Loans Receivable: Troubled Debt Restructurings on Financing Receivables (Details)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2019 USD ($) loan	Jun. 30, 2018 USD ($) loan	Jun. 30, 2019 USD ($) loan	Jun. 30, 2018 USD ($) loan	Dec. 31, 2018 USD ($)
Loan Restructuring, Trial Modifications, Amount \| loan	0	1	9	1
Financing Receivable, Troubled Debt Restructuring	$ 6,700,000		$ 6,700,000		$ 9,400,000
Troubled Debt Restructuring Commitment To Extend Additional Credit	0	$ 0	0	$ 0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default	0		0
Financing Receivable, Troubled Debt Restructuring, Premodification	0	1,124,000	1,518,000	1,124,000
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 0	$ 1,124,000	$ 1,518,000	$ 1,124,000
Minimum [Member]
Troubled Debt Restructuring, Interest Rate Concession Period	1 year	1 year			1 year
Maximum [Member]
Troubled Debt Restructuring, Interest Rate Concession Period	3 years	3 years			3 years
Multifamily [Member] \| Advancement of Maturity Date [Member]
Loan Restructuring, Trial Modifications, Amount \| loan		1		1
Financing Receivable, Troubled Debt Restructuring, Premodification		$ 1,124,000		$ 1,124,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 1,124,000		$ 1,124,000
One to four family residential [Member] \| Principal and Interest with Interest Rate Concession [Member]
Loan Restructuring, Trial Modifications, Amount \| loan	0		6
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0		$ 824,000
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 0		$ 824,000
One to four family residential [Member] \| Advancement of Maturity Date [Member]
Loan Restructuring, Trial Modifications, Amount \| loan	0		3
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0		$ 694,000
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 0		$ 694,000

-- None. No documentation exists for this element. --

No definition available.

Name:	ffnw_TroubledDebtRestructuringCommitmentToExtendAdditionalCredit
Namespace Prefix:	ffnw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Troubled Debt Restructuring, Interest Rate Concession Period

+ References

No definition available.

+ Details

Name:	ffnw_TroubledDebtRestructuringInterestRateConcessionPeriod
Namespace Prefix:	ffnw_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=ffnw_AdvancementofMaturityDateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ffnw_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=ffnw_PrincipalAndInterestWithInterestRateConcessionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: