| Cover Page Cover Page | Jul. 27, 2021 |
| Cover Page [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jul. 27, 2021 |
| Entity Registrant Name | Tremont Mortgage Trust |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 001-38199 |
| Entity Tax Identification Number | 82-1719041 |
| Entity Address, Address Line One | Two Newton Place |
| Entity Address, Address Line Two | 255 Washington Street |
| Entity Address, Address Line Three | Suite 300 |
| Entity Address, City or Town | Newton |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02458-1634 |
| Title of 12(b) Security | Common Shares of Beneficial Interest |
| Trading Symbol | TRMT |
| Security Exchange Name | NASDAQ |
| City Area Code | 617 |
| Local Phone Number | 796-8317 |
| Written Communications | true |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Central Index Key | 0001708405 |
| Amendment Flag | false |
| Entity Ex Transition Period | false |
