Regulatory Matters: Schedule of Compliance with Regulatory Capital Requirements for Mortgage Companies (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)
Timberland Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Actual Amount	$ 188,512	$ 168,937
Tier 1 Leverage Capital, Actual Ratio	0.107	0.111
Tier 1 Leverage Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 70,240	$ 60,993
Tier 1 Leverage Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.040	0.040
Tier 1 Leverage Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 87,801	$ 76,241
Tier 1 Leverage Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Amount	$ 188,512	$ 168,937
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	20.60%	19.70%
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 41,257	$ 38,504
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Adequately Capitalized, Ratio	4.50%	4.50%
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 59,593	$ 55,618
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 Risk-based Capital, Actual Amount	$ 188,512	$ 168,937
Tier 1 Risk-based Capital, Actual Ratio	0.206	0.197
Tier 1 Risk-based Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 55,009	$ 51,339
Tier 1 Risk-based Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.060	0.060
Tier 1 Risk-based Capital, Regulatory Minimum To Be Well Capitalized, Amount	$ 73,345	$ 68,452
Tier 1 Risk-based Capital, Regulatory Minimum To Be Well Capitalized, Ratio	0.080	0.080
Total Capital, Actual Amount	$ 200,002	$ 179,671
Total Capital, Actual Ratio	0.218	0.210
Total Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 73,345	$ 68,452
Total Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.080	0.080
Total Capital, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 91,682	$ 85,566
Total Capital, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Timberland Bancorp
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Actual Amount	$ 191,973	$ 172,000
Tier 1 Leverage Capital, Actual Ratio	0.110	0.113
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Amount	$ 191,973	$ 172,000
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	20.90%	20.10%
Tier 1 Risk-based Capital, Actual Amount	$ 191,973	$ 172,000
Tier 1 Risk-based Capital, Actual Ratio	0.209	0.201
Total Capital, Actual Amount	$ 203,475	$ 182,805
Total Capital, Actual Ratio	0.222	0.213