Note 7 - Deposits (Details Textual) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Time Deposits, at or Above FDIC Insurance Limit	$ 170.7	$ 173.8
Deposits, Public Fund	119.0	114.2
Letters of Credit Outstanding, Amount	60.0
Deposits, Funds Held by Federally Recognized Tribes	20.7	18.4
Asset Pledged as Collateral [Member] \| Funds Held by Federally Recognized Tribes [Member]
Debt Securities	24.2	23.8
Federal Home Loan Bank of Des Moines [Member]
Letters of Credit Outstanding, Amount	$ 60.0	$ 60.0

Represents funds held by federally recognized tribes as deposits.

No definition available.

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Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of public fund deposit liabilities.

+ References

No definition available.

+ Details

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Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 405
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Namespace Prefix:	us-gaap_
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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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