Note 7 - Deposits - Deposits (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Noninterest-bearing demand deposits	$ 252,999	$ 252,083
Interest-bearing demand deposits	$ 167,202	$ 169,418
Interest-bearing demand deposits, weighted-average interest rate	0.60%	0.56%
Money market accounts	$ 433,307	$ 362,205
Money market accounts, weighted-average interest rate	3.18%	1.78%
Savings accounts	$ 212,763	$ 242,148
Savings accounts, weighted-average interest rate	1.45%	1.62%
Certificates of deposit, retail	$ 441,665	$ 443,412
Certificates of deposit, retail, weighted-average interest rate	4.21%	4.04%
Certificates of deposit, brokered	$ 203,705	$ 207,626
Certificates of deposit, brokered, weighted-average interest rate	4.65%	4.85%
Deposits	$ 1,711,641	$ 1,676,892
Total deposits, weighted-average interest rate	2.68%	2.34%

Represents brokered time deposits.

No definition available.

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

- Definition

Represents retail time deposits.

+ References

No definition available.

+ Details

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

- Definition

Represents the weighted average rate of domestic deposits, relating to brokered deposits.

+ References

No definition available.

+ Details

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

- Definition

Represents the weighted average domestic deposit percent relating to retail deposits.

+ References

No definition available.

+ Details

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

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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

- Definition

Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts.

+ References

+ Details

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

- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

+ Details

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

- Definition

The aggregate of all domestic and foreign interest-bearing deposit liabilities.

+ References

+ Details

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

- Definition

The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

+ References

+ Details

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

- Definition

Weighted average interest rate calculated at point in time for domestic deposit liability.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic demand deposit liabilities.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
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- Definition

The weighted average interest rate for domestic money market demand account deposit liabilities.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic savings deposit liabilities.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
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