| Document and Entity Information | Oct. 04, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | FS KKR Capital Corp |
| Amendment Flag | false |
| Entity Central Index Key | 0001422183 |
| Document Type | 8-K |
| Document Period End Date | Oct. 4, 2021 |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 814-00757 |
| Entity Tax Identification Number | 26-1630040 |
| Entity Address, Address Line One | 201 Rouse Boulevard |
| Entity Address, City or Town | Philadelphia |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 19112 |
| City Area Code | (215) |
| Local Phone Number | 495-1150 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock |
| Trading Symbol | FSK |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
