Equity (Cash Distributions Applicable to Period the Distributions Were Earned) (Details) - USD ($) shares in Millions, $ in Millions			2 Months Ended	3 Months Ended								12 Months Ended
		Mar. 01, 2017	Sep. 30, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017	Jul. 30, 2019
Class of Stock [Line Items]
General partner's distributions on general partner units												$ 0.0		$ 0.0	$ 25.0
General partner's distributions on IDRs	[1]											0.0		0.0	303.0
Total distribution on general partner units and IDRs												0.0		0.0	328.0
Total cash distributions declared												2,758.0		2,060.0	1,288.0
Contribution from MPC												346.0		11,980.0	675.0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders												30.0		17.0	7.0
Preferred Class A [Member]
Class of Stock [Line Items]
Limited partners' distributions				$ 20.0	$ 20.0	$ 21.0	$ 20.0	$ 20.0	$ 19.0	$ 20.0	$ 16.0	81.0		75.0	65.0
Limited Partners Common Units
Class of Stock [Line Items]
Limited partners' distributions												2,635.0		1,985.0	895.0
Preferred Class B [Member]
Class of Stock [Line Items]
Limited partners' distributions				11.0	10.0	21.0	0.0					42.0		0.0	0.0
Preferred Partner [Member]
Class of Stock [Line Items]
Total cash distributions declared	[2]														65.0
Preferred Partner [Member] | Preferred Class B [Member]
Class of Stock [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			$ 21.0	$ (21.0)	(21.0)	(21.0)
Preferred Partner [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Total cash distributions declared	[2]											42.0	[3]	0.0	0.0
Limited Partners Common Units
Class of Stock [Line Items]
Total cash distributions declared	[2]											$ 2,635.0		$ 1,985.0	895.0
Public | Limited Partners Common Units
Class of Stock [Line Items]
Limited Partners' Capital Account, Units Issued				392				289				392		289		102
Total cash distributions declared				$ 270.0	266.0	261.0	191.0	$ 187.0	185.0	181.0	179.0
Contribution from MPC												$ 0.0		$ 0.0	0.0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders												$ 0.0		$ 0.0	0.0
Marathon Petroleum Corporation [Member]
Class of Stock [Line Items]
Contribution from MPC		$ 6.0
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Class of Stock [Line Items]
Limited Partners' Capital Account, Units Issued				666				505				666		505		161
Total cash distributions declared				$ 446.0	438.0	$ 431.0	$ 332.0	$ 327.0	$ 322.0	$ 316.0	$ 288.0
Contribution from MPC												$ 315.0		$ 0.0	0.0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders												0.0		$ 0.0	$ 0.0
ANDX LP [Member] | Marathon Petroleum Corporation [Member]
Class of Stock [Line Items]
Contribution from MPC				$ 12.5	$ 12.5							$ 37.5
MPLX LP [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Payment Rate, Variable												LIBOR plus
Preferred Stock, Dividend Rate, Percentage												4.652%

[1] cludes distributions of fourth quarter 2017 income declared on general partner common units issued February 1, 2018 in exchange for the economic general partner interest.

[2] See Note 8 for distribution information.

[3] Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.