Note 2 - Securities - Unrealized Gross Losses and Fair Value of Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Securities available for sale, gross unrealized losses, less than twelve months	$ (229)	$ (176)
Securities available for sale, fair value, less than twelve months	24,200	19,494
Securities available for sale, gross unrealized losses, twelve months or longer	(30,209)	(38,151)
Securities available for sale, fair value, twelve months or longer	242,113	274,306
Securities available for sale, gross unrealized losses, total	(30,438)	(38,327)
Securities available for sale, fair value, total	266,313	293,800
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	0	0
Securities available for sale, fair value, less than twelve months	0	0
Securities available for sale, gross unrealized losses, twelve months or longer	(12,210)	(15,237)
Securities available for sale, fair value, twelve months or longer	81,363	87,461
Securities available for sale, gross unrealized losses, total	(12,210)	(15,237)
Securities available for sale, fair value, total	81,363	87,461
US Government Agencies Debt Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(32)	(65)
Securities available for sale, fair value, less than twelve months	9,520	11,782
Securities available for sale, gross unrealized losses, twelve months or longer	0	0
Securities available for sale, fair value, twelve months or longer	0	0
Securities available for sale, gross unrealized losses, total	(32)	(65)
Securities available for sale, fair value, total	9,520	11,782
Corporate Issued Asset-backed Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(21)	(84)
Securities available for sale, fair value, less than twelve months	4,815	3,771
Securities available for sale, gross unrealized losses, twelve months or longer	0	0
Securities available for sale, fair value, twelve months or longer	0	0
Securities available for sale, gross unrealized losses, total	(21)	(84)
Securities available for sale, fair value, total	4,815	3,771
Corporate Debt Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(50)	0
Securities available for sale, fair value, less than twelve months	1,640	0
Securities available for sale, gross unrealized losses, twelve months or longer	(4,023)	(5,061)
Securities available for sale, fair value, twelve months or longer	52,418	51,454
Securities available for sale, gross unrealized losses, total	(4,073)	(5,061)
Securities available for sale, fair value, total	54,058	51,454
US Small Business Administration Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(7)
Securities available for sale, fair value, less than twelve months	983
Securities available for sale, gross unrealized losses, twelve months or longer	0
Securities available for sale, fair value, twelve months or longer	0
Securities available for sale, gross unrealized losses, total	(7)
Securities available for sale, fair value, total	983
Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(119)	(27)
Securities available for sale, fair value, less than twelve months	7,242	3,941
Securities available for sale, gross unrealized losses, twelve months or longer	(10,321)	(12,391)
Securities available for sale, fair value, twelve months or longer	51,487	59,305
Securities available for sale, gross unrealized losses, total	(10,440)	(12,418)
Securities available for sale, fair value, total	58,729	63,246
Mortgage-Backed Securities, Issued by Private Enterprises [Member]
Securities available for sale, gross unrealized losses, less than twelve months	0	0
Securities available for sale, fair value, less than twelve months	0	0
Securities available for sale, gross unrealized losses, twelve months or longer	(3,655)	(5,462)
Securities available for sale, fair value, twelve months or longer	56,845	76,086
Securities available for sale, gross unrealized losses, total	(3,655)	(5,462)
Securities available for sale, fair value, total	$ 56,845	$ 76,086

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fnwb_CorporateIssuedAssetbackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fnwb_USSmallBusinessAdministrationSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: