Financial Instruments - Derivative Instruments Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Derivative [Line Items]
Derivative Assets Subject to Master Netting Agreement	$ 3	$ 13
Derivative Liabilities Subject to Master Netting Agreement	(1)	(4)
Total	2	9
Unrealized appreciation on foreign currency forward contracts \| Foreign Exchange Forward
Derivative [Line Items]
Derivative Assets Subject to Master Netting Agreement	3	13
Unrealized depreciation on foreign currency forward contracts \| Foreign Exchange Forward
Derivative [Line Items]
Derivative Liabilities Subject to Master Netting Agreement	$ (1)	$ (4)

Fair value, after effect of master netting arrangement, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and contract can be net settled by means outside contract or delivery of asset. Includes derivative subject to master netting arrangement not elected or qualified to offset. Excludes derivative not subject to master netting arrangement or similar agreement.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483466/210-20-50-3

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-22

+ Details

Name:	us-gaap_DerivativeAssetAfterOffsetSubjectToMasterNettingArrangement
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-22

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483466/210-20-50-3

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-7

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-SubTopic 20
-Topic 860
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481326/860-20-50-4D

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=fsk_UnrealizedAppreciationOnForeignCurrencyForwardContractsMember
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_BalanceSheetLocationAxis=fsk_UnrealizedDepreciationOnForeignCurrencyForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: