Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Nov. 02, 2023
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Trading Symbol	TSLX
Entity Registrant Name	Sixth Street Specialty Lending, Inc.
Entity Central Index Key	0001508655
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity File Number	001-36364
Entity Tax Identification Number	27-3380000
Entity Incorporation, State or Country Code	DE
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
Entity Common Stock, Shares Outstanding		87,546,498
Title of 12(b) Security	Common Stock, par value $0.01 per share
Security Exchange Name	NYSE