Regulatory Capital Requirements (Summary Of Capital And Total Risk-Based Capital Ratios) (Details) $ in Thousands	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Capitol Federal Financial Inc [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CBLR, Actual Amount	$ 1,287,854
CBLR, Actual Ratio	0.137
CBLR, For Capital Adequacy Purposes, Amount	$ 754,767
CBLR, For Capital Adequacy Purposes, Ratio	0.080
Tier 1 leverage, Actual Amount		$ 1,336,377
Tier 1 leverage, Actual Ratio		0.138
Tier 1 leverage, For Capital Adequacy Purposes, Amount		$ 387,346
Tier 1 leverage, For Capital Adequacy Purposes, Ratio		0.040
CET1 capital, Actual Amount		$ 1,336,377
CET1 capital, Actual Ratio		0.276
CET1 capital, For Capital Adequacy Purposes, Amount		$ 218,070
CET1 capital, For Capital Adequacy Purposes, Ratio		4.50%
Tier 1 capital, Actual Amount		$ 1,336,377
Tier 1 capital, Actual Ratio		0.276
Tier 1 capital, For Capital Adequacy Purposes, Amount		$ 290,759
Tier 1 capital, For Capital Adequacy Purposes, Ratio		0.060
Total capital, Actual Amount		$ 1,345,603
Total capital, Actual Ratio		0.278
Total capital, For Capital Adequacy Purposes, Amount		$ 387,679
Total capital, For Capital Adequacy Purposes, Ratio		0.080
Capitol Federal Savings Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CBLR, Actual Amount	$ 1,168,808
CBLR, Actual Ratio	0.124
CBLR, For Capital Adequacy Purposes, Amount	$ 754,884
CBLR, For Capital Adequacy Purposes, Ratio	0.080
Tier 1 leverage, Actual Amount		$ 1,169,037
Tier 1 leverage, Actual Ratio		0.121
Tier 1 leverage, For Capital Adequacy Purposes, Amount		$ 387,427
Tier 1 leverage, For Capital Adequacy Purposes, Ratio		0.040
Tier 1 leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 484,284
Tier 1 leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		0.050
CET1 capital, Actual Amount		$ 1,169,037
CET1 capital, Actual Ratio		0.241
CET1 capital, For Capital Adequacy Purposes, Amount		$ 218,042
CET1 capital, For Capital Adequacy Purposes, Ratio		4.50%
CET1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 314,949
CET1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		6.50%
Tier 1 capital, Actual Amount		$ 1,169,037
Tier 1 capital, Actual Ratio		0.241
Tier 1 capital, For Capital Adequacy Purposes, Amount		$ 290,722
Tier 1 capital, For Capital Adequacy Purposes, Ratio		0.060
Tier 1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 387,630
Tier 1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		0.080
Total capital, Actual Amount		$ 1,178,263
Total capital, Actual Ratio		0.243
Total capital, For Capital Adequacy Purposes, Amount		$ 387,630
Total capital, For Capital Adequacy Purposes, Ratio		0.080
Total capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 484,537
Total capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		0.100