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Cover Page - USD ($)
 12 Months Ended
Sep. 30, 2020
Nov. 17, 2020
Mar. 31, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2020    
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    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 1,945,306,499
Entity Common Stock, Shares Outstanding   75,815,930  
Documents Incorporated by Reference
List hereunder the following documents incorporated by reference and the Part of Form 10-K into which the document is incorporated:
(1) Portions of the Registrant’s Annual Report to Shareholders for the fiscal year ended September 30, 2020, are incorporated into Part II, Items 5-8 and Part III, Item 12 of this Form 10-K.
(2) Portions of the Registrant’s definitive proxy statement for its Annual Meeting of Shareholders to be held on January 26, 2021 are incorporated into Part III, Items 10-14 of this Form 10-K.
    
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000936528    
Current Fiscal Year End Date --09-30