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, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 22,409	$ 22,897	$ 24,554	$ 24,383	$ 21,389	$ 22,372	$ 23,330	$ 31,836	$ 94,243	$ 98,927	$ 84,137
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of equipment									9,143	8,458	7,796
Provision for deferred income taxes									(361)	(4,540)	922
Changes in:
Other assets									6,220	1,712	51
Accounts payable and accrued expenses									(19,746)	(639)	(10,743)
Net cash provided by operating activities									80,947	124,163	85,445
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash acquired from acquisition									0	15,685	0
Purchase of equipment									(11,732)	(11,761)	(9,128)
Proceeds from the redemption of common equity securities related to the redemption of junior subordinated debentures									302	0	0
Net cash provided by (used in) investing activities									233,259	(21,567)	146,284
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(134,929)	(118,312)	(117,963)
Stock options exercised									1,485	261	8,843
Net cash used in financing activities									(199,561)	(315,200)	(161,834)
CASH AND CASH EQUIVALENTS:
Beginning of year				139,055					139,055
End of year	220,370				139,055				220,370	139,055
SUPPLEMENTAL DISCLOSURE OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued for acquisition									0	39,113	0
Capitol Federal Financial Inc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									94,243	98,927	84,137
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in excess of distribution over earnings of subsidiary									35,381	34,940	36,085
Depreciation of equipment									37	30	29
Loss on disposal of premises and equipment									8	0	0
Provision for deferred income taxes									0	(35)	(2)
Changes in:
Other assets									54	(53)	(5)
Income taxes receivable/payable									57	(145)	(40)
Accounts payable and accrued expenses									(86)	(257)	(22)
Net cash provided by operating activities									129,694	133,407	120,182
CASH FLOWS FROM INVESTING ACTIVITIES:
Principal collected on note receivable from ESOP									1,314	1,272	1,233
Cash acquired from acquisition									0	18	0
Purchase of equipment									(423)	0	0
Proceeds from the redemption of common equity securities related to the redemption of junior subordinated debentures									302	0	0
Net cash provided by (used in) investing activities									1,193	1,290	1,233
CASH FLOWS FROM FINANCING ACTIVITIES:
Net payment from subsidiary related to restricted stock awards									1,245	253	293
Cash dividends paid									(134,929)	(118,312)	(117,963)
Repayment of other borrowings									(10,052)	0	0
Stock options exercised									1,485	261	8,843
Net cash used in financing activities									(142,251)	(117,798)	(108,827)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS									(11,364)	16,899	12,588
CASH AND CASH EQUIVALENTS:
Beginning of year				$ 137,684				$ 120,785	137,684	120,785	108,197
End of year	$ 126,320				$ 137,684				126,320	137,684	120,785
SUPPLEMENTAL DISCLOSURE OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued for acquisition									0	39,113	0
Capital contribution to subsidiary in conjunction with acquisition of CCB									$ 0	$ 48,798	$ 0