Document And Entity Information - shares	3 Months Ended
	Dec. 31, 2015	Feb. 03, 2016
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-Q
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		137,149,688