| Document and Entity Information | Jun. 17, 2020 |
| Cover [Abstract] | |
| Security Exchange Name | NYSE |
| Amendment Flag | false |
| Entity Central Index Key | 0001540947 |
| Document Type | 8-K |
| Document Period End Date | Jun. 17, 2020 |
| Entity Registrant Name | EQM Midstream Partners, LP |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35574 |
| Entity Tax Identification Number | 37-1661577 |
| 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 |
| Security 12b Title | Common Units Representing Limited Partner Interests |
| Trading Symbol | EQM |
| Entity Emerging Growth Company | false |
