Benefit Plans - Schedule of Expected Benefit Payments (Details) $ in Thousands	Jun. 30, 2024 USD ($)
Atlas Bank Retirement Income Plan ("ABRIP")
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2025	$ 150
2026	151
2027	147
2028	143
2029	138
2030-2034	619
Benefit Equalization Plan ("BEP")
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2025	243
2026	238
2027	232
2028	225
2029	218
2030-2034	973
Postretirement Welfare Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2025	59
2026	72
2027	80
2028	98
2029	113
2030-2034	572
Directors' Consultation and Retirement Plan ("DCRP")
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2025	101
2026	122
2027	143
2028	163
2029	213
2030-2034	1,334
Supplemental Executive Retirement Plan (SERP)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2025	0
2026	0
2027	0
2028	0
2029	0
2030-2034	$ 633

Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

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

- Definition

Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

+ References

+ Details

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

- Definition

Amount of benefit for defined benefit plan expected to be paid in fifth fiscal year following current fiscal year.

+ References

+ Details

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

- Definition

Amount of benefit for defined benefit plan expected to be paid in fourth fiscal year following current fiscal year.

+ References

+ Details

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

- Definition

Amount of benefit for defined benefit plan expected to be paid in third fiscal year following current fiscal year.

+ References

+ Details

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

- Definition

Amount of benefit for defined benefit plan expected to be paid in second fiscal year following current fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PlanNameAxis=krny_AtlasBankRetirementIncomePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=krny_BenefitEqualizationPlanBepMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=krny_PostretirementWelfarePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=krny_DirectorsConsultationAndRetirementPlanDcrpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=krny_SupplementalExecutiveRetirementPlanSerpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: