CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical)	12 Months Ended
Dec. 31, 2017
Common Stock [Member]
Stock dividend declared	4.00%

The percentage rate used to calculate dividend declared on stock.

No definition available.

Name:	fnrn_StockDividendDeclared
Namespace Prefix:	fnrn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration