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Loan Quality and Allowance for Loan Losses - Additional Information (Detail) - Residential Mortgage [Member] $ in Millions	Sep. 30, 2019 USD ($) Property Loan	Jun. 30, 2019 USD ($) Property Loan
Financing Receivable Recorded Investment [Line Items]
Number of loans in process of foreclosure | Loan	10	11
Mortgage loans in process of foreclosure, carrying value | $	$ 2.1	$ 2.1
Real Estate Acquired in Satisfaction of Debt [Member] | Single-family Property [Member]
Financing Receivable Recorded Investment [Line Items]
Number of properties held | Property	0	0

X

- Definition

Mortgage loans in process of foreclosure, number of loans.

+ References

No definition available.

+ Details

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Namespace Prefix:	krny_
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Balance Type:	na
Period Type:	instant

X

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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X

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL49126937-111524

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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- Details

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X

- Details

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X

- Details

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