| Document And Entity Information | Oct. 25, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Period End Date | Oct. 25, 2023 |
| Registrant Name | CAPITOL FEDERAL FINANCIAL, INC. |
| State of incorporation | MD |
| Commission file number | 001-34814 |
| IRS identification number | 27-2631712 |
| Address of principal executive offices location | 700 South Kansas Avenue, |
| City of principal executive offices location | Topeka |
| State of principal executive offices location | KS |
| Zip code of principal executive offices location | 66603 |
| Telephone number - Area code | 785 |
| Telephone number | 235-1341 |
| Written communications indicator | false |
| Soliciting material indicator | false |
| Pre-commencement tender offer indicator | false |
| Pre-commencement issuer tender offer indicator | false |
| Title of security class | Common Stock, par value $0.01 per share |
| Trading symbol | CFFN |
| Name of exchange on which securities are registered | NASDAQ |
| Emerging Growth Company indicator | false |
| Entity Central Index Key | 0001490906 |
| Amendment Description | On October 25, 2023, Capitol Federal Financial, Inc. (the "Company") issued a press release announcing a strategic securities transaction and fiscal year 2023 financial results (the "Original Earnings Release"). A copy of the Original Earnings Release was furnished by the Company with a Current Report on Form 8-K on October 25, 2023 (the “Original 8-K”). On November 15, 2023, the Company issued a press release revising the Original Earnings Release (the “Revised Earnings Release”) to correct the accounting treatment for the net realized losses associated with the Company's sale of securities in October 2023. All the net losses were originally recorded in fiscal year 2023. Following continued review, as indicated in the Revised Earnings Release, the Company determined that a portion of the net losses should be recorded in the first quarter of fiscal year 2024 instead of fiscal year 2023. The sole purpose of this Amendment No. 1 on Form 8-K/A is to amend Items 2.02 and 9.01 of the Original 8-K and furnish the Revised Earnings Release in place of the Original Earnings Release. Except as set forth herein, no other information in the Original 8-K is being amended. |
| Amendment Flag | true |
