Deposits - Components of Deposits (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Aug. 25, 2017
Noninterest-bearing	$ 46,108,000	$ 45,434,000
Interest-bearing demand	40,079,000	38,224,000
Statement savings	24,799,000	28,456,000
Money market	339,047,000	318,636,000
Certificates of deposit, retail (1)	391,174,000	333,264,000
Certificates of deposit, brokered	97,825,000	75,488,000
Total deposits	939,032,000	839,502,000
Opus Bank [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Interest Bearing Deposits			$ 62,534,000
Opus Bank [Member] \| Adjustments [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Interest Bearing Deposits	$ 58,000	$ 107,000	$ (128,000)

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Interest Bearing Deposits

No definition available.

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

- Definition

Certificated of Deposits, Retail

+ References

No definition available.

+ Details

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

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ 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 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts.

+ References

+ Details

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

- Definition

Amount of deposits in interest-bearing transaction accounts at banks or savings and loans, that are restricted in regard to ownership and can usually only be held by individuals, nonprofit entities and governments. Negotiable order of withdrawal accounts differ from money market demand accounts as they typically have higher reserve requirements and no limit on the number of checks that can be written.

+ References

+ Details

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

- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

+ Details

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

- Definition

The aggregate amount of all wholesale deposit accounts including certificates of deposits.

+ References

+ Details

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

- Definition

The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

+ References

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ffnw_OpusBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ffnw_BusinessAcquisitionScenarioAxis=ffnw_AdjustmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: