Parent Company Financial Information (Parent Company Only) (Schedule Of Balance Sheets) (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
ASSETS:
Cash and cash equivalents	$ 185,148	$ 220,370
Other assets	342,532	302,796
Income taxes receivable, net	0	2
TOTAL ASSETS	9,487,218	9,340,018
LIABILITIES:
Accounts payable and accrued expenses	146,942	101,868
Deferred income tax liabilities, net	8,180	14,282
Total liabilities	8,202,359	8,003,692
STOCKHOLDERS' EQUITY:
Preferred stock, $.01 par value; 100,000,000 shares authorized, no shares issued or outstanding	0	0
Common stock, $.01 par value; 1,400,000,000 shares authorized, 138,956,296 and 141,440,030 shares issued and outstanding as of September 30, 2020 and 2019, respectively	1,389	1,414
Additional paid-in capital	1,189,853	1,210,226
Unearned compensation - ESOP	(33,040)	(34,692)
Retained earnings	143,162	174,277
AOCI, net of tax	(16,505)	(14,899)
Total stockholders' equity	1,284,859	1,336,326	$ 1,391,622	$ 1,368,313
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 9,487,218	$ 9,340,018
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	1,400,000,000	1,400,000,000
Common stock, shares issued	138,956,296	141,440,030
Common stock, shares outstanding	138,956,296	141,440,030
Capitol Federal Financial Inc [Member]
ASSETS:
Cash and cash equivalents	$ 82,466	$ 126,320	$ 137,684	$ 120,785
Investment in the Bank	1,165,813	1,168,986
Note receivable - ESOP	38,614	39,971
Other assets	707	711
Income taxes receivable, net	492	429
TOTAL ASSETS	1,288,092	1,336,417
LIABILITIES:
Accounts payable and accrued expenses	3,142	91
Deferred income tax liabilities, net	91	0
Total liabilities	3,233	91
STOCKHOLDERS' EQUITY:
Preferred stock, $.01 par value; 100,000,000 shares authorized, no shares issued or outstanding	0	0
Common stock, $.01 par value; 1,400,000,000 shares authorized, 138,956,296 and 141,440,030 shares issued and outstanding as of September 30, 2020 and 2019, respectively	1,389	1,414
Additional paid-in capital	1,189,853	1,210,226
Unearned compensation - ESOP	(33,040)	(34,692)
Retained earnings	143,162	174,277
AOCI, net of tax	(16,505)	(14,899)
Total stockholders' equity	1,284,859	1,336,326
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 1,288,092	$ 1,336,417
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	1,400,000,000	1,400,000,000
Common stock, shares issued	138,956,296	141,440,030
Common stock, shares outstanding	138,956,296	141,440,030