Document And Entity Information	3 Months Ended
	Dec. 31, 2012	Jan. 28, 2013
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2012
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		152,124,857
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Trading Symbol	cffn