Loans Receivable And Allowance For Credit Losses (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Jun. 30, 2023	Sep. 30, 2022
Loans Receivable [Line Items]
Loan-to-value ratio securing commercial real estate loans, maximum		85.00%
Debt service coverage ratio for commercial real estate loans, minimum		1.15
Loan-to-value ratio securing commercial construction loans, maximum		80.00%
Loans receivable		$ 7,963,360	$ 7,464,208
Amortized cost of loans in process of foreclosure		2,200	2,000
Carrying value of residential OREO		0	328
Off-balance sheet credit exposures		909,700	992,600
Forecast [Member]
Loans Receivable [Line Items]
Unemployment rate	4.00%
Doubtful [Member]
Loans Receivable [Line Items]
Loans receivable		$ 0	$ 0

For commercial real estate loans, the minimum acceptable debt service coverage ratio.

No definition available.

Name:	cffn_DebtServiceCoverageRatioForCommercialRealEstateLoansMinimum
Namespace Prefix:	cffn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Loan To Value Ratio For Securing Commercial Real Estate Loans Maximum

+ References

No definition available.

+ Details

Name:	cffn_LoanToValueRatioForSecuringCommercialRealEstateLoansMaximum
Namespace Prefix:	cffn_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

For commercial construction loans, the maximum acceptable loan to value ratio

+ References

No definition available.

+ Details

Name:	cffn_LoantoValueRatioforSecuringCommercialConstructionLoansMaximum
Namespace Prefix:	cffn_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, undisbursed portions of loans, or financial guarantee not accounted for as insurance.

+ References

No definition available.

+ Details

Name:	cffn_OffBalanceSheetCreditExposures
Namespace Prefix:	cffn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

US National Unemployment Rate

+ References

No definition available.

+ Details

Name:	cffn_UnemploymentRate
Namespace Prefix:	cffn_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 35
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-35

+ Details

Name:	us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481990/310-10-45-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11

+ Details

Name:	us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: