Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available for Sale
Amortized Cost	$ 327,970	$ 317,528
Gross Unrealized Gains	4,156	1,008
Gross Unrealized Losses	(14,606)	(2,956)
Estimated Fair Value	317,520	315,580
Investment Securities
Available for Sale
Amortized Cost	175,620	149,457
Estimated Fair Value	163,254	147,097
Investment Securities \| Municipal bonds
Available for Sale
Amortized Cost	52,238	39,524
Gross Unrealized Gains	708	125
Gross Unrealized Losses	(692)	(367)
Estimated Fair Value	52,254	39,282
Investment Securities \| U.S. government agency issued asset-backed securities (ABS agency)
Available for Sale
Amortized Cost	46,495	29,796
Gross Unrealized Gains	403	0
Gross Unrealized Losses	(4,773)	(938)
Estimated Fair Value	42,125	28,858
Investment Securities \| Corporate issued asset-backed securities (ABS corporate)
Available for Sale
Amortized Cost	41,716	41,728
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7,643)	(873)
Estimated Fair Value	34,073	40,855
Investment Securities \| Corporate issued debt securities (Corporate debt)
Available for Sale
Amortized Cost	9,986	9,986
Gross Unrealized Gains	49	0
Gross Unrealized Losses	(596)	(343)
Estimated Fair Value	9,439	9,643
Investment Securities \| U.S. Small Business Administration securities (SBA)
Available for Sale
Amortized Cost	25,185	28,423
Gross Unrealized Gains	190	72
Gross Unrealized Losses	(12)	(36)
Estimated Fair Value	25,363	28,459
Mortgage-backed Securities
Available for Sale
Amortized Cost	152,350	168,071
Estimated Fair Value	154,266	168,483
Mortgage-backed Securities \| U.S. government agency issued mortgage-backed securities (MBS agency)
Available for Sale
Amortized Cost	142,881	159,697
Gross Unrealized Gains	2,806	811
Gross Unrealized Losses	(548)	(341)
Estimated Fair Value	145,139	160,167
Mortgage-backed Securities \| Corporate issued mortgage-backed securities (MBS corporate)
Available for Sale
Amortized Cost	9,469	8,374
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(342)	(58)
Estimated Fair Value	$ 9,127	$ 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=118939692&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=118939692&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=118939692&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=118942338&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 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: 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=118954083&loc=d3e26610-111562

Reference 3: 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=118939692&loc=d3e27161-111563

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_SecuritiesInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=fnwb_AssetbackedSecuritiesCorporateMember
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_SmallBusinessAdministrationSBAMember
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_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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