LOANS AND ALLOWANCE FOR LOAN LOSSES - Loan modifications (Details) $ in Thousands	3 Months Ended
Mar. 31, 2022 USD ($) loan
Loan Restructuring Modification [Member]
Beginning Balance	1
Ending Balance	0
Beginning Balance \| $	$ 9,423
CARES Act Section 4013 Program For Loan Modifications [Member] \| Commercial and Industrial [Member]
Percent of Total Section 4013 CARES Act modifications	0.00%
Percent of Total Section 4013 CARES Act modifications to Total Loans	0.00%
CARES Act Section 4013 Program For Loan Modifications [Member] \| Commercial Real Estate
Beginning Balance	1
Section 4013 CARES Act modifications returned to normal payment status (a)	(1)
Beginning Balance \| $	$ 9,423
Section 4013 CARES Act modifications returned to normal payment status (a) \| $	$ (9,423)
Percent of Total Section 4013 CARES Act modifications	0.00%
Percent of Total Section 4013 CARES Act modifications to Total Loans	0.00%
CARES Act Section 4013 Program For Loan Modifications [Member] \| Residential Real Estate
Percent of Total Section 4013 CARES Act modifications	0.00%
Percent of Total Section 4013 CARES Act modifications to Total Loans	0.00%
CARES Act Section 4013 Program For Loan Modifications [Member] \| Consumer [Member]
Percent of Total Section 4013 CARES Act modifications	0.00%
Percent of Total Section 4013 CARES Act modifications to Total Loans	0.00%
CARES Act Section 4013 Program For Loan Modifications [Member] \| Total Loans[Member]
Beginning Balance	1
Section 4013 CARES Act modifications returned to normal payment status (a)	(1)
Beginning Balance \| $	$ 9,423
Section 4013 CARES Act modifications returned to normal payment status (a) \| $	$ (9,423)
Percent of Total Section 4013 CARES Act modifications	0.00%
Percent of Total Section 4013 CARES Act modifications to Total Loans	0.00%

Amount of financing receivable modified in compliance with Section 4013 CARES Act, not classified as TDRs.

No definition available.

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

- Definition

Represents the number of loans that were modified in accordance with Section 4013 of the CARES Act that have returned to normal payment status.

+ References

No definition available.

+ Details

Name:	fkys_FinancingReceivablesLoansReturnedToNormalPaymentStatus
Namespace Prefix:	fkys_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified, returned to normal payment status.

+ References

No definition available.

+ Details

Name:	fkys_FinancingReceivablesLoansReturnedToNormalPaymentStatusAmount
Namespace Prefix:	fkys_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the modified loans as a percentage of total loans.

+ References

No definition available.

+ Details

Name:	fkys_FinancingReceivablesModifiedLoansAsPercentOfTotalLoans
Namespace Prefix:	fkys_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of total section 4013 CARES Act modifications.

+ References

No definition available.

+ Details

Name:	fkys_FinancingReceivablesPercentageOfTotalSection4013CaresActModifications
Namespace Prefix:	fkys_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number loans not classified as TDRs that were modified in response to COVID-19 in accordance with Section 4013 of the CAREs Act and whose modifications were still active as of the reporting period.

+ References

No definition available.

+ Details

Name:	fkys_NumberOfLoanModificationsApprovedAcrossLoanSegments
Namespace Prefix:	fkys_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=fkys_DeferPrincipalAndOrInterestOrModifyInterestRatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationNameAxis=fkys_CaresActSection4013ProgramForLoanModificationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fkys_LoanModificationTotalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: