Note 2 - Securities - Unrealized Gross Losses and Fair Value of Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Securities available for sale, gross unrealized losses, less than twelve months	$ (1,820)	$ (118)
Securities available for sale, fair value, less than twelve months	126,789	39,688
Securities available for sale, gross unrealized losses, twelve months or longer	(724)	(1,351)
Securities available for sale, fair value, twelve months or longer	29,603	59,749
Securities available for sale, gross unrealized losses, total	(2,544)	(1,469)
Securities available for sale, fair value, total	156,392	99,437
US States and Political Subdivisions Debt Securities [Member] \| Securities Investment [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(306)	(15)
Securities available for sale, fair value, less than twelve months	23,125	5,214
Securities available for sale, gross unrealized losses, twelve months or longer	(34)	(2)
Securities available for sale, fair value, twelve months or longer	1,475	1,319
Securities available for sale, gross unrealized losses, total	(340)	(17)
Securities available for sale, fair value, total	24,600	6,533
Asset-backed Securities, Issued by US Government Agencies [Member] \| Securities Investment [Member]
Securities available for sale, gross unrealized losses, less than twelve months		0
Securities available for sale, fair value, less than twelve months		0
Securities available for sale, gross unrealized losses, twelve months or longer		(354)
Securities available for sale, fair value, twelve months or longer		21,430
Securities available for sale, gross unrealized losses, total		(354)
Securities available for sale, fair value, total		21,430
Asset-backed Securities, Issued by Private Enterprises [Member] \| Securities Investment [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(67)	0
Securities available for sale, fair value, less than twelve months	10,976	0
Securities available for sale, gross unrealized losses, twelve months or longer	0	(418)
Securities available for sale, fair value, twelve months or longer	0	27,283
Securities available for sale, gross unrealized losses, total	(67)	(418)
Securities available for sale, fair value, total	10,976	27,283
US Government-sponsored Enterprises Debt Securities [Member] \| Securities Investment [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(27)
Securities available for sale, fair value, less than twelve months	1,920
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	(27)
Securities available for sale, fair value, total	1,920
Corporate Debt Securities [Member] \| Securities Investment [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(333)	(8)
Securities available for sale, fair value, less than twelve months	18,890	5,892
Securities available for sale, gross unrealized losses, twelve months or longer	(234)	(577)
Securities available for sale, fair value, twelve months or longer	9,752	9,409
Securities available for sale, gross unrealized losses, total	(567)	(585)
Securities available for sale, fair value, total	28,642	15,301
US Small Business Administration Securities [Member] \| Securities Investment [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	63
Securities available for sale, gross unrealized losses, twelve months or longer	0	0
Securities available for sale, fair value, twelve months or longer	69	47
Securities available for sale, gross unrealized losses, total	0	0
Securities available for sale, fair value, total	69	110
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] \| Collateralized Mortgage Backed Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(713)	(52)
Securities available for sale, fair value, less than twelve months	39,029	18,516
Securities available for sale, gross unrealized losses, twelve months or longer	(450)	0
Securities available for sale, fair value, twelve months or longer	12,802	261
Securities available for sale, gross unrealized losses, total	(1,163)	(52)
Securities available for sale, fair value, total	51,831	18,777
Mortgage-backed Securities, Issued by Private Enterprises [Member] \| Collateralized Mortgage Backed Securities [Member]
Securities available for sale, gross unrealized losses, less than twelve months	(374)	(43)
Securities available for sale, fair value, less than twelve months	32,849	10,003
Securities available for sale, gross unrealized losses, twelve months or longer	(6)	0
Securities available for sale, fair value, twelve months or longer	5,505	0
Securities available for sale, gross unrealized losses, total	(380)	(43)
Securities available for sale, fair value, total	$ 38,354	$ 10,003

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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ 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

+ 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ 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

+ 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_InvestmentTypeAxis=us-gaap_SecuritiesInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
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_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesMember
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: