ACCOUNTING POLICIES (Details)	6 Months Ended
Jun. 30, 2018 Property
Impact of Recently Issued Accounting Standards [Abstract]
Number of properties leased	10

This element represents the number of properties leased by the entity under operating lease arrangement.

No definition available.

Name:	fnrn_OperatingLeasesNumberOfPropertiesLeased
Namespace Prefix:	fnrn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration