Loans Receivable - Loans on Non-accrual Status (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual percent of total loans	0.48%	0.46%
Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 53,808	$ 49,580
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	100.00%	100.00%
Single-family residential \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 25,319	$ 27,930
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	47.10%	56.30%
Construction \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 2,296	$ 0
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	4.30%	0.00%
Construction - custom \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 0	$ 91
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	0.00%	0.20%
Land - acquisition & development \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 2,004	$ 296
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	3.70%	0.60%
Land - consumer lot loans \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 1,045	$ 605
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	1.90%	1.20%
Multi-family \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 0	$ 139
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	0.00%	0.30%
Commercial real estate \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 9,527	$ 11,815
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	17.70%	23.80%
Commercial & industrial \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 13,362	$ 8,082
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	24.80%	16.30%
HELOC \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 217	$ 531
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	0.40%	1.10%
Consumer \| Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 38	$ 91
Ratio of nonaccrual loan by portfolio segment to total loans on nonaccrual status	0.10%	0.20%

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.

No definition available.

+ Details

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

- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

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

- Definition

Loans and Leases Receivable, Ratio of Type of Nonperforming Loans to All Nonperforming Loans

+ References

No definition available.

+ Details

Name:	wfsl_LoansAndLeasesReceivableRatioOfTypeOfNonperformingLoansToAllNonperformingLoans
Namespace Prefix:	wfsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest, Percent of Total Loans

+ References

No definition available.

+ Details

Name:	wfsl_LoansandLeasesReceivableNonperformingNonaccrualofInterestPercentofTotalLoans
Namespace Prefix:	wfsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=wfsl_NonAcquiredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_HomeEquityMember
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: