Summary Of Significant Accounting Policies (Policies)	6 Months Ended
Mar. 31, 2026
Accounting Policies [Abstract]
Basis of Presentation	The consolidated financial statements include the accounts of Capitol Federal Financial, Inc.® (the "Company") and its wholly-owned subsidiary, Capitol Federal Savings Bank (the "Bank"). The Bank has two wholly-owned subsidiaries, Capitol Funds, Inc. and Capital City Investments, Inc. Capitol Funds, Inc. has a wholly-owned subsidiary, Capitol Federal Mortgage Reinsurance Company. Capital City Investments, Inc. is a real estate and investment holding company. All intercompany accounts and transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company's Annual Report on Form 10-K for the fiscal year ended September 30, 2025, filed with the Securities and Exchange Commission ("SEC"). Interim results are not necessarily indicative of results for a full year.
Recent Accounting Pronouncements	In October 2023, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2023-06, Disclosure Improvements - Codification Amendments in Response to the SEC's Disclosure Update and Simplification Initiative. This ASU incorporates a variety of Topics into the FASB Accounting Standards Codification (the "Codification") that are currently included in SEC Regulations S-X and S-K. The ASU is intended to align the accounting standards of GAAP with SEC Regulations S-X and S-K. Each amendment in the ASU will only become effective for the Company if the SEC removes the related disclosure or presentation requirement from its existing regulations by June 30, 2027. The amendments will be applied prospectively by the Company. The adoption of this ASU may result in disclosures currently presented outside of the Company's financial statements being relocated to the Company's financial statements. If the SEC has not removed the applicable requirements from Regulation S-X or S-K by June 30, 2027, the pending content of the related amendment will be removed from the Codification and will not become effective for the Company. The ASU is not expected to have a material impact on the Company's disclosures as the Company is currently subject to SEC Regulations S-X and S-K. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740) - Improvements to Income Tax Disclosures. This ASU requires public business entities to provide additional annual disclosures regarding specific categories of the income tax rate reconciliation using both percentages and currency amounts with certain reconciling items being further broken out by nature and jurisdiction to the extent those items exceed a certain quantitative threshold. The ASU also requires annual disclosures of income taxes paid (net of refunds received) disaggregated by federal, state, and foreign taxes and the amount of income taxes paid (net of refunds received) disaggregated by individual jurisdictions that meet a certain quantitative threshold. This ASU also discontinues certain other income tax disclosures. The ASU is effective for public business entities for annual periods beginning after December 15, 2024 which is the fiscal year ending September 30, 2026 for the Company. Early adoption is permitted for annual financial statements that have not yet been issued or made available for issuance. This ASU should be applied on a prospective basis; however, retrospective application is permitted. The Company's financial condition, results of operations and cash flows will not be impacted by this guidance; however, the guidance will impact the Company's income tax footnote disclosures. The Company is currently evaluating the effect this ASU will have on the Company's income tax footnote disclosures. In March 2024, the FASB issued ASU 2024-02, Codification Improvements - Amendments to Remove References to the Concepts Statements. This ASU removes references to various FASB Concept Statements to simplify the Codification and provide a distinction between authoritative and nonauthoritative literature. This ASU is effective for the Company on October 1, 2025, starting with its Form 10-K for the fiscal year ending September 30, 2026. This ASU is not expected to have a material impact on the Company's consolidated financial statements and disclosures. In November 2024, the FASB issued ASU 2024-03, Income Statement - Reporting Comprehensive Income - Expense Disaggregation Disclosures. This ASU requires additional expense disclosures by public entities in the notes to the financial statements. The ASU outlines the specific costs that are required to be disclosed, which include costs such as: purchases of inventory, employee compensation, depreciation, intangible asset amortization, selling costs, and depreciation, depletion, and amortization related to oil and gas production. It also requires qualitative descriptions of the amounts remaining in the relevant expense income statement captions that are not separately disaggregated quantitatively in the notes to the financial statements and the entity's definition of selling expenses. The disclosures are required for each interim and annual reporting period. The ASU is effective for fiscal years beginning after December 15, 2026, which is the fiscal year ending September 30, 2028 for the Company. In January 2025, the FASB issued ASU 2025-1, Income Statement - Reporting Comprehensive Income - Expense Disaggregation Disclosures: Clarifying the Effective Date. The FASB clarified the interim reporting date when an entity adopts ASU 2024-03. Per ASU 2025-01, ASU 2024-03 is effective for interim periods within fiscal years beginning after December 15, 2027, which is the quarter ending December 31, 2028 for the Company. The Company is currently evaluating the effect this ASU will have on the Company's expense disclosures in the notes to the consolidated financial statements. In September 2025, the FASB issued ASU 2025-06, Intangibles - Goodwill and Other - Internal-use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software. This ASU updates Subtopic 350-40 by modernizing the accounting for internal-use software costs, including clarifying when entities should begin capitalizing eligible costs related to developing or obtaining software for internal use, implementing a cloud computing arrangement as a customer, and developing websites. The ASU is effective for fiscal years beginning after December 15, 2027, which is the fiscal year ending September 30, 2029 for the Company, and interim periods within fiscal years beginning after December 15, 2027, which is the quarter ending December 31, 2028 for the Company. Early adoption is permitted for any interim or annual period for which financial statements have not yet been issued or made available for issuance as of the beginning of the entity's fiscal year. The Company is currently evaluating the effect this ASU will have on the Company's consolidated financial statements and disclosures, but it is not expected to have a material impact. In December 2025, the FASB issued ASU 2025-11, Interim Reporting (Topic 270): Narrow-Scope Improvements. This ASU does not change the overall purpose of interim reporting or alter the scope of existing disclosure requirements; rather, the ASU is intended to provide more clarity and make interim disclosure requirements under Topic 270 easier to navigate. The ASU also requires entities to disclose events occurring after the end of the most recent annual reporting period that have a material impact on the entity. The ASU is effective for interim reporting periods within annual reporting periods beginning after December 15, 2027, which is the quarter ended December 31, 2028 for the Company. The Company is currently evaluating the impact this ASU will have on the Company's interim consolidated financial statements and disclosures. In December 2025, the FASB issued ASU 2025-12, Codification Improvements. This ASU is intended to enhance the Codification by refining guidance and simplifying its application through technical corrections, helping to enhance consistency and make the standards easier for preparers and users to interpret. The ASU is effective for fiscal years beginning after December 15, 2026, which is the fiscal year ending September 30, 2028 for the Company, and the interim periods within fiscal years beginning after December 15, 2026, which is the quarter ended December 31, 2027 for the Company. The Company is currently evaluating the impact this ASU will have on the Company's consolidated financial statements and disclosures.

