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Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Nov. 30, 2019
Mar. 31, 2019
Document and Entity Information      
Entity Registrant Name TIMBERLAND BANCORP INC,    
Entity Central Index Key 0001046050    
Current Fiscal Year End Date --09-30    
Entity Filer Category Accelerated Filer    
Document Type 10-K    
Document Period End Date Sep. 30, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Small Business true    
Entity Shell Company false    
Entity Emerging Growth Company false    
Entity Common Stock, Shares Outstanding   8,345,069  
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 233.3