| Cover | Jul. 28, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jul. 28, 2023 |
| Entity Registrant Name | AG Twin Brook BDC, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 814-01259 |
| Entity Tax Identification Number | 83-4184014 |
| Entity Address, Address Line One | 245 Park Avenue, 26th Floor |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10167 |
| City Area Code | 212 |
| Local Phone Number | 692-2000 |
| Written Communications | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001666384 |
| Amendment Flag | false |
| Soliciting Material | false |
