Summary Of Significant Accounting Policies (Policies)	3 Months Ended
Dec. 31, 2021
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, 2021, filed with the Securities and Exchange Commission ("SEC"). Interim results are not necessarily indicative of results for a full year.
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	Cash, cash equivalents, restricted cash and restricted cash equivalents reported in the statement of cash flows include cash and cash equivalents of $135.5 million and $42.3 million at December 31, 2021 and September 30, 2021, respectively, and restricted cash and cash equivalents of $23.1 million and $28.0 million at December 31, 2021 and September 30, 2021, respectively, which was included in other assets on the consolidated balance sheet. The restricted cash and cash equivalents relate to the collateral postings to/from the Bank's derivative counterparties associated with the Bank's interest rate swaps. See additional discussion regarding the interest rate swaps in Note 5. Borrowed Funds.

Summary Of Significant Accounting Policies (Policies)

3 Months Ended

Dec. 31, 2021

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, 2021, filed with the Securities and Exchange Commission ("SEC"). Interim results are not necessarily indicative of results for a full year.

Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents

Cash, cash equivalents, restricted cash and restricted cash equivalents reported in the statement of cash flows include cash and cash equivalents of $135.5 million and $42.3 million at December 31, 2021 and September 30, 2021, respectively, and restricted cash and cash equivalents of $23.1 million and $28.0 million at December 31, 2021 and September 30, 2021, respectively, which was included in other assets on the consolidated balance sheet. The restricted cash and cash equivalents relate to the collateral postings to/from the Bank's derivative counterparties associated with the Bank's interest rate swaps. See additional discussion regarding the interest rate swaps in Note 5. Borrowed Funds.