| Document and Entity Information | Oct. 31, 2019 |
| Document And Entity Information [Abstract] | |
| Entity Central Index Key | 0001552000 |
| Amendment Flag | false |
| Document Type | 8-K |
| Document Period End Date | Oct. 31, 2019 |
| Entity Registrant Name | MPLX LP |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35714 |
| Entity Tax Identification Number | 27-0005456 |
| Entity Address, Address Line One | 200 E. Hardin Street |
| Entity Address, City or Town | Findlay |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 45840 |
| City Area Code | 419 |
| Local Phone Number | 421-2414 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Units Representing Limited Partnership Interests |
| Trading Symbol | MPLX |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
