Derivative Asset, Average Notional Balance

No definition available.

Name:	fsk_DerivativeAssetAverageNotionalBalance
Namespace Prefix:	fsk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
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-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

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-Topic 835
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-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(a)(1))
-Publisher FASB
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-Topic 470
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-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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/disclosureRef
-Topic 815
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-Section 50
-Paragraph 5
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-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
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-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Variable interest rate in effect as of the balance sheet date related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeVariableInterestRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=fsk_A6.875UnsecuredNotesDue2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fsk_InterestRateSwapOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fsk_InterestRateSwapTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments - Narrative (Details) - USD ($)	9 Months Ended
Financial Instruments - Narrative (Details) - USD ($)	Sep. 30, 2024	Sep. 30, 2023	Jun. 27, 2024	Jun. 13, 2024	Jun. 06, 2024	Dec. 31, 2023
Derivative [Line Items]
Derivative, notional amount	$ 148,900,000					$ 167,500,000
6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Principal amount					$ 600,000,000
Stated interest rate	6.875%	6.875%			6.875%
Foreign Exchange Forward
Derivative [Line Items]
Average notional balance	$ 174,200,000	$ 214,700,000
Interest Rate Swap \| 6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Stated interest rate	6.875%				6.875%
Derivative, notional amount					$ 600,000,000
Interest Rate Swap One
Derivative [Line Items]
Derivative, notional amount	$ 200,000,000
Fixed interest rate	6.875%
Interest Rate Swap One \| 6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Derivative, notional amount				$ 200,000,000
Fixed interest rate				6.875%
Floating interest rate				2.754%
Interest Rate Swap Two
Derivative [Line Items]
Derivative, notional amount	$ 400,000,000
Fixed interest rate	6.875%
Interest Rate Swap Two \| 6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Derivative, notional amount			$ 400,000,000
Fixed interest rate			6.875%
Floating interest rate			2.788%