Regulatory Capital Requirements (Summary Of Capital And Total Risk-Based Capital Ratios) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Capitol Federal Financial Inc [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 leverage, Actual Amount	$ 1,336,377	$ 1,381,791
Tier 1 leverage, Actual Ratio	13.80%	14.90%
Tier 1 leverage, For Capital Adequacy Purposes, Amount	$ 387,346	$ 370,475
Tier 1 leverage, For Capital Adequacy Purposes, Ratio	4.00%	4.00%
CET1 capital, Actual Amount	$ 1,336,377	$ 1,381,791
CET1 capital, Actual Ratio	27.60%	28.60%
CET1 capital, For Capital Adequacy Purposes, Amount	$ 218,070	$ 215,793
CET1 capital, For Capital Adequacy Purposes, Ratio	4.50%	4.50%
Tier 1 capital, Actual Amount	$ 1,336,377	$ 1,381,791
Tier 1 capital, Actual Ratio	27.60%	28.80%
Tier 1 capital, For Capital Adequacy Purposes, Amount	$ 290,759	$ 287,724
Tier 1 capital, For Capital Adequacy Purposes, Ratio	6.00%	6.00%
Total capital, Actual Amount	$ 1,345,603	$ 1,390,255
Total capital, Actual Ratio	27.80%	29.00%
Total capital, For Capital Adequacy Purposes, Amount	$ 387,679	$ 383,632
Total capital, For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Capitol Federal Savings Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 leverage, Actual Amount	$ 1,169,037	$ 1,202,125
Tier 1 leverage, Actual Ratio	12.10%	13.00%
Tier 1 leverage, For Capital Adequacy Purposes, Amount	$ 387,427	$ 370,559
Tier 1 leverage, For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 484,284	$ 463,199
Tier 1 leverage, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%
CET1 capital, Actual Amount	$ 1,169,037	$ 1,202,125
CET1 capital, Actual Ratio	24.10%	25.10%
CET1 capital, For Capital Adequacy Purposes, Amount	$ 218,042	$ 215,764
CET1 capital, For Capital Adequacy Purposes, Ratio	4.50%	4.50%
CET1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 314,949	$ 311,659
CET1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 capital, Actual Amount	$ 1,169,037	$ 1,202,125
Tier 1 capital, Actual Ratio	24.10%	25.10%
Tier 1 capital, For Capital Adequacy Purposes, Amount	$ 290,722	$ 287,685
Tier 1 capital, For Capital Adequacy Purposes, Ratio	6.00%	6.00%
Tier 1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 387,630	$ 383,580
Tier 1 capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Total capital, Actual Amount	$ 1,178,263	$ 1,210,589
Total capital, Actual Ratio	24.30%	25.20%
Total capital, For Capital Adequacy Purposes, Amount	$ 387,630	$ 383,580
Total capital, For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Total capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 484,537	$ 479,475
Total capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%

The minimum Common Equity Tier 1 Capital Ratio required for capital adequacy purposes as defined by the regulators.

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The Common Equity Tier 1 Capital Ratio required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Amount of Common Equity Tier One Risk-based Capital, as defined by regulatory framework.

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Percentage of Common Equity Tier One Risk-based Capital to total risk-weighted assets, as defined by regulatory framework.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as adequately capitalized under regulatory framework for prompt corrective action.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as well-capitalized under regulatory framework for prompt corrective action.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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