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Revenue from Contracts with Customers - Additional Information (Details)	12 Months Ended
	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Revenue streams percentage, contract with customer	5.00%

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- References

No definition available.

+ Details

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

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- Definition

Revenue From Contract With Customer, Percentage

+ References

No definition available.

+ Details

Name:	wfsl_RevenueFromContractWithCustomerPercentage
Namespace Prefix:	wfsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration