Note 14 - Fair Value Measurements - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Investment securities available for sale	$ 344,212,000	$ 364,296,000
Accrued interest receivable	5,289,000	6,966,000
Servicing rights on sold loans, net	3,800,000	2,200,000
Reported Value Measurement [Member]
Cash and cash equivalents	126,016,000	65,155,000
Investment securities available for sale	344,212,000	364,296,000
Loans held for sale	760,000	3,753,000
Loans receivable, net	1,350,260,000	1,141,969,000
FHLB stock	5,196,000	5,977,000
Accrued interest receivable	5,289,000	6,966,000
Servicing rights on sold loans, net	3,282,000	2,120
Equity investments	3,071,000	1,260
Borrowings	80,000,000	109,977
Subordinated debt, net	39,280,000
Accrued interest payable	393,000	53,000
Reported Value Measurement [Member] \| Demand Deposits [Member]
deposits	1,333,337,000	1,024,748,000
Reported Value Measurement [Member] \| Time Deposits [Member]
deposits	247,243,000	308,769,000
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	126,016,000	65,155,000
Investment securities available for sale	347,283,000	365,556,000
Loans held for sale	760,000	3,753,000
Loans receivable, net	1,328,589,000	1,129,570,000
FHLB stock	5,196,000	5,977,000
Accrued interest receivable	5,289,000	6,966,000
Servicing rights on sold loans, net	3,820,000	2,189
Equity investments	3,071,000	1,260
Borrowings	80,192,000	111,462
Subordinated debt, net	39,144,000
Accrued interest payable	393,000	53,000
Estimate of Fair Value Measurement [Member] \| Demand Deposits [Member]
deposits	1,333,337,000	1,024,748,000
Estimate of Fair Value Measurement [Member] \| Time Deposits [Member]
deposits	247,217,000	310,992,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	126,016,000	65,155,000
Investment securities available for sale	11,963,000	0
Loans held for sale	0	0
Loans receivable, net	0	0
FHLB stock	0	0
Accrued interest receivable	0	0
Servicing rights on sold loans, net	0	0
Equity investments	0	0
Borrowings	0	0
Subordinated debt, net	0
Accrued interest payable	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| Demand Deposits [Member]
deposits	1,333,337,000	1,024,748,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| Time Deposits [Member]
deposits	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Investment securities available for sale	335,320,000	356,644,000
Loans held for sale	760,000	3,753,000
Loans receivable, net	0	0
FHLB stock	5,196,000	5,977,000
Accrued interest receivable	5,289,000	6,966,000
Servicing rights on sold loans, net	0	0
Equity investments	3,071,000	1,260
Borrowings	0	111,462
Subordinated debt, net	0
Accrued interest payable	393,000	53,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Demand Deposits [Member]
deposits	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Time Deposits [Member]
deposits	0	310,992,000
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents	0	0
Investment securities available for sale	0	8,912,000
Loans held for sale	0	0
Loans receivable, net	1,328,589,000	1,129,570,000
FHLB stock	0	0
Accrued interest receivable	0	0
Servicing rights on sold loans, net	3,820,000	2,189
Equity investments	0	0
Borrowings	80,192,000	0
Subordinated debt, net	39,144,000
Accrued interest payable	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member] \| Demand Deposits [Member]
deposits	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member] \| Time Deposits [Member]
deposits	$ 247,217,000	$ 0

The fair value portion of interest payable.

No definition available.

Name:	fnwb_InterestPayableFairValueDisclosure
Namespace Prefix:	fnwb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

+ Details

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

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of investments accounted under the equity method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=125513658&loc=d3e32014-111567

+ Details

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

- Definition

Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

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

- Definition

Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=125521744&loc=d3e122625-111746

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_DemandDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=fnwb_TimeDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: