Disclosures About Fair Value of Assets and Liabilities (Details 2) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Sep. 30, 2017	Jun. 30, 2017
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Foreclosed and repossessed assets, Fair Value	$ 70	$ 71
Loan one- to four-family [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Foreclosed and repossessed assets, Fair Value	70
Loan one- to four-family [Member] \| Repossessed assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Foreclosed and repossessed assets, Fair Value	$ 70
Valuation Technique(s)	Sales comparison approach
Unobservable Input(s)	Adjustments for differences between comparable sales
Loan one- to four-family [Member] \| Repossessed assets [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)	(23.50%)
Loan one- to four-family [Member] \| Repossessed assets [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)	13.80%
Loan one- to four-family [Member] \| Repossessed assets [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)	(1.50%)
One- to four-family [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Foreclosed and repossessed assets, Fair Value		$ 103
One- to four-family [Member] \| Repossessed assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique(s)		Sales comparison approach
Unobservable Input(s)		Adjustments for differences between comparable sales
Range (Weighted Average)		9.50%
One- to four-family [Member] \| Repossessed assets [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)	28.50%	3.60%
Land [Member] \| Repossessed assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique(s)		Sales comparison approach
Unobservable Input(s)		Adjustments for differences between comparable sales
Range (Weighted Average)		5.00%
Land [Member] \| Repossessed assets [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)		3.50%
Land [Member] \| Repossessed assets [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Range (Weighted Average)		6.60%

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average expected volatility of stock price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsWeightedAverageVolatilityRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of where the gains (losses) for the period arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) are included in other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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

- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

+ Details

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

- Details

Name:	kffb_ResidentialRealEstateFamilyAxis=kffb_LoansOneToFourFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=kffb_RepossessedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	kffb_ResidentialRealEstateFamilyAxis=kffb_ResidentialRealEstateOneToFourFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	kffb_ResidentialRealEstateFamilyAxis=kffb_ResidentialRealEstateLandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: