Aggregate dividends declared during the period for each share of common stock outstanding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Definition

The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ffnw_Derivative1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ffnw_Derivative7Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ffnw_Derivative2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ffnw_Derivative3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ffnw_Derivative8Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Subsequent Events (Details) - Subsequent Event [Member] - USD ($)	Mar. 10, 2020	Mar. 02, 2020	Feb. 14, 2020	Jan. 10, 2020	Jan. 06, 2020
Subsequent Event [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased					513,000
Equity securities				$ 2,400,000
Common Stock, Dividends, Per Share, Declared			$ 0.10
Notional amount		$ 15,000,000
Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount	$ 25,000,000.0
0.911% Derivative [Member] \| Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount		$ 15,000,000
Fixed rate		0.911%
Term		6 years
0.793% Derivative [Member] \| Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount	$ 15,000,000
Fixed rate	0.793%
Term	7 years
0.937% Derivative [Member] \| Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount		$ 15,000,000,000
Fixed rate		93700.00%
Term		7 years
0.984 % Derivative [Member] \| Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount		$ 15,000,000,000
Fixed rate		98400.00%
Term		8 years
0.800% Derivative [Member] \| Cash Flow Hedging [Member]
Subsequent Event [Line Items]
Notional amount	$ 10,000,000
Fixed rate	80000.00%
Term	8 years