Loans Receivable - Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2017 USD ($) contract	Jun. 30, 2016 USD ($) contract	Jun. 30, 2017 USD ($) contract	Jun. 30, 2016 USD ($) contract
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| contract	11	9	33	24
Pre-modification outstanding recorded investment	$ 1,836	$ 3,050	$ 6,114	$ 5,845
Post-modification outstanding recorded investment	$ 1,836	$ 3,050	$ 6,114	$ 5,845
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| contract	11	7	31	17
Pre-modification outstanding recorded investment	$ 1,836	$ 1,492	$ 5,682	$ 3,322
Post-modification outstanding recorded investment	$ 1,836	$ 1,492	$ 5,682	$ 3,322
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| contract			1	0
Pre-modification outstanding recorded investment			$ 204	$ 0
Post-modification outstanding recorded investment			$ 204	$ 0
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| contract	0	2	0	7
Pre-modification outstanding recorded investment	$ 0	$ 1,558	$ 0	$ 2,523
Post-modification outstanding recorded investment	$ 0	$ 1,558	$ 0	$ 2,523
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| contract			1	0
Pre-modification outstanding recorded investment			$ 228	$ 0
Post-modification outstanding recorded investment			$ 228	$ 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.

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

+ Details

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

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

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

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
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=us-gaap_CommercialRealEstatePortfolioSegmentMember
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: