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, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 20,602	$ 21,370	$ 21,587	$ 20,578	$ 20,698	$ 20,551	$ 21,527	$ 20,718	$ 84,137	$ 83,494	$ 78,093
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of equipment									7,796	7,141	6,844
Provision for deferred income taxes									922	470	3,201
Changes in:
Other assets									(680)	1,807	3,878
Accounts payable and accrued expenses									(10,743)	(6,840)	(6,215)
CASH FLOWS FROM FINANCING ACTIVITIES:
Dividends paid									(117,963)	(111,767)	(114,162)
Repurchase of common stock									0	0	(50,034)
Stock options exercised									8,843	4,070	267
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									69,895	(490,868)	(38,208)
CASH AND CASH EQUIVALENTS:
Beginning of year				281,764				772,632	281,764	772,632	810,840
End of year	351,659				281,764				351,659	281,764	772,632
Capitol Federal Financial Inc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									84,137	83,494	78,093
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in excess of distribution over earnings of subsidiary									36,085	34,070	37,422
Depreciation of equipment									29	30	30
Provision for deferred income taxes									(2)	2	428
Changes in:
Other assets									(5)	1	35
Income taxes receivable/payable									(40)	445	3,300
Accounts payable and accrued expenses									(22)	14	1
Net cash flows provided by operating activities									120,182	118,056	119,309
CASH FLOWS FROM INVESTING ACTIVITIES:
Principal collected on notes receivable from ESOP									1,233	1,194	1,156
Net cash flows provided by investing activities									1,233	1,194	1,156
CASH FLOWS FROM FINANCING ACTIVITIES:
Net payment from subsidiary related to restricted stock awards									293	473	95
Dividends paid									(117,963)	(111,767)	(114,162)
Repurchase of common stock									0	0	(50,034)
Stock options exercised									8,843	4,070	267
Net cash flows used in financing activities									(108,827)	(107,224)	(163,834)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									12,588	12,026	(43,369)
CASH AND CASH EQUIVALENTS:
Beginning of year				$ 108,197				$ 96,171	108,197	96,171	139,540
End of year	$ 120,785				$ 108,197				$ 120,785	$ 108,197	$ 96,171