Document And Entity Information - shares	9 Months Ended
	Jun. 30, 2021	Aug. 02, 2021
Cover [Abstract]
Document Type	10-Q
Quarterly Report indicator	true
Period End Date	Jun. 30, 2021
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
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
Shell company indicator	false
Entity Common Stock, Shares Outstanding		138,833,184
Entity Central Index Key	0001490906
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false