| Document and Entity Information | Mar. 31, 2023 |
| Document And Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Mar. 31, 2023 |
| Entity Registrant Name | MAGENTA THERAPEUTICS, INC. |
| Entity incorporation state code | DE |
| Entity File Number | 001-38541 |
| Entity Tax Identification Number | 81-0724163 |
| Entity Address, Address Line One | 300 Technology Square |
| Entity Address, Address Line Two | 8th Floor |
| 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 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001690585 |
| Amendment Flag | false |
| Former Address [Member] | |
| Document And Entity Information [Line Items] | |
| 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 |
| Common Stock, $0.001 Par Value [Member] | |
| Document And Entity Information [Line Items] | |
| Title of 12(b) Security | Common Stock, $0.001 Par Value |
| Trading Symbol | MGTA |
| Security Exchange Name | NASDAQ |
| Preferred Stock Purchase Rights [Member] | |
| Document And Entity Information [Line Items] | |
| Title of 12(b) Security | Preferred Stock Purchase Rights |
| Security Exchange Name | NASDAQ |
| No Trading Symbol Flag | true |
