Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Nov. 17, 2015	Mar. 31, 2015
Document And Entity Information [Abstract]
Entity Registrant Name	Capitol Federal Financial Inc
Trading Symbol	cffn
Entity Central Index Key	0001490906
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		137,130,588
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,730