MPLX (MPLX) 10-Q 6 Nov 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 29, 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	Sep. 30, 2020
Amendment Flag	false
Entity File Number	001-35714
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
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		1,040,169,324
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

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Restructuring Charges		$ 36		$ 0		$ 36		$ 0
Revenues and other income:
Total revenues from contracts with customers		1,756		1,760		5,129		5,185
Rental income		102		99		296		291
Rental income - related parties		241		293		712		904
Income/(loss) from equity method investments(1)		83		95		(1,012)	[1],[2]	255
Other income		2		2		5		6
Other income - related parties		63		31		190		84
Revenues		2,247		2,280		5,320		6,725
Costs and expenses:
Rental cost of sales		33		37		101		103
Rental cost of sales - related parties		32		45		119		124
Purchases - related parties		297		303		853		894
Depreciation and amortization	[3]	346		302		992		916
Goodwill and Intangible Asset Impairment		0		0		2,165		0
General and administrative expenses		96		102		289		293
Other taxes		33		29		94		84
Costs and Expenses		1,348		1,354		6,029		4,002
Income/(loss) from operations		899		926		(709)		2,723
Related party interest and other financial costs		0		5		4		8
Interest expense (net of amounts capitalized of $8 million, $13 million, $31 million and $36 million, respectively)		207		212		624		640
Other financial costs		17		16		49		38
Income/(loss) before income taxes		675		693		(1,386)		2,037
Provision for income taxes		1		4		1		2
Net income		674		689		(1,387)		2,035
Less: Net income (loss) attributable to noncontrolling interest		9		8		24		20
Income (Loss) Attributable to Predecessor		0		52		0		401
Net income (loss) attributable to MPLX LP		665	[4]	629	[5]	(1,411)	[6]	1,614	[7]
Limited partners' interest in net income (loss) attributable to MPLX LP		635		602		(1,503)		1,546
Weighted average limited partner units outstanding:
Equity Method Investment, Other than Temporary Impairment						1,264
Limited Partners Common Units
Costs and expenses:
Net income (loss) attributable to MPLX LP		$ 635	[4]	$ 599	[5]	$ (1,503)	[6]	$ 1,522	[7]
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
Common - diluted (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
Weighted average limited partner units outstanding:
Common - basic (in shares)		1,046		974	[8]	1,054		855	[9]
Common - diluted (in shares)		1,047		975	[8]	1,054		855	[9]
Service [Member]
Revenues and other income:
Total revenues from contracts with customers		$ 604		$ 632		$ 1,779		$ 1,865
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		909		899		2,694		2,549
Service, Other [Member]
Revenues and other income:
Total revenues from contracts with customers		41		26		102		86
Product [Member]
Revenues and other income:
Total revenues from contracts with customers		165		171		454		576
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		37		32		100		109
Oil and Gas, Refining and Marketing [Member]
Costs and expenses:
Cost of Goods and Services Sold		323		407		1,006		1,099
Natural Gas, Midstream [Member]
Costs and expenses:
Cost of Goods and Services Sold		152		129		374		489
Series A Preferred Stock [Member] | Preferred Partner [Member]
Costs and expenses:
Net income (loss) attributable to MPLX LP		20	[4]	20	[5]	61	[6]	61	[7]
Dividends, Preferred Stock		20		20		61		61
Series B Preferred Stock [Member] | Preferred Partner [Member]
Costs and expenses:
Net income (loss) attributable to MPLX LP		10	[4]	10	[5]	31	[6]	31	[7]
Dividends, Preferred Stock		10		7		31		7
MPC
Revenues and other income:
Rental income - related parties		241		293		712		904
Other income - related parties		48		14		144		34
MPC | Service [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		908		899		2,692		2,549
MPC | Product [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[10]	$ 37		$ 32		$ 100		$ 109
[1]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[2]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .
[3]	Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019
[4]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[5]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[6]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[7]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[8]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire three months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.
[9]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire nine months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.
[10]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2020 , these sales totaled $107 million and $332 million , respectively. For the three and nine months ended September 30, 2019 , these sales totaled $301 million and $819 million , respectively.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Interest Costs Capitalized	$ 8	$ 13	$ 31	$ 36

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net (loss)/income	$ 674	$ 689	$ (1,387)	$ 2,035
Other comprehensive income/(loss), net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	0	0	(1)	1
Comprehensive income/(loss)	674	689	(1,388)	2,036
Less comprehensive income attributable to:
Noncontrolling interests	9	8	24	20
Comprehensive Income (loss), Net of Tax, Attributable to Predecessor	0	52	0	401
Comprehensive income/(loss) attributable to MPLX LP	$ 665	$ 629	$ (1,412)	$ 1,615

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Assets
Cash and cash equivalents		$ 28	$ 15
Receivables, net		477	593
Current Assets, Related Parties		591	656
Inventories		117	110
Other current assets		59	110
Total current assets		1,272	1,484
Equity method investments		4,081	5,275
Property, plant and equipment, net		21,615	22,145
Intangibles, net		991	1,270
Goodwill		7,657	9,536
Operating Lease, Right-of-Use Asset		323	365
Other Assets, Related Parties, Noncurrent		675	303
Other noncurrent assets		48	52
Total assets		36,662	40,430
Current liabilities:
Accounts payable		140	242
Accrued Liabilities, Current		169	187
Current Liabilities, Related Parties		364	1,008
Accrued Property Plant and Equipment Current		121	283
Accrued interest payable		207	210
Operating Lease, Liability, Current		65	66
Other current liabilities		158	127
Total current liabilities		1,531	2,132
Long-term deferred revenue		291	217
Liabilities, Related Parties, Noncurrent		281	290
Long-term debt		20,042	19,704
Deferred income taxes		12	12
Operating Lease, Liability, Noncurrent		258	302
Debt, Current		307	9
Deferred credits and other liabilities		184	192
Total liabilities		22,599	22,849
Commitments and contingencies (see Note 21)
Series A preferred units		968	968
Equity
Total MPLX LP partners’ capital		12,848	16,364
Accumulated other comprehensive loss	[1]	(16)	(15)
Noncontrolling interests		247	249
Total equity		13,095	16,613
Total liabilities, preferred units and equity		36,662	40,430
MPC
Assets
Other current assets		1	0
Operating Lease, Right-of-Use Asset		231	232
Current liabilities:
Operating Lease, Liability, Current		1	1
Operating Lease, Liability, Noncurrent		230	230
Series B Preferred Stock [Member]
Equity
Total MPLX LP partners’ capital		601	611
Limited Partners Common Units | Public
Equity
Total MPLX LP partners’ capital		9,436	10,800
Total equity		9,436	10,800
Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital		2,827	4,968
Total equity		2,827	4,968
LOOP and Explorer
Equity
Accumulated other comprehensive loss		(16)	(15)
Retained Earnings [Member] | MPC
Equity
Total equity		$ 0	$ 0
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Sep. 30, 2020	Dec. 31, 2019
Limited Partners Common Units | Public
Units issued	393	392
Units Outstanding	393	392
Limited Partners Common Units | MPC
Units issued	647	666
Units Outstanding	647	666
Series B Preferred Stock [Member]
Units issued	0.6	0.6
Units Outstanding	0.6	0.6

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019
Operating activities:
Net (loss)/income		$ (1,387)		$ 2,035
Adjustments to reconcile net income/(loss) to net cash provided by operating activities:
Amortization of deferred financing costs		44		29
Depreciation and amortization	[1]	992		916
Goodwill and Intangible Asset Impairment		2,165		0
Deferred income taxes		(1)		1
Retirement Expenditures		0		1
Loss/(gain) on disposal of assets		1		(3)
Loss/(income) from equity method investments(1)		1,012	[2],[3]	(255)
Distributions from unconsolidated affiliates		350		379
Changes in:
Current receivables		69		38
Inventories		(8)		(3)
Fair value of derivatives		1		(4)
Current accounts payable and accrued liabilities		(27)		(81)
Current assets/current liabilities - related parties		36		(148)
Increase (Decrease) in Other Operating Assets and Liabilities, Net		(2)		6
Deferred revenue		85		58
All other, net		6		23
Net cash provided by operating activities		3,336		2,990
Investing activities:
Additions to property, plant and equipment		(982)		(1,720)
Proceeds from Contributions from Affiliates		0		6
Disposal of assets		54		14
Investments in unconsolidated affiliates		(244)		(494)
Distributions from unconsolidated affiliates - return of capital		112		2
Payments for (Proceeds from) Other Investing Activities		0		(3)
Net cash used in investing activities		(1,060)		(2,189)
Financing activities:
Long-term debt - borrowings		5,990		8,674
Long-term debt - repayments		(5,372)		(7,423)
Related party debt - borrowings		4,870		7,708
Related party debt - repayments		(5,464)		(7,583)
Distributions to noncontrolling interests		(26)		(20)
Distributions to unitholders and general partner		(2,162)		(1,731)
Proceeds from Contributions from Parent		34		52
Contributions from noncontrolling interests		0		94
All other, net		(8)		(12)
Net cash used in financing activities		(2,263)		(845)
Net increase/(decrease) in cash, cash equivalents and restricted cash		13		(44)
Cash, cash equivalents and restricted cash at beginning of period		15		85
Cash, cash equivalents and restricted cash at end of period		28		41
Equity Method Investment, Other than Temporary Impairment		1,264
Payments of Debt Issuance Costs		23		20
Series A Preferred Stock [Member] | Preferred Partner [Member]
Financing activities:
Distributions to preferred unitholders		(61)		(61)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Financing activities:
Distributions to preferred unitholders		(41)		(21)
Payments of Distributions on Preferred Units from Predecessor		$ 0		$ 502
[1]	Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[3]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($)	Total	Preferred Class B [Member]	PublicLimited Partners Common Units	MPCLimited Partners Common Units	Accumulated Other Comprehensive Loss	Noncontrolling Interests	Equity of PredecessorMPC
Beginning Balance at Dec. 31, 2018	$ 17,731,000,000	$ 0	$ 8,336,000,000	$ (1,612,000,000)	$ (16,000,000)	$ 156,000,000	$ 10,867,000,000
Distributions to:	(669,000,000)	0	176,000,000	307,000,000	0	(6,000,000)	(180,000,000)
Contributions from:	15,000,000	0	0	0	0	0	15,000,000
Unitholders and general partner	(776,000,000)	0	(188,000,000)	(327,000,000)	0	0	(261,000,000)
Noncontrolling interests	(6,000,000)	0	0	0	0	(6,000,000)	0
Contributions from noncontrolling interests	(94,000,000)	0	0	0	0	(94,000,000)	0
Other	3,000,000	0	2,000,000	0	1,000,000	0	0
Ending Balance at Mar. 31, 2019	17,730,000,000	0	8,326,000,000	(1,632,000,000)	(15,000,000)	250,000,000	10,801,000,000
Beginning Balance at Dec. 31, 2018	17,731,000,000	0	8,336,000,000	(1,612,000,000)	(16,000,000)	156,000,000	10,867,000,000
Contributions from noncontrolling interests	(94,000,000)
Ending Balance at Sep. 30, 2019	17,892,000,000	601,000,000	11,289,000,000	5,767,000,000	(15,000,000)	250,000,000	0
Beginning Balance at Mar. 31, 2019	17,730,000,000	0	8,326,000,000	(1,632,000,000)	(15,000,000)	250,000,000	10,801,000,000
Distributions to:	(636,000,000)	0	168,000,000	293,000,000	0	(6,000,000)	(169,000,000)
Contributions from:	13,000,000	0	0	0	0	0	13,000,000
Unitholders and general partner	(764,000,000)	0	(191,000,000)	(332,000,000)	0	0	241,000,000
Noncontrolling interests	(6,000,000)	0	0	0	0	(6,000,000)	0
Other	2,000,000	0	2,000,000	0	0	0	0
Ending Balance at Jun. 30, 2019	17,611,000,000	0	8,305,000,000	(1,671,000,000)	(15,000,000)	250,000,000	10,742,000,000
Distributions to:	(669,000,000)	(7,000,000)	(222,000,000)	(380,000,000)	0	(8,000,000)	(52,000,000)
Stockholders' Equity, Change in Reporting Entity	0	615,000,000	2,983,000,000	7,199,000,000	0	0	(10,797,000,000)
Contributions from:	295,000,000	0	0	292,000,000	0	0	3,000,000
Unitholders and general partner	(715,000,000)	(21,000,000)	(262,000,000)	(432,000,000)	0	0	0
Noncontrolling interests	(8,000,000)	0	0	0	0	(8,000,000)	0
Partners' Capital Account, Exchanges and Conversions	36,000,000	0	36,000,000	0	0	0	0
Other	4,000,000	0	5,000,000	(1,000,000)	0	0	0
Ending Balance at Sep. 30, 2019	17,892,000,000	601,000,000	11,289,000,000	5,767,000,000	(15,000,000)	250,000,000	0
Beginning Balance at Dec. 31, 2019	16,613,000,000	611,000,000	10,800,000,000	4,968,000,000	(15,000,000)	249,000,000	0
Distributions to:	2,736,000,000	(11,000,000)	(1,022,000,000)	(1,733,000,000)	0	(8,000,000)	0
Contributions from:	225,000,000	0	0	225,000,000	0	0	0
Unitholders and general partner	(738,000,000)	(21,000,000)	(271,000,000)	(446,000,000)	0	0	0
Noncontrolling interests	(9,000,000)	0	0	0	0	(9,000,000)	0
Other	1,000,000	0	2,000,000	0	(1,000,000)	0	0
Ending Balance at Mar. 31, 2020	13,356,000,000	601,000,000	9,509,000,000	3,014,000,000	(16,000,000)	248,000,000	0
Beginning Balance at Dec. 31, 2019	16,613,000,000	611,000,000	10,800,000,000	4,968,000,000	(15,000,000)	249,000,000	0
Distributions to:		(31,000,000)
Unitholders and general partner		(41,000,000)
Contributions from noncontrolling interests	0
Ending Balance at Sep. 30, 2020	13,095,000,000	601,000,000	9,436,000,000	2,827,000,000	(16,000,000)	247,000,000	0
Beginning Balance at Mar. 31, 2020	13,356,000,000	601,000,000	9,509,000,000	3,014,000,000	(16,000,000)	248,000,000	0
Distributions to:	(634,000,000)	(10,000,000)	229,000,000	388,000,000	0	(7,000,000)	0
Contributions from:	6,000,000	0	0	6,000,000	0	0	0
Unitholders and general partner	(728,000,000)	0	(270,000,000)	(458,000,000)	0	0	0
Noncontrolling interests	(8,000,000)	0	0	0	0	(8,000,000)	0
Other	2,000,000	0	1,000,000	1,000,000	0	0	0
Ending Balance at Jun. 30, 2020	13,262,000,000	611,000,000	9,469,000,000	2,951,000,000	(16,000,000)	247,000,000	0
Distributions to:	(654,000,000)	(10,000,000)	236,000,000	399,000,000	0	(9,000,000)	0
Stockholders' Equity, Change in Reporting Entity	(90,000,000)	0	0	(90,000,000)	0	0	0
Contributions from:	13,000,000	0	0	13,000,000	0	0	0
Unitholders and general partner	(736,000,000)	(20,000,000)	(271,000,000)	(445,000,000)	0	0	0
Noncontrolling interests	(9,000,000)	0	0	0	0	(9,000,000)	0
Other	1,000,000	0	2,000,000	(1,000,000)	0	0	0
Ending Balance at Sep. 30, 2020	$ 13,095,000,000	$ 601,000,000	$ 9,436,000,000	$ 2,827,000,000	$ (16,000,000)	$ 247,000,000	$ 0

Description of the Business and

Description of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 30, 2019, MPLX completed its acquisition by merger (the “Merger”) of Andeavor Logistics LP (“ANDX”). At the effective time of the Merger, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units. ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. See Note 3 for additional information regarding the Merger. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (WRW”) to Western Refining Southwest, Inc. (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – The outbreak of COVID-19 and its development into a pandemic in March 2020 resulted in significant economic disruption globally. Actions taken by various governmental authorities, individuals and companies around the world to prevent the spread of COVID-19 through social distancing have restricted travel, many business operations, public gatherings and the overall level of individual movement and in-person interaction across the globe. Although there have been some signs of economic improvement, these events significantly reduced global economic activity and resulted in a decline in the demand for the midstream services we provide beginning with the first quarter of 2020. Macroeconomic conditions and global geopolitical events have also resulted in significant price volatility related to those aforementioned products. The overall deterioration in the economy and the environment in which MPLX and its customers operate, as well as a sustained decrease in unit price, were considered triggering events resulting in impairments of the carrying value of certain assets during the first quarter of 2020. We recognized impairments related to goodwill, certain equity method investments and certain long-lived assets (including intangibles), within our G&P segment. Many of our producer customers refined and updated production forecasts in response to the current environment, which impacted their current and expected future demand for our services, including the future utilization of our assets. Additionally, certain of our contracts have commodity price exposure, including NGL prices, which have experienced increased volatility as noted above. The table below provides information related to the impairments recognized during the first quarter of 2020 as well as the corresponding footnote where additional information can be found. No additional events or circumstances arose during the second or third quarters of 2020 which would indicate the need for any additional impairment beyond those recognized during the first quarter. (In millions) Impairment Footnote Reference Goodwill $ 1,814 12 Equity method investments 1,264 4 Intangibles, net 177 12 Property, plant and equipment, net 174 11 Total impairments $ 3,429 Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2019 . The results of operations for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the full year. In relation to the Merger described above and in Note 3 , ANDX’s assets, liabilities and results of operations prior to the Merger are collectively included in what we refer to as the “Predecessor” from October 1, 2018, which was the date that MPC acquired Andeavor. MPLX’s acquisition of ANDX is considered a transfer between entities under common control due to MPC’s relationship with ANDX prior to the Merger. As an entity under common control with MPC, MPLX recorded the assets acquired and liabilities assumed on its consolidated balance sheets at MPC’s historical carrying value. For the acquiring entity, transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. Accordingly, the accompanying financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of ANDX beginning October 1, 2018. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .

Accounting Standards Accounting

Accounting Standards Accounting Standards	9 Months Ended
	Sep. 30, 2020
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Update and Change in Accounting Principle [Text Block]	Recently Adopted ASU 2016-13, Credit Losses - Measurement of Credit Losses on Financial Instruments Effective January 1, 2020, we adopted ASU 2016-13 using the modified retrospective transition method. This ASU requires entities to consider a broader range of information to estimate expected credit losses, which may result in earlier recognition of losses. The ASU requires the company to utilize an expected loss methodology in place of the incurred loss methodology for financial instruments, including trade receivables, and off-balance sheet credit exposures. Adoption of the standard did not have a material impact on our financial statements. We are exposed to credit losses, primarily as a result of the midstream services that we provide. We assess each customer’s ability to pay through our credit review process, which considers various factors such as external credit ratings; a review of financial statements to determine liquidity, leverage, trends and business specific risks; market information; pay history and our business strategy. We monitor our ongoing credit exposure through timely review of customer payment activity. At September 30, 2020 , we reported $477 million of accounts receivable, net of allowances of $4 million . We also adopted the following ASUs during the first nine months of 2020, which did not have a material impact to our financial statements or financial statement disclosures: ASU Effective Date 2018-13 Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement January 1, 2020 2020-04 Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting April 1, 2020

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2020
Business Combinations [Abstract]
Acquisitions	Acquisitions and Dispositions Wholesale Exchange On July 31, 2020, MPLX entered into a Redemption Agreement (the “Redemption Agreement”) with WRSW, an Arizona corporation and wholly owned subsidiary of MPC, pursuant to which MPLX agreed to transfer to WRSW all of the outstanding membership interests in WRW in exchange for the redemption of MPLX common units held by WRSW. The transaction effects the transfer to MPC of the Western wholesale distribution business that MPLX acquired as a result of its acquisition of ANDX as described below. Per the terms of the Redemption Agreement, MPLX redeemed 18,582,088 common units (the “Redeemed Units”) held by WRSW on July 31, 2020. The number of Redeemed Units was calculated by dividing WRW’s aggregate valuation of $340 million by the simple average of the volume weighted average NYSE prices of an MPLX common unit for the ten trading days ending at market close on July 27, 2020. MPLX canceled the Redeemed Units immediately following the Wholesale Exchange. The carrying value of the net assets of WRW transferred to MPC was approximately $90 million as of July 31, 2020, resulting in $250 million being recorded to “Common Unit-holder MPC” within the Consolidated Statements of Equity, in addition to the fair value of the redeemed units. Included within the $90 million carrying value of the WRW net assets was approximately $65 million of goodwill. Acquisition of Andeavor Logistics LP On May 7, 2019, ANDX, Tesoro Logistics GP, LLC (then the general partner of ANDX (“TLGP”)), MPLX, MPLX GP LLC (the general partner of MPLX (“MPLX GP”)), and MPLX MAX LLC, a wholly owned subsidiary of MPLX (“Merger Sub”), entered into an Agreement and Plan of Merger (the “Merger Agreement”) that provided for, among other things, the merger of Merger Sub with and into ANDX. On July 30, 2019, the Merger was completed, and ANDX survived the Merger as a wholly owned subsidiary of MPLX. At the effective time of the Merger, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units. ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. See Note 7 for information on units issued in connection with the Merger. Additionally, as a result of the Merger, each ANDX TexNew Mex Unit issued and outstanding immediately prior to the effective time of the Merger was converted into a right for WRSW, a wholly owned subsidiary of MPC, as the holder of all such units, to receive a unit representing a substantially equivalent limited partner interest in MPLX (the “MPLX TexNew Mex Units”). By virtue of the conversion, all ANDX TexNew Mex Units were cancelled and ceased to exist as of the effective time of the Merger. The MPLX TexNew Mex Units are a new class of units in MPLX substantially equivalent to the ANDX TexNew Mex Units, including substantially equivalent rights, powers, duties and obligations that the ANDX TexNew Mex Units had immediately prior to the closing of the Merger. As a result of the Merger, the ANDX Special Limited Partner Interest outstanding immediately prior to the effective time of the Merger was converted into a right for WRSW, as the holder of all such interest, to receive a substantially equivalent special limited partner interest in MPLX (the “MPLX Special Limited Partner Interest”). By virtue of the conversion, the ANDX Special Limited Partner Interest was cancelled and ceased to exist as of the effective time of the Merger. For information on ANDX’s preferred units, please see Note 7 . The assets of ANDX consist of a network of owned and operated crude oil, refined product and natural gas pipelines; crude oil and water gathering systems; refining logistics assets; terminals with crude oil and refined products storage capacity; rail facilities; marine terminals including storage; bulk petroleum distribution facilities; a trucking fleet; and natural gas processing and fractionation systems and complexes. The assets are located in the western and inland regions of the United States and complement MPLX’s existing business and assets. MPC accounted for its October 1, 2018 acquisition of Andeavor (through which it acquired control of ANDX), using the acquisition method of accounting, which required Andeavor assets and liabilities to be recorded by MPC at the acquisition date fair value. The Merger was closed on July 30, 2019, and the results of ANDX have been incorporated into the results of MPLX as of October 1, 2018, which is the date that common control was established. As a result of MPC’s relationship with both MPLX and ANDX, the Merger has been treated as a common control transaction, which requires the recasting of MPLX’s historical results and the recognition of assets acquired and liabilities assumed using MPC’s historical carrying value. We recognized $14 million in acquisition costs during the first nine months of 2019 related to the Merger, which are reflected in general and administrative expenses. For the three and nine months ended September 30, 2019, we recognized $612 million and $1,789 million of revenues and other income, respectively, related to ANDX. For the three and nine months ended September 30, 2019 we recognized $191 million and $539 million

Equity Method Investments

Equity Method Investments	9 Months Ended
	Sep. 30, 2020
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests [Text Block]	Investments and Noncontrolling Interests The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at September 30, September 30, December 31, (In millions, except ownership percentages) 2020 2020 2019 L&S MarEn Bakken Company LLC (1) 25% $ 469 $ 481 Illinois Extension Pipeline Company, L.L.C. 35% 261 265 LOOP LLC 41% 249 238 Andeavor Logistics Rio Pipeline LLC (2) 67% 195 202 Minnesota Pipe Line Company, LLC 17% 189 190 Whistler Pipeline LLC (2) 38% 184 134 Explorer Pipeline Company 25% 78 83 W2W Holdings LLC (2)(3) 50% 77 — Wink to Webster Pipeline LLC (2)(3) 15% — 126 Other (2) 100 55 Total L&S 1,802 1,774 G&P MarkWest Utica EMG, L.L.C. (2) 57% 720 1,984 Sherwood Midstream LLC (2) 50% 560 537 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 307 302 Rendezvous Gas Services, L.L.C. (2) 78% 164 170 Sherwood Midstream Holdings LLC (2) 51% 151 157 Centrahoma Processing LLC 40% 147 153 Other (2) 230 198 Total G&P 2,279 3,501 Total $ 4,081 $ 5,275 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL. (2) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (3) During the nine months ended September 30, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC. For those entities that have been deemed to be VIEs, neither MPLX nor any of its subsidiaries have been deemed to be the primary beneficiary due to voting rights on significant matters. While we have the ability to exercise influence through participation in the management committees which make all significant decisions, we have equal influence over each committee as a joint interest partner and all significant decisions require the consent of the other investors without regard to economic interest and as such we have determined that these entities should not be consolidated and apply the equity method of accounting with respect to our investments in each entity. Sherwood Midstream has been deemed the primary beneficiary of Sherwood Midstream Holdings due to its controlling financial interest through its authority to manage the joint venture. As a result, Sherwood Midstream consolidates Sherwood Midstream Holdings. Therefore, MPLX also reports its portion of Sherwood Midstream Holdings’ net assets as a component of its investment in Sherwood Midstream. As of September 30, 2020 , MPLX has a 24.51 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream. MPLX’s maximum exposure to loss as a result of its involvement with equity method investments includes its equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of its compensation received for the performance of the operating services. MPLX did not provide any financial support to equity method investments that it was not contractually obligated to provide during the nine months ended September 30, 2020 . During the first quarter of 2020, we recorded an other than temporary impairment for three joint ventures in which we have an interest as discussed in Note 1 . Impairment of these investments was $1,264 million , of which $1,251 million was related to MarkWest Utica EMG and its investment in Ohio Gathering Company, L.L.C. The fair value of the investments was determined based upon applying the discounted cash flow method, which is an income approach. The discounted cash flow fair value estimate is based on known or knowable information at the interim measurement date. The significant assumptions that were used to develop the estimate of the fair value under the discounted cash flow method include management’s best estimates of the expected future cash flows, including prices and volumes, the weighted average cost of capital and the long-term growth rate. Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As such, the fair value of these equity method investments represents a Level 3 measurement. As a result, there can be no assurance that the estimates and assumptions made for purposes of the impairment test will prove to be an accurate prediction of the future. The impairment was recorded through “Income from equity method investments.” The impairments were largely due to a reduction in forecasted volumes gathered and processed by the systems operated by the joint ventures. There were no additional impairments recorded during the second or third quarters of 2020. Summarized financial information for MPLX’s equity method investments for the nine months ended September 30, 2020 and 2019 is as follows: Nine Months Ended September 30, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ 132 $ 933 $ 1,065 Costs and expenses 308 405 713 Income from operations (176 ) 528 352 Net income (230 ) 477 247 (Loss)/income from equity method investments (1) $ (1,138 ) $ 126 $ (1,012 ) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million . Nine Months Ended September 30, 2019 (In millions) VIEs Non-VIEs Total Revenues and other income $ 479 $ 1,116 $ 1,595 Costs and expenses 251 434 685 Income from operations 228 682 910 Net income 192 605 797 Income from equity method investments (1) $ 89 $ 166 $ 255 (1) Includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for MPLX’s equity method investments as of September 30, 2020 and December 31, 2019 is as follows: September 30, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 774 $ 367 $ 1,141 Noncurrent assets 6,398 5,024 11,422 Current liabilities 304 194 498 Noncurrent liabilities $ 1,611 $ 857 $ 2,468 December 31, 2019 (In millions) VIEs Non-VIEs Total Current assets $ 534 $ 330 $ 864 Noncurrent assets 5,862 5,134 10,996 Current liabilities 192 245 437 Noncurrent liabilities $ 305 $ 822 $ 1,127 As of September 30, 2020 , the underlying net assets of MPLX’s investees in the G&P segment exceeded the carrying value of its equity method investments by approximately $58 million . At December 31, 2019 , the carrying value of MPLX’s equity method investments in the G&P segment exceeded the underlying net assets of its investees by approximately $1.0 billion . As of September 30, 2020 and December 31, 2019 , the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $331 million and $329 million , respectively. At September 30, 2020 and December 31, 2019 , the G&P basis difference was being amortized into net income over the remaining estimated useful lives of the underlying assets, except for $31 million and $498 million of excess related to goodwill, respectively. At September 30, 2020 and December 31, 2019 , the L&S basis difference was being amortized into net income over the remaining estimated useful lives of the underlying assets, except for $167 million

Related Party Agreements and Tr

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions MPLX engages in transactions with both MPC and certain of its equity method investments as part of its normal business; however, transactions with MPC make up the majority of MPLX’s related party transactions. Transactions with related parties are further described below. MPLX has various long-term, fee-based commercial agreements with MPC. Under these agreements, MPLX provides transportation, terminal, fuels distribution, marketing, storage, management, operational and other services to MPC. MPC has committed to provide MPLX with minimum throughput volumes on crude oil and refined products and other fees for storage capacity; operating and management fees; as well as reimbursements for certain direct and indirect costs. MPC has also committed to provide a fixed fee for 100 percent of available capacity for boats, barges and third-party chartered equipment under the marine transportation service agreement. MPLX also has a keep-whole commodity agreement with MPC under which MPC pays us a processing fee for NGLs related to keep-whole agreements and delivers shrink gas to the producers on our behalf. We pay MPC a marketing fee in exchange for assuming the commodity risk. Additionally, MPLX has obligations to MPC for services provided to MPLX by MPC under omnibus and employee services-type agreements as well as other various agreements. MPC has also been advancing certain strategic priorities to lay a foundation for long-term success, including plans to optimize its assets and structurally lower costs in 2021 and beyond, which included an involuntary workforce reduction plan. The workforce reduction plan, together with employee reductions resulting from MPC's indefinite idling of its Martinez, California and Gallup, New Mexico refineries, affected approximately 2,050 employees. All of the employees that conduct MPLX’s business are directly employed by affiliates of MPC, and certain of those employees were affected by MPC’s workforce reductions. During the third quarter of 2020, MPLX reimbursed MPC for $36 million related to severance and employee benefits related expenses that MPC recorded in connection with its workforce reductions. These costs are shown on the Consolidated Statements of Income as “Restructuring expenses.” Related Party Loan MPLX is party to a loan agreement with MPC Investment LLC (“MPC Investment”) (the “MPC Loan Agreement”). Under the terms of the agreement, MPC Investment makes a loan or loans to MPLX on a revolving basis as requested by MPLX and as agreed to by MPC Investment. In connection with the Merger, on July 31, 2019, MPLX and MPC Investment amended and restated the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement to $1.5 billion in aggregate principal amount of all loans outstanding at any one time. The entire unpaid principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), shall become due and payable on July 31, 2024 , provided that MPC Investment may demand payment of all or any portion of the outstanding principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), at any time prior to July 31, 2024 . Borrowings under the MPC Loan Agreement prior to July 31, 2019 bore interest at LIBOR plus 1.50 percent while borrowings as of and after July 31, 2019 bear interest at LIBOR plus 1.25 percent or such lower rate as would be applicable to such loans under the MPLX Credit Agreement as discussed in Note 15 . Activity on the MPC Loan Agreement was as follows: (In millions) Nine Months Ended September 30, 2020 Year Ended December 31, 2019 Borrowings $ 4,870 $ 8,540 Average interest rate of borrowings 2.380 % 3.441 % Repayments $ 5,464 $ 7,946 Outstanding balance at end of period (1) $ — $ 594 (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets. Prior to the Merger, ANDX was also party to a loan agreement with MPC (“ANDX-MPC Loan Agreement”). This facility was entered into on December 21, 2018, with a borrowing capacity of $500 million . In connection with the Merger, on July 31, 2019, MPLX repaid the entire outstanding balance and terminated the ANDX-MPC Loan Agreement. Activity on the ANDX-MPC Loan Agreement prior to the Merger was as follows: (In millions) Year Ended December 31, 2019 Borrowings $ 773 Average interest rate of borrowings 4.249 % Repayments $ 773 Outstanding balance at end of period $ — Related Party Revenue Related party sales to MPC consist of crude oil and refined products pipeline and trucking transportation services based on tariff/contracted rates; storage, terminal and fuels distribution services based on contracted rates; and marine transportation services. Related party sales to MPC also consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Service revenues - related parties MPC $ 908 $ 899 $ 2,692 $ 2,549 Other 1 — 2 — Total Service revenue - related parties 909 899 2,694 2,549 Rental income - related parties MPC 241 293 712 904 Product sales - related parties (1) MPC 37 32 100 109 Other income - related parties MPC 48 14 144 34 Other 15 17 46 50 Total Other income - related parties $ 63 $ 31 $ 190 $ 84 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2020 , these sales totaled $107 million and $332 million , respectively. For the three and nine months ended September 30, 2019 , these sales totaled $301 million and $819 million , respectively. Related Party Expenses MPC provides executive management services and certain general and administrative services to MPLX under the terms of our omnibus agreements (“Omnibus charges”). Omnibus charges included in “Rental cost of sales - related parties” primarily relate to services that support MPLX’s rental operations and maintenance of assets available for rent. Omnibus charges included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Omnibus charges included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. MPLX also obtains employee services from MPC under employee services agreements (“ESA charges”). ESA charges for personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties.” ESA charges for personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” ESA charges for personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses.” In addition to these agreements, MPLX purchases products from MPC, makes payments to MPC in its capacity as general contractor to MPLX, and has certain lease agreements with MPC. Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Rental cost of sales - related parties MPC $ 32 $ 45 $ 119 $ 124 Purchases - related parties MPC 293 297 840 878 Other 4 6 13 16 Total Purchase - related parties 297 303 853 894 General and administrative expenses MPC 63 59 195 174 Restructuring expenses MPC $ 36 $ — $ 36 $ — Some charges incurred under the omnibus and ESA agreements are related to engineering services and are associated with assets under construction. These charges are added to “Property, plant and equipment, net” on the Consolidated Balance Sheets. For the three and nine months ended September 30, 2020 , these charges totaled $29 million and $80 million , respectively. For the three and nine months ended September 30, 2019 , these charges totaled $48 million and $127 million , respectively. Related Party Assets and Liabilities Assets and liabilities with related parties appearing on the Consolidated Balance Sheets are detailed in the table below. This table identifies the various components of related party assets and liabilities, including those associated with leases (see Note 20 for additional information) and deferred revenue on minimum volume commitments. If MPC fails to meet its minimum committed volumes, MPC will pay MPLX a deficiency payment based on the terms of the agreement. The deficiency amounts are recorded as “Current liabilities - related parties.” In many cases, MPC may then apply the amount of any such deficiency payments as a credit for volumes in excess of its minimum volume commitment in future periods under the terms of the applicable agreements. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes in excess of minimum quarterly volume commitments, where it is probable the customer will not use the credit in future periods or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Current liabilities - related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements or in some cases as an equity contribution from its sponsor. (In millions) September 30, 2020 December 31, 2019 Current assets - related parties Receivables - MPC $ 543 $ 621 Receivables - Other 7 22 Prepaid - MPC 11 9 Other - MPC 1 — Lease Receivables - MPC 29 4 Total 591 656 Noncurrent assets - related parties Long-term receivables - MPC 32 21 Right of use assets - MPC 231 232 Long-term lease receivables - MPC 390 43 Unguaranteed residual asset - MPC 22 7 Total 675 303 Current liabilities - related parties Payables - MPC 247 911 Payables - Other 42 37 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 54 42 Deferred revenue - Project reimbursements - MPC 19 16 Deferred revenue - Project reimbursements - Other 1 1 Total 364 1,008 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 230 230 Long-term deferred revenue - Project reimbursements - MPC 45 53 Long-term deferred revenue - Project reimbursements - Other 6 7 Total $ 281 $ 290 Other Related Party Transactions From time to time, MPLX may also sell to or purchase from related parties, assets and inventory at the lesser of average unit cost or net realizable value. Sales to and purchases from related parties were immaterial for the nine months ended September 30, 2020 and 2019 , respectively.

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Income Per Limited Partner Unit	Net Income/(Loss) Per Limited Partner Unit Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding. Classes of participating securities for the three and nine months ended September 30, 2020 and 2019 include: Nine Months Ended September 30, 2020 2019 Common Units ü ü Equity-based compensation awards ü ü Series A preferred units ü ü Series B preferred units ü ü For the three and nine months ended September 30, 2020 and 2019 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three and nine months ended September 30, 2020 and 2019 were less than 1 million . Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Net income/(loss) attributable to MPLX LP $ 665 $ 629 $ (1,411 ) $ 1,614 Less: Distributions declared on Series A preferred units (1) 20 20 61 61 Distributions declared on Series B preferred units (1) 10 10 31 31 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1)(2) 715 704 2,158 1,919 Undistributed net loss attributable to MPLX LP $ (80 ) $ (105 ) $ (3,661 ) $ (397 ) (1) See Note 7 for distribution information. (2) The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates. Three Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 715 $ 20 $ 10 $ 745 Undistributed net loss attributable to MPLX LP (80 ) — — (80 ) Net income attributable to MPLX LP (1) $ 635 $ 20 $ 10 $ 665 Weighted average units outstanding: Basic 1,046 Diluted 1,047 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended September 30, 2019 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 704 $ 20 $ 10 $ 734 Undistributed net loss attributable to MPLX LP (105 ) — — (105 ) Net income attributable to MPLX LP (1) $ 599 $ 20 $ 10 $ 629 Weighted average units outstanding: Basic (2) 974 Diluted (2) 975 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire three months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units. Nine Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 2,158 $ 61 $ 31 $ 2,250 Undistributed net loss attributable to MPLX LP (3,661 ) — — (3,661 ) Net (loss)/income attributable to MPLX LP (1) $ (1,503 ) $ 61 $ 31 $ (1,411 ) Weighted average units outstanding: Basic 1,054 Diluted 1,054 Net income attributable to MPLX LP per limited partner unit: Basic $ (1.43 ) Diluted $ (1.43 ) (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Nine Months Ended September 30, 2019 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,919 $ 61 $ 31 $ 2,011 Undistributed net loss attributable to MPLX LP (397 ) — — (397 ) Net income attributable to MPLX LP (1) $ 1,522 $ 61 $ 31 $ 1,614 Weighted average units outstanding: Basic (2) 855 Diluted (2) 855 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.78 Diluted $ 1.78 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire nine months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.

Equity

Equity	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Equity	Equity The changes in the number of common units outstanding during the nine months ended September 30, 2020 are summarized below: (In units) Common Balance at December 31, 2019 1,058,355,471 Unit-based compensation awards 395,091 Units redeemed in Wholesale Exchange (18,582,088 ) Balance at September 30, 2020 1,040,168,474 Wholesale Exchange and Merger In connection with the Wholesale Exchange as discussed in Note 3 , 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. In connection with the Merger and as discussed in Note 3 , each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units while ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. This resulted in the issuance of MPLX common units of approximately 102 million units to public unitholders and approximately 161 million units to MPC on July 30, 2019. Series B Preferred Units Prior to the Merger, ANDX had outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of ANDX at a price to the public of $1,000 per unit. Upon completion of the Merger, the ANDX preferred units converted to preferred units of MPLX representing substantially equivalent limited partnership interests in MPLX (the “Series B preferred units”). The Series B preferred units are pari passu with the Series A preferred units with respect to distribution rights and rights upon liquidation. Distributions on the Series B preferred units are payable semi-annually in arrears on the 15th day, or the first business day thereafter, of February and August of each year up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent . The changes in the Series B preferred unit balance from December 31, 2019 through September 30, 2020 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Equity of Predecessor” for the period prior to the Merger and within “Series B preferred units” for the period following the Merger. (In millions) Series B Preferred Units Balance at December 31, 2019 $ 611 Net income allocated 31 Distributions received by Series B preferred unitholders (41 ) Balance at September 30, 2020 $ 601 TexNew Mex Units - Prior to the Merger, MPC held 80,000 ANDX TexNew Mex units, representing all outstanding units. At the time of the Merger, each ANDX TexNew Mex unit was automatically converted into TexNew Mex units of MPLX with substantially the same rights and obligations as the ANDX TexNew Mex units. The TexNew Mex units represent the right to receive quarterly distribution payments in an amount calculated using the distributable cash flow generated by a particular portion of the TexNew Mex pipeline system, in excess of a base amount and adjusted for previously agreed upon stipulations and contingencies. Distributions earned by TexNew Mex units during the fourth quarter of 2019 and during the six months ended June 30, 2020 were immaterial. Distributions of $5 million were earned by TexNew Mex units during the three months ended September 30, 2020. Cash distributions – In accordance with the MPLX partnership agreement, on October 27, 2020 , MPLX declared a quarterly cash distribution for the third quarter of 2020 , totaling $715 million , or $0.6875 per common unit. This rate will also be received by Series A preferred unitholders. These distributions will be paid on November 13, 2020 to common unitholders of record on November 6, 2020 . Series B preferred unitholders are entitled to receive a fixed distribution of $68.75 per unit, per annum, payable semi-annually in arrears on February 15 and August 15, or the first business day thereafter, up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent , in each case assuming a distribution is declared by the Board of Directors. Accordingly, MPLX made a cash distribution payment totaling $21 million to Series B unitholders on August 17, 2020 . Quarterly distributions for 2020 and 2019 are summarized below: (Per common unit) 2020 2019 March 31, $ 0.6875 $ 0.6575 June 30, 0.6875 0.6675 September 30, $ 0.6875 $ 0.6775 The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three and nine months ended September 30, 2020 and 2019 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 715 $ 704 $ 2,158 $ 1,919 Series A preferred unit distributions 20 20 61 61 Series B preferred unit distributions 10 10 31 31 Total cash distributions declared $ 745 $ 734 $ 2,250 $ 2,011 The distribution on common units for the three and nine months ended September 30, 2019 includes the impact of the issuance of approximately 102 million units issued to public unitholders and approximately 161 million units issued to MPC in connection with the Merger. Due to the timing of the closing, distributions presented in the table above include second quarter distributions for MPLX common units and Series B preferred units issued to former ANDX unitholders in connection with the Merger. The distributions on common units exclude $12.5 million of waived distributions for the three months ended September 30, 2019 and $25 million

