Regulatory Matters: Schedule of Compliance with Regulatory Capital Requirements for Mortgage Companies (Details) $ in Thousands	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Timberland Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Actual Amount	$ 218,749	$ 202,438
Tier 1 Leverage Capital, Actual Ratio	0.120	0.109
Tier 1 Leverage Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 72,983	$ 74,039
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	$ 91,229	$ 92,549
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	$ 218,749	$ 202,438
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	18.00%	18.00%
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 54,549	$ 50,551
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	$ 78,792	$ 73,018
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	$ 218,749	$ 202,438
Tier 1 Risk-based Capital, Actual Ratio	0.180	0.180
Tier 1 Risk-based Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 72,731	$ 67,402
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	$ 96,975	$ 89,869
Tier 1 Risk-based Capital, Regulatory Minimum To Be Well Capitalized, Ratio	0.080	0.080
Total Capital, Actual Amount	$ 233,914	$ 216,446
Total Capital, Actual Ratio	0.193	0.193
Total Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 96,975	$ 89,869
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	$ 121,219	$ 112,336
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	$ 219,851	$ 204,659
Tier 1 Leverage Capital, Actual Ratio	0.121	0.110
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Amount	$ 219,851	$ 204,659
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	18.10%	18.20%
Tier 1 Risk-based Capital, Actual Amount	$ 219,851	$ 204,659
Tier 1 Risk-based Capital, Actual Ratio	0.181	0.182
Total Capital, Actual Amount	$ 235,023	$ 218,667
Total Capital, Actual Ratio	0.194	0.195