Summary Of Significant Accounting Policies (Policies)	9 Months Ended
	Jun. 30, 2023
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, 2022, filed with the Securities and Exchange Commission ("SEC"). Interim results are not necessarily indicative of results for a full year.
Cash, Cash Equivalents and Restricted Cash	Cash, cash equivalents and restricted cash reported in the statement of cash flows consisted entirely of cash and cash equivalents of $329.4 million and $49.2 million at June 30, 2023 and September 30, 2022, respectively. At times, the Company holds restricted cash, which is reported in other assets on the consolidated balance sheet, related to collateral postings to/from the Bank's derivative counterparties associated with the Bank's interest rate swaps.  There was no restricted cash at June 30, 2023 or September 30, 2022. See additional discussion regarding the interest rate swaps in Note 5. Borrowed Funds.
Net Presentation of Cash Flows Related to Borrowings	At times, the Bank enters into FHLB advances with contractual maturities of 90 days or less. Cash flows related to these advances are reported on a net basis in the consolidated statements of cash flows.
Recent Accounting Pronouncements	In March 2022, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2022-02, Financial Instruments - Credit Losses (Topic 326): Troubled Debt Restructurings ("TDRs") and Vintage Disclosures. This ASU eliminates the accounting guidance for TDRs by creditors, while enhancing disclosure requirements for certain loan refinancings and restructurings by creditors when a borrower is experiencing financial difficulty. Additionally, this ASU requires that an entity disclose current-period gross write-offs by year of origination for financing receivables within the scope of Accounting Standards Codification ("ASC") 326-20, Financial Instruments-Credit Losses-Measured at Amortized Cost. This ASU is effective for the Company on October 1, 2023. While the adoption of this ASU is expected to result in enhanced disclosures, the Company does not expect the adoption of this ASU to have a material impact on the Company's consolidated financial condition or results of operations.