Document And Entity Information - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2024	Nov. 21, 2024	Mar. 31, 2024
Cover [Abstract]
Document Type	10-K
Annual Report indicator	true
Period End Date	Sep. 30, 2024
Transition Report indicator	false
Commission file number	001-34814
Registrant Name	Capitol Federal Financial, Inc.
State of incorporation	MD
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
Title of security class	Common Stock, par value $0.01 per share
Trading symbol	CFFN
Name of exchange on which securities are registered	NASDAQ
Well-known Seasoned Issuer indicator	Yes
Voluntary Filer indicator	No
Entity current reporting status indicator	Yes
Interactive data current reporting status indicator	Yes
Filer category	Large Accelerated Filer
Smaller Reporting Company indicator	false
Emerging Growth Company indicator	false
ICFR Auditor Attestation indicator	true
Financial statement error correction indicator	false
Shell company indicator	false
Entity Public Float			$ 772.2
Entity Common Stock, Shares Outstanding		132,774,365
Documents Incorporated by Reference [Text Block]	Part III of Form 10-K - Portions of the proxy statement for the Annual Meeting of Stockholders for the year ended September 30, 2024.
Entity Central Index Key	0001490906
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY
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