Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 01, 2020
Document Information [Line Items]
Document Transition Report	false
Document Quarterly Report	true
Title of 12(b) Security	Common Units Representing Limited Partnership Interests
Entity Incorporation, State or Country Code	DE
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Amendment Flag	false
Entity File Number	001-35714
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	0001552000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		1,058,603,592
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Security Exchange Name	NYSE
Entity Tax Identification Number	27-0005456