ACL and Reserve for Off-Balance Sheet Credit Exposures

No definition available.

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

- Definition

Summary Of Significant Accounting Policies [Line Items]

+ References

No definition available.

+ Details

Name:	cffn_SummaryOfSignificantAccountingPoliciesLineItems
Namespace Prefix:	cffn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (c)

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121611286&loc=d3e11927-110247

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82919202-210446

+ Details

Name:	us-gaap_OffBalanceSheetCreditLossLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cffn_OneToFourFamilyLoansOriginatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cffn_OneToFourFamilyLoansCorrespondentPurchasedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cffn_OneToFourFamilyLoansBulkPurchasedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary Of Significant Accounting Policies (Cumulative-Effect Adjustment) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Summary Of Significant Accounting Policies [Line Items]
ACL		$ 31,527	$ 9,226	$ 8,463
ACL	$ 19,823
Reserve for off-balance sheet credit exposures	5,743	0
ACL and Reserve for Off-Balance Sheet Credit Exposures		31,527
Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(4,761)
Reserve for off-balance sheet credit exposures		7,788
ACL and Reserve for Off-Balance Sheet Credit Exposures		3,027
Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		26,766
Reserve for off-balance sheet credit exposures		7,788
ACL and Reserve for Off-Balance Sheet Credit Exposures		34,554
One- to Four-Family Segment [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		9,243	3,890	5,739
ACL	3,978
One- to Four-Family Segment [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(4,383)
One- to Four-Family Segment [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		4,860
One- to Four-Family Segment [Member] \| Originated [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		6,085	2,000	2,953
ACL	1,612
One- to Four-Family Segment [Member] \| Originated [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(4,452)
One- to Four-Family Segment [Member] \| Originated [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		1,633
One- to Four-Family Segment [Member] \| Correspondent Purchased [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		2,691	1,203	1,861
ACL	2,062
One- to Four-Family Segment [Member] \| Correspondent Purchased [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(367)
One- to Four-Family Segment [Member] \| Correspondent Purchased [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		2,324
One- to Four-Family Segment [Member] \| Bulk Purchased [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		467	687	925
ACL	304
One- to Four-Family Segment [Member] \| Bulk Purchased [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		436
One- to Four-Family Segment [Member] \| Bulk Purchased [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		903
Commercial Segment [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		21,800	5,171	2,556
ACL	15,652
Commercial Segment [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(193)
Commercial Segment [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		21,607
Commercial Segment [Member] \| Commercial Real Estate [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		20,349
Commercial Segment [Member] \| Commercial Real Estate [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		699
Commercial Segment [Member] \| Commercial Real Estate [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		21,048
Commercial Segment [Member] \| Commercial and Industrial [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		1,451
Commercial Segment [Member] \| Commercial and Industrial [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(892)
Commercial Segment [Member] \| Commercial and Industrial [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		559
Consumer Segment [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		484	$ 165	$ 168
ACL	$ 193
Consumer Segment [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(185)
Consumer Segment [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		299
Consumer Segment [Member] \| Home Equity [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		370
Consumer Segment [Member] \| Home Equity [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		(289)
Consumer Segment [Member] \| Home Equity [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		81
Consumer Segment [Member] \| Other [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		114
Consumer Segment [Member] \| Other [Member] \| Cumulative effect adjustment - Adoption of CECL [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		104
Consumer Segment [Member] \| Other [Member] \| Balance at October 1, 2020 [Member] \| Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
ACL		$ 218