FAIR VALUE MEASUREMENTS - Nonrecurring - Impaired loans measured at FV (Details) - USD ($)	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
FAIR VALUE MEASUREMENTS
Minimum impaired loan balance that requires an appraisal to be obtained and reviewed annually for impaired loan valuation procedure	$ 250,000
Maximum impaired loan balance for which the bank completes a Certificate of Inspection	250,000
Transfer in or out of Level 3	0	$ 0
Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	12,786,000	1,603,000
Fair Value, Inputs, Level 1 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Fair Value, Inputs, Level 2 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Fair Value, Inputs, Level 3 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	12,786,000	1,603,000
Real Estate \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	12,776,000	1,603,000
Real Estate \| Fair Value, Inputs, Level 1 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Real Estate \| Fair Value, Inputs, Level 2 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Real Estate \| Fair Value, Inputs, Level 3 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	12,776,000	$ 1,603,000
Commercial and Industrial \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	10,000
Commercial and Industrial \| Fair Value, Inputs, Level 1 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0
Commercial and Industrial \| Fair Value, Inputs, Level 2 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0
Commercial and Industrial \| Fair Value, Inputs, Level 3 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	$ 10,000

The fair value of financing receivables that were individually evaluated for impairment.

No definition available.

Name:	fkys_FinancingReceivableIndividuallyEvaluatedForImpairmentFairValueDisclosure
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impaired Loan Balance that Require Annual Bank Certificate of Inspection.

+ References

No definition available.

+ Details

Name:	fkys_ImpairedLoanBalanceThatRequireAnnualBankCertificateOfInspection
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impaired Loan Balance That Requires Appraisal or Certificate of Inspection For Impaired Loan Valuation Procedure.

+ References

No definition available.

+ Details

Name:	fkys_ImpairedLoanBalanceThatRequireAppraisalForImpairedLoanValuationProcedure
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

+ Details

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

- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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