| Document And Entity Information | Jan. 26, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Period End Date | Jan. 26, 2021 |
| 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 Flag | false |
