DEPOSITS - Additional Information (Details) - USD ($)	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
DEPOSITS
Increase (decrease) in deposits	$ 39,470,000	$ (1,196,000)
Deposits	1,019,909,000		$ 980,439,000
Increase (decrease) in retail CDs	33,251,000
Increase (decrease) in brokered CDs	4,930,000
Increase (decrease) in municipal deposits	22,277,000
Increase (decrease) in savings deposits	$ (15,994,000)

Represents information pertaining to increase (decrease) in brokered certificates of deposits.

No definition available.

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Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents information pertaining to increase (decrease) in municipal deposits.

+ References

No definition available.

+ Details

Name:	fkys_IncreaseDecreaseInMunicipalDeposits
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents information pertaining to increase (decrease) in retail certificates of deposits.

+ References

No definition available.

+ Details

Name:	fkys_IncreaseDecreaseInRetailCds
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1

+ 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

The net cash inflow or outflow for the increase (decrease) in the beginning and end of period deposits balances.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-8

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 942
-SubTopic 230
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481160/942-230-45-1

+ Details

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

- Definition

The net cash inflow or outflow in the total of all savings account deposits, which may include passbook, statement savings accounts and money-market deposit accounts (MMDAs), classified as cash flows from financing activities.

+ References

+ Details

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