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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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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-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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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
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-Section 50
-Paragraph 1B
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-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/2009/role/commonPracticeRef
-Topic 815
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6A
-Publisher FASB
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-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/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
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-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_DebtInstrumentAxis=fsk_A6.125UnsecuredNotesDue2031Member
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

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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:
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Period Type:

Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended
Financial Instruments - Narrative (Details) - USD ($)	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Sep. 25, 2025	Sep. 18, 2025	Dec. 27, 2024	Dec. 20, 2024	Nov. 20, 2024	Nov. 13, 2024	Jun. 27, 2024	Jun. 13, 2024	Jun. 06, 2024
Derivative [Line Items]
Derivative, notional amount	$ 242,200,000		$ 220,000,000.0
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						$ 100,000,000		$ 600,000,000
Stated interest rate (as percent)	6.125%	6.125%	6.125%			6.125%
6.125% Unsecured Notes due 2031 \| Unsecured Notes
Derivative [Line Items]
Principal amount				$ 400,000,000
Stated interest rate (as percent)	6.125%	6.125%	6.125%	6.125%
Foreign currency forward contracts
Derivative [Line Items]
Average notional balance	$ 211,100,000	$ 171,000,000.0
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 \| 6.125% Unsecured Notes due 2031 \| Unsecured Notes
Derivative [Line Items]
Stated interest rate (as percent)	6.125%			6.125%
Derivative, notional amount				$ 400,000,000
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 One \| 6.125% Unsecured Notes due 2031 \| Unsecured Notes
Derivative [Line Items]
Derivative, notional amount					$ 400,000,000
Fixed interest rate (as percent)					6.125%
Floating interest rate (as percent)					2.748%
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%