Summary Of Significant Accounting Policies (Policies)	6 Months Ended
Summary Of Significant Accounting Policies (Policies)	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: The accompanying unaudited consolidated financial statements for Timberland Bancorp, Inc. (“Company”) were prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and with instructions for Form 10-Q and, therefore, do not include all disclosures necessary for a complete presentation of consolidated financial condition, results of operations, and cash flows in conformity with GAAP. However, all adjustments which are, in the opinion of management, necessary for a fair presentation of the interim consolidated financial statements have been included. All such adjustments are of a normal recurring nature. The unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended September 30, 2017 (“2017 Form 10-K”). The unaudited consolidated results of operations for the six months ended March 31, 2018 are not necessarily indicative of the results that may be expected for the entire fiscal year ending September 30, 2018.
Principles of Consolidation	Principles of Consolidation: The unaudited consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, Timberland Bank (“Bank”), and the Bank’s wholly-owned subsidiary, Timberland Service Corporation. All significant intercompany transactions and balances have been eliminated in consolidation.
Operating Segment	Operating Segment: The Company has one reportable operating segment which is defined as community banking in western Washington under the operating name, “Timberland Bank.”
Use of Estimates	The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates.
Reclassification	Certain prior period amounts have been reclassified to conform to the March 31, 2018 presentation with no change to net income or total shareholders’ equity as previously reported.
Recent Accounting Pronouncements	In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, Revenue from Contracts with Customers (Topic 606). The core principle of this ASU is that an entity recognizes revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In general, the new guidance requires companies to use more judgment and make more estimates than under current guidance, including identifying performance obligations in the contract and estimating the amount of variable consideration to include in the transaction price related to each separate performance obligation. This ASU is effective for annual periods beginning after December 15, 2017, including interim periods within that reporting period. The Company's primary source of revenue is interest income, which is recognized when earned and is deemed to be in compliance with this ASU. Accordingly, the adoption of ASU No. 2014-09 is not expected to have a material impact on the Company's future consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. The main provisions of this ASU address the valuation and impairment of certain equity investments along with simplified disclosures about the fair value of financial instruments. The amendments in this ASU are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. Management is in the planning stages of developing processes and procedures to comply with the disclosure requirements of this ASU, which could impact the disclosures the Company makes related to the fair value of its financial instruments; however, the adoption of ASU No. 2016-01 is not expected to have a material impact on the Company's future consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). This ASU is intended to increase transparency and comparability among organizations by requiring the recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The principal change required by this ASU relates to lessee accounting, and is that for operating leases, a lessee is required to (1) recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, in the statement of financial position, (2) recognize a single lease cost, calculated so that the cost of the lease is allocated over the lease term on a generally straight-line basis, and (3) classify all cash payments within operating activities in the statement of cash flows. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize lease assets and lease liabilities. If a lessee makes this election, it should recognize lease expense for such leases generally on a straight-line basis over the lease term. This ASU also changes disclosure requirements related to leasing activities and requires certain qualitative disclosures along with specific quantitative disclosures. The amendments in this ASU are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. Early application of the amendments in this ASU is permitted. The effect of adoption will depend on leases at the time of adoption. Once adopted, the Company expects to report higher assets and liabilities as a result of including right-of-use assets and lease liabilities related to certain banking offices and certain equipment under non-cancelable operating lease agreements; however, based on current leases the adoption of ASU No. 2016-02 is not expected to have a material impact on the Company's future consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses. This ASU replaces the existing incurred losses methodology with a current expected losses methodology with respect to most financial assets measured at amortized cost and certain other instruments, including trade and other receivables, loans, held to maturity investment securities and off-balance sheet commitments. In addition, this ASU requires credit losses relating to available for sale debt securities to be recorded through an allowance for credit losses rather than as a reduction of the carrying amount. ASU No. 2016-13 also changes the accounting for purchased credit-impaired debt securities and loans. The standard retains many of the