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Covered Assets - Additional Information (Details) (USD $)
 6 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Covered Assets [Line Items]    
Covered assets 153,673,000wfsl_CoveredAssets $ 200,558,000wfsl_CoveredAssets
Covered loans, gross 159,018,000wfsl_LoansAndLeasesReceivableCoveredLoansGross 213,203,000wfsl_LoansAndLeasesReceivableCoveredLoansGross
Discount 21,013,000us-gaap_ReceivableWithImputedInterestDiscount 34,483,000us-gaap_ReceivableWithImputedInterestDiscount
Covered Loans, Allowance for Losses 0wfsl_CoveredLoansAllowanceForLosses 2,244,000wfsl_CoveredLoansAllowanceForLosses
Single-Family Residential [Member]    
Covered Assets [Line Items]    
Remaining term 5 years  
Horizon Bank [Member]    
Covered Assets [Line Items]    
Covered assets 83,077,000wfsl_CoveredAssets
/ us-gaap_BusinessAcquisitionAxis
= wfsl_HorizonBankMember		  
Home Valley Bank [Member]    
Covered Assets [Line Items]    
Covered assets 53,181,000wfsl_CoveredAssets
/ us-gaap_BusinessAcquisitionAxis
= wfsl_HomeValleyBankMember