Deposits - Schedule of Deposits (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jun. 30, 2018
Deposits [Abstract]
Non-interest bearing demand: Amount	$ 309,063	$ 311,938
Interest-bearing demand: Amount	843,432	1,000,989
Savings and club: Amount	790,658	744,039
Certificates of deposit: Amount	2,204,457	2,016,638
Total deposits	$ 4,147,610	$ 4,073,604
Non-interest-bearing demand, Weighted Average Interest Rate	0.00%	0.00%
Interest-bearing demand, Weighted Average Interest Rate	0.94%	0.92%
Savings and club, Weighted Average Interest Rate	0.73%	0.36%
Certificates of deposit, Weighted Average Interest Rate	2.16%	1.62%
Total deposits, Weighted Average Interest Rate	1.48%	1.09%

Weighted average rate domestic deposit non interest bearing demand.

No definition available.

Name:	krny_WeightedAverageRateDomesticDepositNoninterestBearingDemand
Namespace Prefix:	krny_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

+ Details

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

- Definition

Amount of money in interest-bearing domestic accounts that entitle the depositor to withdraw funds at any time without prior notice, also known as demand deposits.

+ References

+ Details

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

- Definition

Amount of domestic noninterest-bearing deposits held by the entity, which may include demand deposits, checking, brokered and retail deposits.

+ References

+ Details

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

- Definition

Amount of time deposit liabilities, including certificates of deposit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=50488274&loc=d3e64164-112818

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Weighted average interest rate calculated at point in time for domestic deposit liability.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDeposit
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic certificates of deposit liabilities.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDepositCertificatesOfDeposit
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic demand deposit liabilities.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDepositDemand
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average interest rate for domestic savings deposit liabilities.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDepositSavings
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant