Note 9 - Employee Benefits (Details Textual)	3 Months Ended
Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)
Employee Stock Ownership Plan (ESOP), Minimum Service Period, Hours	1,000
Employee Stock Ownership Plan (ESOP), Requisite Service Period (Month)	12 months
Employee Stock Ownership Plan (ESOP), Debt Structure, Amortization Period (Year)	20 years
Employee Stock Ownership Plan (ESOP), Debt Structure, Estimated Interest Rate	2.46%
Employee Stock Ownership Plan (ESOP), Principal and Interest Payments from ESOP	$ 0	$ 0
Employee Stock Ownership Plan (ESOP), Compensation Expense, Net of Dividends Received	$ 127,000	$ 140,000

The amount of plan compensation cost recognized during the period, net of dividends received.

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopCompensationExpenseNetOfDividendsReceived
Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amortization period of the loan made by a third-party lender directly to the ESOP.

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopDebtStructureAmortizationPeriod
Namespace Prefix:	fnwb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The percentage of estimated interest paid by the ESOP to a lender.

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopDebtStructureEstimatedInterestRate
Namespace Prefix:	fnwb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

The number of minimum service period hours required for the employee stock ownership plan.

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopMinimumServicePeriodHours
Namespace Prefix:	fnwb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

The amount of principal and interest paid by the ESOP to a lender.

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopPrincipalAndInterestPaymentsFromEsop
Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The requisite service period pursuant to the employee stock ownership plan (ESOP).

No definition available.

Name:	fnwb_EmployeeStockOwnershipPlanEsopRequisiteServicePeriod
Namespace Prefix:	fnwb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration