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Document and Entity Information - USD ($)
 12 Months Ended
Sep. 30, 2019
Nov. 15, 2019
Mar. 29, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2019    
Document Transition Report false    
Entity File Number 001-34654    
Entity Registrant Name WASHINGTON FEDERAL INC    
Entity Incorporation, State or Country Code WA    
Entity Tax Identification Number 91-1661606    
Entity Address, Address Line One 425 Pike Street    
Entity Address, City or Town Seattle    
Entity Address, State or Province WA    
Entity Address, Postal Zip Code 98101    
City Area Code 206    
Local Phone Number 624-7930    
Title of 12(b) Security Common Stock, $1.00 par value per share    
Trading Symbol WAFD    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 2,300,567,209
Entity Common Stock, Shares Outstanding   78,448,561  
Amendment Flag false    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000936528    
Current Fiscal Year End Date --09-30