| Cover | Jan. 26, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jan. 26, 2021 |
| Entity File Number | 001-38629 |
| Entity Registrant Name | Equitrans Midstream Corporation |
| Entity Central Index Key | 0001747009 |
| Entity Tax Identification Number | 83-0516635 |
| Entity Incorporation, State or Country Code | PA |
| Entity Address, Address Line One | 2200 Energy Drive |
| Entity Address, City or Town | Canonsburg |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 15317 |
| City Area Code | 724 |
| Local Phone Number | 271-7600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, no par value |
| Trading Symbol | ETRN |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
