| Cover | Jul. 27, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 27, 2022 |
| Entity File Number | 814-01175 |
| Entity Registrant Name | BAIN CAPITAL SPECIALTY FINANCE, INC. |
| Entity Central Index Key | 0001655050 |
| Entity Tax Identification Number | 81-2878769 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 37th Floor |
| Entity Address, Address Line Two | 200 Clarendon Street |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02116 |
| City Area Code | 617 |
| Local Phone Number | 516-2000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $.001 par value |
| Trading Symbol | BCSF |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
