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Acquisitions - Additional Information (Details) (Bank of America, National Association [Member], USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
branch
May 02, 2014
Oct. 31, 2013
branch
Dec. 06, 2013
branch
Business Acquisition [Line Items]        
Number of branches 74wfsl_BusinessCombinationNumberofBranches      
Customer accounts   $ 1,853,798wfsl_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDepositAccounts    
Loans receivable, net   12,881us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables    
Property and equipment, net   25,097us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment    
Premium on deposits   1.99%wfsl_BusinessCombinationPremiumonDeposits    
Cash   1,776,660us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents    
Core deposit intangible   11,040us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles    
Goodwill   $ 31,225us-gaap_Goodwill    
New Mexico        
Business Acquisition [Line Items]        
Number of branches     11wfsl_BusinessCombinationNumberofBranches
/ us-gaap_BusinessAcquisitionAxis
= wfsl_BankofAmericaNationalAssociationMember
/ us-gaap_StatementGeographicalAxis
= stpr_NM
 
Washington, Oregon, and Idaho        
Business Acquisition [Line Items]        
Number of branches       40wfsl_BusinessCombinationNumberofBranches
/ us-gaap_BusinessAcquisitionAxis
= wfsl_BankofAmericaNationalAssociationMember
/ us-gaap_StatementGeographicalAxis
= wfsl_WashingtonOregonandIdahoMember
Arizona and Nevada        
Business Acquisition [Line Items]        
Number of branches   23wfsl_BusinessCombinationNumberofBranches
/ us-gaap_BusinessAcquisitionAxis
= wfsl_BankofAmericaNationalAssociationMember
/ us-gaap_StatementGeographicalAxis
= wfsl_ArizonaandNevadaMember