| Document and Entity Information | Nov. 05, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001690585 |
| Document Type | 8-K |
| Document Period End Date | Nov. 5, 2020 |
| Entity Registrant Name | MAGENTA THERAPEUTICS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38541 |
| Entity Tax Identification Number | 81-0724163 |
| Entity Address, Address Line One | 100 Technology Square |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02139 |
| City Area Code | (857) |
| Local Phone Number | 242-0170 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 Par Value |
| Trading Symbol | MGTA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