current disclosure requirements in GAAP and expands disclosure requirements. ASU No. 2016-13 is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Upon adoption, the Company expects a change in the processes and procedures to calculate the allowance for loan losses, including changes in the assumptions and estimates to consider expected credit losses over the life of the loan versus the current accounting practice that utilizes the incurred loss model. In addition, the current policy for other-than-temporary impairment on investment securities available for sale will be replaced with an allowance approach. The Company is reviewing the requirements of ASU No. 2016-13 and expects to begin developing and implementing processes and procedures to ensure it is fully compliant with the amendments at the adoption date. At this time, the Company anticipates the allowance for loan losses will increase as a result of the implementation of this ASU; however, until its evaluation is complete, the magnitude of the increase will be unknown. In January 2017, the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other: Simplifying the Test for Goodwill Impairment. This ASU simplifies the subsequent measurement of goodwill and eliminates Step 2 from the goodwill impairment test. In computing the implied fair value of goodwill under Step 2, an entity had to perform procedures to determine the fair value of its assets and liabilities (including unrecognized assets and liabilities) at the impairment testing date following the procedure that would be required in determining the fair value of assets acquired and liabilities assumed in a business combination. Under ASU No. 2017-04, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. Additionally, an entity should consider income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss, if applicable. ASU No. 2017-04 is effective for annual or interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early application of this ASU is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The adoption of ASU No. 2017-04 is not expected to have a material impact on the Company's future consolidated financial statements. In March 2017, the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. The ASU shortens the amortization period for certain callable debt securities held at a premium. This ASU is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. The adoption of ASU No. 2017-08 is not expected to have a material impact on the Company's future consolidated financial statements. In May 2017, the FASB issued ASU No. 2017-09, Compensation--Stock Compensation (Topic 718): Scope of Modification Accounting. This ASU was issued to provide clarity as to when to apply modification accounting when there is a change in the terms or conditions of a share-based payment award. According to the ASU, an entity should account for the effects of a modification unless the fair value, vesting conditions, and balance sheet classification of the award are the same after the modification as compared to the original award prior to modification. ASU No. 2017-09 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017, with early adoption permitted. The adoption of ASU No. 2017-09 is not expected to have a material impact on the Company's future consolidated financial statements. In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740). This ASU was issued to provide guidance on the income tax accounting implications of the Tax Cuts and Jobs Act (the "Tax Act"), and allows for entities to report provisional amounts for specific income tax effects of the Tax Act for which the accounting under Topic 740 was not yet complete but a reasonable estimate could be determined. A measurement period of one-year is allowed to complete the accounting effects under Topic 740 and revise any previous estimates reported. Any provisional amounts or subsequent adjustments included in an entity's financial statements during the measurement period should be included in income from continuing operations as an adjustment to tax expense in the period the amounts are determined. The Company adopted this ASU with the provisional adjustments as reported in the consolidated financial statements on Form 10-Q as of December 31, 2017. As of March 31, 2018, the Company did not incur any adjustments to the provisional recognition. (10) U.S. TAX REFORM On December 22, 2017, the U.S. government enacted the Tax Act. The Tax Act significantly revises the future ongoing U.S. corporate income tax by, among other things, decreasing U.S. corporate income tax rates to 21.0% from 35.0%. As the Company has a September 30 fiscal year-end, the lower corporate income tax rate will be phased in, resulting in a blended U.S. statutory federal rate of approximately 24.5% for the Company's fiscal year ending September 30, 2018, and 21.0% for subsequent fiscal years. In addition, the reduction of the corporate tax rate required the Company to revalue its deferred tax assets and liabilities based on the lower federal tax rate of 21.0%. As a result of the new legislation, during the quarter ended December 31, 2017, the Company recorded a one-time income tax expense of $548,000 in conjunction with writing down its net deferred tax assets. The impact of using the 24.5% blended federal tax rate for the six months ended March 31, 2018 versus a 35.0% rate reduced the provision for income taxes by approximately $1.05 million, which was partially offset by the $548,000 one-time net deferred tax asset write-down. The changes included in the Tax Act are broad and complex. The final transition impacts of the Tax Act may differ from the above estimates, possibly materially, due to, among other things, changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, and any changes in accounting standards for income taxes or related interpretations in response to the Tax Act.