Parent Company Financial Information (Parent Company Only) (Schedule Of Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 18,279	$ 19,474	$ 4,276	$ 22,511	$ 22,409	$ 22,897	$ 24,554	$ 24,383	$ 64,540	$ 94,243	$ 98,927
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of equipment									9,133	9,143	8,458
Provision for deferred income taxes									(5,588)	(361)	(4,540)
Changes in:
Other assets									9,105	6,220	1,712
Accounts payable and accrued expenses									(8,231)	(19,746)	(639)
Net cash provided by operating activities									92,928	80,947	124,163
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash acquired from acquisition									0	0	15,685
Purchase of equipment									(14,742)	(11,732)	(11,761)
Proceeds from the redemption of common equity securities related to the redemption of junior subordinated debentures									0	302	0
Net cash provided by investing activities									(151,290)	233,259	(21,567)
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(93,862)	(134,929)	(118,312)
Repurchase of common stock									(20,767)	0	0
Stock options exercised									638	1,485	261
Net cash used in financing activities									44,370	(199,561)	(315,200)
CASH AND CASH EQUIVALENTS:
Beginning of year				220,370					220,370
End of year	185,148				220,370				185,148	220,370
SUPPLEMENTAL DISCLOSURE OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued for acquisition									0	0	39,113
Capitol Federal Financial Inc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									64,540	94,243	98,927
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in excess of distribution over earnings of subsidiary									3,895	35,381	34,940
Depreciation of equipment									45	37	30
Loss on disposal of premises and equipment									0	8	0
Provision for deferred income taxes									91	0	(35)
Changes in:
Other assets									(60)	54	(53)
Income taxes receivable/payable									(63)	57	(145)
Accounts payable and accrued expenses									13	(86)	(257)
Net cash provided by operating activities									68,461	129,694	133,407
CASH FLOWS FROM INVESTING ACTIVITIES:
Principal collected on note receivable from ESOP									1,357	1,314	1,272
Cash acquired from acquisition									0	0	18
Purchase of equipment									0	(423)	0
Proceeds from the redemption of common equity securities related to the redemption of junior subordinated debentures									0	302	0
Net cash provided by investing activities									1,357	1,193	1,290
CASH FLOWS FROM FINANCING ACTIVITIES:
Net payment from subsidiary related to restricted stock awards									319	1,245	253
Cash dividends paid									(93,862)	(134,929)	(118,312)
Repurchase of common stock									(20,767)	0	0
Repayment of other borrowings									0	(10,052)	0
Stock options exercised									638	1,485	261
Net cash used in financing activities									(113,672)	(142,251)	(117,798)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS									(43,854)	(11,364)	16,899
CASH AND CASH EQUIVALENTS:
Beginning of year				$ 126,320				$ 137,684	126,320	137,684	120,785
End of year	$ 82,466				$ 126,320				82,466	126,320	137,684
SUPPLEMENTAL DISCLOSURE OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued for acquisition									0	0	39,113
Capital contribution to subsidiary in conjunction with acquisition of CCB									$ 0	$ 0	$ 48,798