Securities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Securities [Abstract]
Securities callable within one year, amortized cost	$ 598.9

Amortized cost of securities that have call or pre-refund dates within one year of the balance sheet date.

No definition available.

Name:	cffn_SecuritiesCallableWithinOneYearAmortizedCost
Namespace Prefix:	cffn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant