Loans Receivable - Schedule of Loans on Non-accrual Status (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Sep. 30, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual percent of total loans	0.61%	0.63%
Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	$ 123,859	$ 128,628
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	93,795	103,540
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Commercial loans \| Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	34,385	19,121
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Commercial loans \| Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	4,682	69,972
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Commercial loans \| Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	54,728	11,047
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Commercial loans \| Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	0	3,400
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	30,064	25,088
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans \| Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	760	760
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans \| Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	28,188	23,741
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans \| Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	325	23
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans \| HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	593	412
Non-accrual with no ACL	0	0
90 days or more past due and accruing	0	0
Financial Asset Originated \| Consumer loans \| Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	198	152
Non-accrual with no ACL	0	0
90 days or more past due and accruing	$ 0	$ 0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable 90 days or more past due and still accruing. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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

- Definition

Financing Receivable, Nonaccrual, Percent of Total Loans

+ References

No definition available.

+ Details

Name:	wfsl_FinancingReceivableNonaccrualPercentOfTotalLoans
Namespace Prefix:	wfsl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetOriginatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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