LOANS RECEIVABLE - Amortized Cost of Loans to Borrowers, Modified (Details) $ in Thousands	3 Months Ended
Sep. 30, 2024 USD ($)
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 11,626
Payment Delay
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	8,960
Term Extension
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	0
Payment Delay, Term Extension, and Interest Rate Reductions
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	2,666
Commercial \| Multi-family mortgage
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 11,448
Financing receivable, percent of total class	0.43%
Commercial \| Multi-family mortgage \| Payment Delay
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 8,782
Commercial \| Multi-family mortgage \| Term Extension
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	0
Commercial \| Multi-family mortgage \| Payment Delay, Term Extension, and Interest Rate Reductions
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	2,666
Commercial \| Nonresidential mortgage
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 178
Financing receivable, percent of total class	0.02%
Commercial \| Nonresidential mortgage \| Payment Delay
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 178
Commercial \| Nonresidential mortgage \| Term Extension
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	0
Commercial \| Nonresidential mortgage \| Payment Delay, Term Extension, and Interest Rate Reductions
Financing Receivable Modifications [Line Items]
Financing receivable, modified in period	$ 0

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=krny_PaymentDeferralExtendedMaturityAndInterestRateReductionMember
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=krny_MultiFamilyMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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