| Document and Entity Information | Feb. 13, 2025 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001508655 |
| Document Type | 8-K |
| Document Period End Date | Feb. 13, 2025 |
| Entity Registrant Name | Sixth Street Specialty Lending, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-36364 |
| Entity Tax Identification Number | 27-3380000 |
| Entity Address, Address Line One | 2100 McKinney Avenue |
| Entity Address, Address Line Two | Suite 1500 |
| Entity Address, City or Town | Dallas |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 75201 |
| City Area Code | (469) |
| Local Phone Number | 621-3001 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | TSLX |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
