Parent Company Financial Information (Parent Company Only) (Schedule Of Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 38,010	$ (101,659)	$ 84,453
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of equipment	7,929	9,039	9,365
Provision for deferred income taxes	3,546	(45,520)	2,047
Changes in:
Other assets	8,227	2,528	6,661
Other liabilities	(22,831)	(4,372)	(10,823)
Net cash provided by operating activities	31,184	47,531	97,301
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash provided by investing activities	629,109	(329,103)	(167,576)
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid	(44,522)	(83,172)	(103,131)
Repurchase of common stock	(19,448)	(23,453)	0
Net cash used in financing activities	(688,591)	477,983	49,177
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(28,298)	196,411	(21,098)
CASH AND CASH EQUIVALENTS:
Beginning of year	245,605	49,194	70,292
End of year	217,307	245,605	49,194
Capitol Federal Financial Inc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	38,010	(101,659)	84,453
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in excess of distribution over earnings of subsidiary	(11,891)	188,166	27,109
Depreciation of equipment	23	45	46
Provision for deferred income taxes	(2)	(12)	(10)
Changes in:
Receivable from/payable to the Bank	2,489	(2,519)	18,239
Income taxes receivable/payable	(92)	123	13
Other assets	(11)	(2)	(10)
Other liabilities	193	11	0
Net cash provided by operating activities	28,719	84,153	129,840
CASH FLOWS FROM INVESTING ACTIVITIES:
Principal collected on note receivable from ESOP	1,542	1,494	1,446
Net cash provided by investing activities	1,542	1,494	1,446
CASH FLOWS FROM FINANCING ACTIVITIES:
Net payment from subsidiary related to restricted stock awards	419	401	269
Cash dividends paid	(44,522)	(83,172)	(103,131)
Repurchase of common stock	(19,448)	(23,453)	0
Net cash used in financing activities	(63,551)	(106,224)	(102,862)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(33,290)	(20,577)	28,424
CASH AND CASH EQUIVALENTS:
Beginning of year	83,400	103,977	75,553
End of year	$ 50,110	$ 83,400	$ 103,977