Document And Entity Information	6 Months Ended
	Mar. 31, 2014	Apr. 23, 2014
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2014
Entity Registrant Name	Capitol Federal Financial Inc
Entity Central Index Key	0001490906
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		143,120,893
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q2
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Trading Symbol	cffn