Note 2 - Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Securities available for sale, amortized cost	$ 365,079	$ 317,528
Securities available for sale, gross unrealized gains	7,022	1,008
Securities available for sale, gross unrealized losses	(2,990)	(2,956)
Securities available for sale, estimated fair value	369,111	315,580
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale, amortized cost	94,006	39,524
Securities available for sale, gross unrealized gains	3,232	125
Securities available for sale, gross unrealized losses	(95)	(367)
Securities available for sale, estimated fair value	97,143	39,282
Asset-backed Securities, Issued by US Government Agencies [Member]
Securities available for sale, amortized cost	73,915	29,796
Securities available for sale, gross unrealized gains	1,123	0
Securities available for sale, gross unrealized losses	(1,420)	(938)
Securities available for sale, estimated fair value	73,618	28,858
Asset-backed Securities, Issued by Private Enterprises [Member]
Securities available for sale, amortized cost	33,553	41,728
Securities available for sale, gross unrealized gains	7	0
Securities available for sale, gross unrealized losses	(813)	(873)
Securities available for sale, estimated fair value	32,747	40,855
Corporate Debt Securities [Member]
Securities available for sale, amortized cost	33,401	9,986
Securities available for sale, gross unrealized gains	283	0
Securities available for sale, gross unrealized losses	(454)	(343)
Securities available for sale, estimated fair value	33,230	9,643
US Small Business Administration Securities [Member]
Securities available for sale, amortized cost	23,623	28,423
Securities available for sale, gross unrealized gains	242	72
Securities available for sale, gross unrealized losses	(1)	(36)
Securities available for sale, estimated fair value	23,864	28,459
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available for sale, amortized cost	90,281	159,697
Securities available for sale, gross unrealized gains	2,133	811
Securities available for sale, gross unrealized losses	(12)	(341)
Securities available for sale, estimated fair value	92,402	160,167
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Securities available for sale, amortized cost	16,300	8,374
Securities available for sale, gross unrealized gains	2	0
Securities available for sale, gross unrealized losses	(195)	(58)
Securities available for sale, estimated fair value	$ 16,107	$ 8,316

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost 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 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

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=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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).

+ 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=121558606&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=121553693&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=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
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=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_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: