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Customer Accounts - Narrative (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Sep. 30, 2020
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Banking and Thrift, Interest [Abstract] |  |
Customer accounts over $250,000 | $ 7,198,111 | $ 5,491,395

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- References No definition available. + Details
Name: | us-gaap_BankingAndThriftInterestAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Customer Accounts Greater than $250,000 + References No definition available. + Details
Name: | wfsl_CustomerAccountsGreaterThan250000
Namespace Prefix: | wfsl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant