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FHLB Advances and Other Borrowings - Additional Information (Details) | 12 Months Ended
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Sep. 30, 2018
Federal Home Loan Bank of Des Moines |
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items] |
Federal home loan bank, percent of assets | 45.00%

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

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- Definition Federal Home Loan Bank, Line of Credit, Maximum Borrowing Capacity, Percentage of Assets + References No definition available. + Details
Name: | wfsl_FederalHomeLoanBankLineofCreditMaximumBorrowingCapacityPercentageofAssets
Namespace Prefix: | wfsl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfDesMoinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |