	$ 7,483	$ 4,812
Operating Lease, Liability, Statement of Financial Position [Extensible List]	fnrn:InterestPayableAndOtherLiabilities	fnrn:InterestPayableAndOtherLiabilities