Note 5 - Deposits - Deposits (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Noninterest-bearing demand deposits	$ 238,901	$ 245,760
Interest-bearing demand deposits	$ 157,565	$ 143,166
Interest-bearing demand deposits, weighted-average interest rate	0.20%	0.19%
Money market accounts	$ 449,353	$ 451,143
Money market accounts, weighted-average interest rate	2.11%	2.12%
Savings accounts	$ 246,533	$ 239,258
Savings accounts, weighted-average interest rate	1.45%	1.39%
Certificates of deposit, customer	$ 445,110	$ 433,264
Certificates of deposit, customer, weighted average interest rate	3.60%	3.63%
Certificates of deposit, brokered	$ 64,120	$ 86,510
Certificates of deposit, brokered, weighted-average interest rate	4.20%	4.22%
Deposits	$ 1,601,582	$ 1,599,101
Total deposits, weighted-average interest rate	2.00%	2.04%

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

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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://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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+ 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

Name:	us-gaap_DepositsMoneyMarketDeposits
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
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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

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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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 405
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478353/942-405-50-1

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Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic demand deposit liabilities.

+ References

+ Details

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Data Type:	dtr-types:percentItemType
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Period Type:	instant

- Definition

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

+ References

+ Details

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Data Type:	dtr-types:percentItemType
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- Definition

The weighted average interest rate for domestic savings deposit liabilities.

+ References

+ Details

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