Series A Preferred Units

Series A Preferred Units	9 Months Ended
	Sep. 30, 2020
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units	Series A Preferred Units On May 13, 2016, MPLX LP issued approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The Series A preferred units rank senior to all common units and pari passu with all Series B preferred units with respect to distributions and rights upon liquidation. The holders of the Series A preferred units are entitled to receive, when and if declared by the board, a quarterly distribution equal to the greater of $0.528125 per unit or the amount of distributions they would have received on an as converted basis. On October 27, 2020 , MPLX declared a quarterly cash distribution of $0.6875 per common unit for the third quarter of 2020 . Holders of the Series A preferred units will receive the common unit rate in lieu of the lower $0.528125 base amount. The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate. Approximately 29.6 million Series A preferred units remaining outstanding as of September 30, 2020 . The changes in the redeemable preferred balance from December 31, 2019 through September 30, 2020 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2019 $ 968 Net income allocated 61 Distributions received by Series A preferred unitholders (61 ) Balance at September 30, 2020 $ 968 The Series A preferred units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside MPLX’s control. Therefore, they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The Series A preferred units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value and declared distributions decrease the carrying value of the Series A preferred units. As the Series A preferred units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the Series A preferred units would become redeemable.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Segment Information MPLX’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews MPLX’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. MPLX has two reportable segments: L&S and G&P. Each of these segments is organized and managed based upon the nature of the products and services it offers. • L&S – transports, stores, distributes and markets crude oil, asphalt, refined petroleum products and water. Also includes an inland marine business, terminals, rail facilities, storage caverns and refining logistics. • G&P – gathers, processes and transports natural gas; and gathers, transports, fractionates, stores and markets NGLs. Our CEO evaluates the performance of our segments using Segment Adjusted EBITDA. Amounts included in net income and excluded from Segment Adjusted EBITDA include: (i) depreciation and amortization; (ii) provision/(benefit) for income taxes; (iii) amortization of deferred financing costs; (iv) extinguishment of debt; (v) non-cash equity-based compensation; (vi) impairment expense; (vii) net interest and other financial costs; (viii) income/(loss) from equity method investments; (ix) distributions and adjustments related to equity method investments; (x) unrealized derivative gains/(losses); (xi) acquisition costs; (xii) noncontrolling interest; and (xiii) other adjustments as deemed necessary. These items are either: (i) believed to be non-recurring in nature; (ii) not believed to be allocable or controlled by the segment; or (iii) are not tied to the operational performance of the segment. The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 L&S Service revenue $ 989 $ 976 $ 2,924 $ 2,787 Rental income 249 304 737 935 Product related revenue 9 22 49 57 Income from equity method investments 36 60 126 159 Other income 51 17 154 45 Total segment revenues and other income (1) 1,334 1,379 3,990 3,983 Segment Adjusted EBITDA (2) 893 766 2,604 1,895 Restructuring expenses 27 — 27 — Capital expenditures 118 272 410 700 Investments in unconsolidated affiliates 4 95 132 163 G&P Service revenue 524 555 1,549 1,627 Rental income 94 88 271 260 Product related revenue 234 207 607 714 Income/(loss) from equity method investments 47 35 (1,138 ) 96 Other income 14 16 41 45 Total segment revenues and other income (1) 913 901 1,330 2,742 Segment Adjusted EBITDA (2) 442 399 1,252 1,120 Restructuring expenses 9 — 9 — Capital expenditures 131 321 375 953 Investments in unconsolidated affiliates $ 18 $ 76 $ 112 $ 331 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $139 million and $443 million for the three and nine months ended September 30, 2020 , respectively, and $182 million and $498 million for the three and nine months ended September 30, 2019 , respectively. Third party revenues for the G&P segment were $858 million and $1,181 million for the three and nine months ended September 30, 2020 , respectively, and $843 million and $2,581 million for the three and nine months ended September 30, 2019 , respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to net income. (In millions) September 30, 2020 December 31, 2019 Segment assets Cash and cash equivalents $ 28 $ 15 L&S 21,144 20,810 G&P 15,490 19,605 Total assets $ 36,662 $ 40,430 The table below provides a reconciliation between net (loss)/income and Segment Adjusted EBITDA. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 893 $ 766 $ 2,604 $ 1,895 G&P Segment Adjusted EBITDA 442 399 1,252 1,120 Total reportable segments 1,335 1,165 3,856 3,015 Depreciation and amortization (1) (346 ) (302 ) (992 ) (916 ) Provision for income taxes (1 ) (4 ) (1 ) (2 ) Amortization of deferred financing costs (15 ) (10 ) (44 ) (29 ) Non-cash equity-based compensation (4 ) (5 ) (12 ) (17 ) Impairment expense — — (2,165 ) — Net interest and other financial costs (223 ) (223 ) (647 ) (657 ) Gain on extinguishment of debt 14 — 14 — Income/(loss) from equity method investments 83 95 (1,012 ) 255 Distributions/adjustments related to equity method investments (130 ) (145 ) (369 ) (399 ) Unrealized derivative (losses)/gains (2) (10 ) 11 (1 ) 7 Acquisition costs — (9 ) — (14 ) Restructuring expenses (36 ) — (36 ) — Other (3 ) (1 ) (5 ) (1 ) Adjusted EBITDA attributable to noncontrolling interests 10 9 27 23 Adjusted EBITDA attributable to Predecessor (3) — 108 — 770 Net income/(loss) $ 674 $ 689 $ (1,387 ) $ 2,035 (1) Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019 , respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (3) The adjusted EBITDA adjustments related to Predecessor are excluded from adjusted EBITDA attributable to MPLX LP prior to the Merger.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) September 30, 2020 December 31, 2019 NGLs $ 3 $ 5 Line fill 11 10 Spare parts, materials and supplies 103 95 Total inventories $ 117 $ 110

Property, Plant and Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) Estimated Useful Lives September 30, 2020 December 31, 2019 L&S Pipelines 2-51 years $ 6,001 $ 5,572 Refining Logistics 13-40 years 2,196 2,870 Terminals 4-40 years 1,534 1,109 Marine 15-20 years 961 906 Land, building and other 1-61 years 1,576 1,817 Construction-in progress 403 660 Total L&S property, plant and equipment 12,671 12,934 G&P Gathering and transportation 5-40 years 7,413 7,159 Processing and fractionation 10-40 years 5,944 5,545 Land, building and other 3-40 years 509 484 Construction-in-progress 382 745 Total G&P property, plant and equipment 14,248 13,933 Total property, plant and equipment 26,919 26,867 Less accumulated depreciation (1) 5,304 4,722 Property, plant and equipment, net $ 21,615 $ 22,145 (1) The September 30, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below. Long-lived assets used in operations are assessed for impairment whenever changes in facts and circumstances indicate that the carrying value of the assets may not be recoverable based on the expected undiscounted future cash flow of an asset group. For purposes of impairment evaluation, long-lived assets must be grouped at the lowest level for which independent cash flows can be identified, which is at least at the segment level and in some cases for similar assets in the same geographic region where cash flows can be separately identified. If the sum of the undiscounted cash flows is less than the carrying value of an asset group, fair value is calculated, and the carrying value is written down if greater than the calculated fair value. No impairment triggers were identified in the second or third quarters of 2020; however, during the first quarter of 2020, we did identify an impairment trigger relating to asset groups within our Western G&P reporting unit as a result of significant impacts to forecasted cash flows for these asset groups resulting from the first quarter events and circumstances as discussed in Note 1 . The cash flows associated with these assets were significantly impacted by volume declines reflecting decreased forecasted producer customer production as a result of lower commodity prices. After assessing each asset group within the Western G&P reporting unit for impairment, only the East Texas G&P asset group had a carrying value in excess of the fair value of its underlying assets. As a result, an impairment of $174 million