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740) - Improvements to Income Tax Disclosures. This ASU requires public business entities to provide additional annual disclosures regarding specific categories of the income tax rate reconciliation using both percentages and currency amounts with certain reconciling items being further broken out by nature and jurisdiction to the extent those items exceed a certain quantitative threshold. The ASU also requires annual disclosures of income taxes paid (net of refunds received) disaggregated by federal, state, and foreign taxes and the amount of income taxes paid (net of refunds received) disaggregated by individual jurisdictions that meet a certain quantitative threshold. This ASU also discontinues certain other income tax disclosures. The ASU is effective for public business entities for annual periods beginning after December 15, 2024 which is the fiscal year ending September 30, 2026 for the Company. Early adoption is permitted for annual financial statements that have not yet been issued or made available for issuance. This ASU should be applied on a prospective basis; however, retrospective application is permitted. The Company's financial condition, results of operations and cash flows will not be impacted by this guidance; however, the guidance will impact the Company's income tax footnote disclosures. The Company is currently evaluating the effect this ASU will have on the Company's income tax footnote disclosures.

In March 2024, the FASB issued ASU 2024-02, Codification Improvements - Amendments to Remove References to the Concepts Statements. This ASU removes references to various FASB Concept Statements to simplify the Codification and provide a distinction between authoritative and nonauthoritative literature. This ASU is effective for the Company on October 1, 2025, starting with its Form 10-K for the fiscal year ending September 30, 2026. This ASU is not expected to have a material impact on the Company's consolidated financial statements and disclosures.

In November 2024, the FASB issued ASU 2024-03, Income Statement - Reporting Comprehensive Income - Expense Disaggregation Disclosures. This ASU requires additional expense disclosures by public entities in the notes to the financial statements. The ASU outlines the specific costs that are required to be disclosed, which include costs such as: purchases of inventory, employee compensation, depreciation, intangible asset amortization, selling costs, and depreciation, depletion, and amortization related to oil and gas production. It also requires qualitative descriptions of the amounts remaining in the relevant expense income statement captions that are not separately disaggregated quantitatively in the notes to the financial statements and the entity's definition of selling expenses. The disclosures are required for each interim and annual reporting period. The ASU is effective for fiscal years beginning after December 15, 2026, which is the fiscal year ending September 30, 2028 for the Company. In January 2025, the FASB issued ASU 2025-1, Income Statement - Reporting Comprehensive Income - Expense Disaggregation Disclosures: Clarifying the Effective Date. The FASB clarified the interim reporting date when an entity adopts ASU 2024-03. Per ASU 2025-01, ASU 2024-03 is effective for interim periods within fiscal years beginning after December 15, 2027, which is the quarter ending December 31, 2028

In September 2025, the FASB issued ASU 2025-06, Intangibles - Goodwill and Other - Internal-use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software. This ASU updates Subtopic 350-40 by modernizing the accounting for internal-use software costs, including clarifying when entities should begin capitalizing eligible costs related to developing or obtaining software for internal use, implementing a cloud computing arrangement as a customer, and developing websites. The ASU is effective for fiscal years beginning after December 15, 2027, which is the fiscal year ending September 30, 2029 for the Company, and interim periods within fiscal years beginning after December 15, 2027, which is the quarter ending December 31, 2028 for the Company. Early adoption is permitted for any interim or annual period for which financial statements have not yet been issued or made available for issuance as of the beginning of the entity's fiscal year. The Company is currently evaluating the effect this ASU will have on the Company's consolidated financial statements and disclosures, but it is not expected to have a material impact.

In December 2025, the FASB issued ASU 2025-11, Interim Reporting (Topic 270): Narrow-Scope Improvements. This ASU does not change the overall purpose of interim reporting or alter the scope of existing disclosure requirements; rather, the ASU is intended to provide more clarity and make interim disclosure requirements under Topic 270 easier to navigate. The ASU also requires entities to disclose events occurring after the end of the most recent annual reporting period that have a material impact on the entity. The ASU is effective for interim reporting periods within annual reporting periods beginning after December 15, 2027, which is the quarter ended December 31, 2028 for the Company. The Company is currently evaluating the impact this ASU will have on the Company's interim consolidated financial statements and disclosures.

In December 2025, the FASB issued ASU 2025-12, Codification Improvements. This ASU is intended to enhance the Codification by refining guidance and simplifying its application through technical corrections, helping to enhance consistency and make the standards easier for preparers and users to interpret. The ASU is effective for fiscal years beginning after December 15, 2026, which is the fiscal year ending September 30, 2028 for the Company, and the interim periods within fiscal years beginning after December 15, 2026, which is the quarter ended December 31, 2027 for the Company. The Company is currently evaluating the impact this ASU will have on the Company's consolidated financial statements and disclosures.