Loans Receivable - Loans Impaired, Loan Commitments and Loans Serviced (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Receivables [Abstract]
Recorded investment in impaired loans	$ 251,274	$ 285,243
TDRs included in impaired loans	207,377	261,531
Allocated reserves on impaired loans	1,363	1,980
Specific reserves on impaired loans	126	366
Average balance of impaired loans for year ended	274,530	301,685
Interest income from impaired loans for year ended	11,736	12,843
Outstanding fixed-rate origination commitments	425,130	331,947
Gross loans serviced for others	$ 77,119	$ 80,896

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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Period Type:	duration

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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Period Type:	duration

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Allocated Reserves on Impaired Loans

+ References

No definition available.

+ Details

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Namespace Prefix:	wfsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing Receivable, Average Balance of Impaired Loans

+ References

No definition available.

+ Details

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Namespace Prefix:	wfsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loans and Leases Receivable, Loan Origination Commitments Outstanding

+ References

No definition available.

+ Details

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Namespace Prefix:	wfsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Loans Serviced for Others

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Recorded investment in impaired loans

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant