Regulatory Capital Requirements Regulatory Capital Requirements (Narrative) (Details) $ in Millions	Sep. 30, 2024 USD ($)
Banking Regulation, Tier 1 Leverage Capital [Abstract]
Liquidation account balance	$ 70.9

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