Parent Company Financial Information (Parent Company Only) (Schedule Of Statements Of Income) (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
INTEREST AND DIVIDEND INCOME:
Total interest and dividend income	$ 71,606	$ 74,381	$ 78,955	$ 80,036	$ 83,285	$ 82,211	$ 82,037	$ 82,421	$ 304,978	$ 329,954	$ 321,892
INTEREST EXPENSE									115,643	123,564	123,119
NET INTEREST INCOME	45,683	46,287	48,668	48,697	49,811	51,681	52,597	52,301	189,335	206,390	198,773
NON-INTEREST INCOME									19,599	21,958	22,035
NON-INTEREST EXPENSE:
Salaries and employee benefits									52,996	53,145	46,563
Regulatory and outside services									5,762	5,813	5,709
Other non-interest expense									5,514	6,006	3,356
Total non-interest expense									106,004	106,944	96,902
INCOME BEFORE INCOME TAX EXPENSE AND EQUITY IN EXCESS OF DISTRIBUTION OVER EARNINGS OF SUBSIDIARY									80,630	120,654	123,906
INCOME TAX EXPENSE (BENEFIT)									16,090	26,411	24,979
NET INCOME	$ 18,279	$ 19,474	$ 4,276	$ 22,511	$ 22,409	$ 22,897	$ 24,554	$ 24,383	64,540	94,243	98,927
Capitol Federal Financial Inc [Member]
INTEREST AND DIVIDEND INCOME:
Dividend income from the Bank									68,329	129,409	134,540
Interest income from other investments									2,036	2,428	1,951
Total interest and dividend income									70,365	131,837	136,491
INTEREST EXPENSE									0	403	62
NET INTEREST INCOME									70,365	131,434	136,429
NON-INTEREST INCOME									0	14	0
NON-INTEREST EXPENSE:
Salaries and employee benefits									988	829	1,031
Regulatory and outside services									292	286	1,129
Other non-interest expense									622	652	581
Total non-interest expense									1,902	1,767	2,741
INCOME BEFORE INCOME TAX EXPENSE AND EQUITY IN EXCESS OF DISTRIBUTION OVER EARNINGS OF SUBSIDIARY									68,463	129,681	133,688
INCOME TAX EXPENSE (BENEFIT)									28	57	(179)
INCOME BEFORE EQUITY IN EXCESS OF DISTRIBUTION OVER EARNINGS OF SUBSIDIARY									68,435	129,624	133,867
EQUITY IN EXCESS OF DISTRIBUTION OVER EARNINGS OF SUBSIDIARY									(3,895)	(35,381)	(34,940)
NET INCOME									$ 64,540	$ 94,243	$ 98,927