Regulatory Capital Requirements Regulatory Capital Requirements (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Banking Regulation, Tier One Leverage Capital [Abstract]
Liquidation account balance	$ 115.4
Required Capital Conservation Buffer		2.50%

The amount equal to the former mutual holding company's ownership interest in the retained earnings of the former company, as determined as of a specific date, for the benefit of certain depositors of the Bank.

No definition available.

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