Note 9 - Fair Value Accounting and Measurement (Details Textual) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Measurement with Unobservable Inputs Reconciliation, Discount	$ 0	$ 0

Fair value of discount of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

No definition available.

Name:	fnwb_FairValueMeasurementWithUnobservableInputsReconciliationDiscount
Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant