Income Taxes - Summary of Tax Character of Shareholder Distributions (Parenthetical) (Details)	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Percentage of ordinary income qualified as interest related dividend exempt from US withholding tax applicable to non-US shareholder	86.81%	87.01%	87.86%

Percentage of ordinary income qualified as interest related dividend exempt from domestic withholding tax applicable to foreign shareholder.

No definition available.

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Namespace Prefix:	tslx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration