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
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-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
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-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
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-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
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
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-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-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_DebtInstrumentAxis=fsk_A6.125UnsecuredNotesDue2030Member
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:

- Details

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

- Details

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

Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended
Financial Instruments - Narrative (Details) - USD ($)	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024	Dec. 27, 2024	Dec. 20, 2024	Nov. 13, 2024	Jun. 27, 2024	Jun. 13, 2024	Jun. 06, 2024
Derivative [Line Items]
Derivative, notional amount	$ 197,900,000		$ 146,800,000
6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Principal amount									$ 600,000,000
Stated interest rate (as percent)	6.875%	6.875%	6.875%						6.875%
6.125% Notes due 2030 \| Unsecured Notes
Derivative [Line Items]
Principal amount				$ 700,000,000
Stated interest rate (as percent)	6.125%	6.125%	6.125%	6.125%
Foreign Exchange Forward
Derivative [Line Items]
Average notional balance	$ 171,000,000.0	$ 177,000,000.0
Interest Rate Swap \| 6.875% Notes due 2029 \| Unsecured Notes
Derivative [Line Items]
Stated interest rate (as percent)	6.875%								6.875%
Derivative, notional amount									$ 600,000,000
Interest Rate Swap \| 6.125% Notes due 2030 \| Unsecured Notes
Derivative [Line Items]
Stated interest rate (as percent)				6.125%					6.125%
Derivative, notional amount				$ 700,000,000
Interest Rate Swap One
Derivative [Line Items]
Derivative, notional amount	$ 200,000,000
Fixed interest rate (as percent)	6.875%		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 (as percent)								6.875%
Floating interest rate (as percent)								2.754%
Interest Rate Swap Two
Derivative [Line Items]
Derivative, notional amount	$ 400,000,000
Fixed interest rate (as percent)	6.875%		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 (as percent)							6.875%
Floating interest rate (as percent)							2.788%
Interest Rate Swap Three
Derivative [Line Items]
Derivative, notional amount	$ 600,000,000
Fixed interest rate (as percent)	6.125%		6.125%
Interest Rate Swap Three \| 6.125% Notes due 2030 \| Unsecured Notes
Derivative [Line Items]
Derivative, notional amount						$ 600,000,000
Fixed interest rate (as percent)						6.125%
Floating interest rate (as percent)						2.1374%
Interest Rate Swap Four
Derivative [Line Items]
Derivative, notional amount	$ 100,000,000
Fixed interest rate (as percent)	6.125%		6.125%
Interest Rate Swap Four \| 6.125% Notes due 2030 \| Unsecured Notes
Derivative [Line Items]
Derivative, notional amount					$ 100,000,000
Fixed interest rate (as percent)					6.125%
Floating interest rate (as percent)					2.0614%