Goodwill and Intangibles (Notes

Goodwill and Intangibles (Notes)	9 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets Disclosure [Text Block]	Goodwill and Intangibles Goodwill MPLX annually evaluates goodwill for impairment as of November 30, as well as whenever events or changes in circumstances indicate it is more likely than not that the fair value of a reporting unit with goodwill is less than its carrying amount. There were no events or changes in circumstances noted in the second or third quarters of 2020 which would indicate it is more likely than not that the fair value of our reporting units with goodwill is less than their carrying amount. During the first quarter of 2020, we determined that an interim impairment analysis of the goodwill recorded was necessary based on consideration of a number of first quarter events and circumstances as discussed in Note 1 . Our producer customers in our Eastern G&P region reduced production forecasts and drilling activity in response to the global economic downturn. Additionally, a decline in NGL prices impacted our future revenue forecast. After performing our evaluations related to the interim impairment of goodwill during the first quarter of 2020, we recorded an impairment of $1,814 million within the Eastern G&P reporting unit, which was recorded to “Impairment expense” on the Consolidated Statements of Income. The impairment was primarily driven by additional guidance related to the slowing of drilling activity, which has reduced production growth forecasts from our producer customers. This resulted in goodwill totaling approximately $7.7 billion as of March 31, 2020 within four reporting units. The fair value of the remaining reporting units with goodwill were in excess of their carrying value by percentages ranging from 8.5 percent to 270.0 percent . The reporting unit whose fair value exceeded its carrying amount by 8.5 percent , our Crude Gathering reporting unit, had goodwill totaling $1.1 billion at March 31, 2020. The operations which make up this reporting unit were acquired through the merger with ANDX. MPC accounted for its October 1, 2018 acquisition of Andeavor (through which it acquired control of ANDX), using the acquisition method of accounting, which required Andeavor assets and liabilities to be recorded by MPC at the acquisition date fair value. The Merger was closed on July 30, 2019 and has been treated as a common control transaction, which required the recognition of assets acquired and liabilities assumed using MPC’s historical carrying value. As such, given the short amount of time from when fair value was established to the date of the impairment test, the amount by which the fair value exceeded the carrying value within this reporting unit was not unexpected. Our reporting units are one level below our operating segments and are determined based on the way in which segment management operates and reviews each operating segment. The fair value of our six reporting units was determined based on applying both a discounted cash flow or income approach as well as a market approach. The discounted cash flow fair value estimate is based on known or knowable information at the measurement date. The significant assumptions that were used to develop the estimates of the fair values under the discounted cash flow method included management’s best estimates of the expected future results and discount rate, which ranged from 9.5 percent to 11.5 percent . Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As a result, there can be no assurance that the estimates and assumptions made for purposes of the interim goodwill impairment test will prove to be an accurate prediction of the future. The fair value measurements for the individual reporting units represent Level 3 measurements. After performing our evaluations related to the impairment of goodwill during the fiscal year ending December 31, 2019, we recorded an impairment of $1,197 million within the Western G&P reporting unit. The remainder of the reporting units’ fair values were in excess of their carrying values. The impairment was primarily driven by updated guidance related to the slowing of drilling activity, which has reduced production growth forecasts from our producer customers. This resulted in goodwill totaling approximately $9.5 billion as of December 31, 2019, with all but one of our six reporting units having goodwill. The changes in carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2018 $ 7,234 $ 2,912 $ 10,146 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2018 7,234 2,782 10,016 Impairment losses — (1,197 ) (1,197 ) Acquisitions 488 229 717 Balance as of December 31, 2019 7,722 1,814 9,536 Impairment losses — (1,814 ) (1,814 ) Wholesale Exchange (Note 3) (65 ) — (65 ) Balance as of September 30, 2020 7,657 — 7,657 Gross goodwill as of September 30, 2020 7,657 3,141 10,798 Accumulated impairment losses — (3,141 ) (3,141 ) Balance as of September 30, 2020 $ 7,657 $ — $ 7,657 Intangible Assets During the first quarter of 2020, we also determined that an impairment analysis of intangibles within our Western G&P reporting unit was necessary. See Note 11 for additional information regarding our assessment around the Western G&P reporting unit, and more specifically our East Texas G&P asset group. The fair value of the intangibles in our East Texas G&P asset group was determined based on applying the multi-period excess earnings method, which is an income approach. Key assumptions included management’s best estimates of the expected future cash flows from existing customers, customer attrition rates and the discount rate. After performing our evaluations related to the impairment of intangible assets associated with our East Texas G&P asset group during the first quarter of 2020, we recorded an impairment of $177 million to “Impairment expense” on the Consolidated Statements of Income related to our customer relationships. MPLX’s remaining intangible assets are comprised of customer contracts and relationships. Gross intangible assets with accumulated amortization as of September 30, 2020 and December 31, 2019 is shown below: September 30, 2020 December 31, 2019 (In millions) Useful Lives Gross Accumulated Amortization (1)(2) Net Gross Accumulated Amortization Net L&S 6 - 8 years $ 283 $ (72 ) $ 211 $ 283 $ (45 ) $ 238 G&P 6 - 25 years 1,288 (508 ) 780 1,288 (256 ) 1,032 $ 1,571 $ (580 ) $ 991 $ 1,571 $ (301 ) $ 1,270 (1) Amortization expense attributable to the G&P and L&S segments for the nine months ended September 30, 2020 was $75 million and $27 million , respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization. Estimated future amortization expense related to the intangible assets at September 30, 2020 is as follows: (In millions) 2020 $ 32 2021 128 2022 128 2023 128 2024 124 Thereafter 451 Total $ 991

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values – Recurring Fair value measurements and disclosures relate primarily to MPLX’s derivative positions as discussed in Note 14 . The following table presents the financial instruments carried at fair value on a recurring basis as of September 30, 2020 and December 31, 2019 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. September 30, 2020 December 31, 2019 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (61 ) $ — $ (60 ) Total carrying value on Consolidated Balance Sheets $ — $ (61 ) $ — $ (60 ) Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.46 to $0.97 per gallon with a weighted average of $0.58 per gallon and (2) the probability of renewal of 100 percent for the first five-year term and second five-year term of the gas purchase commitment and related keep-whole processing agreement, respectively. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability, respectively. Beyond the embedded derivative discussed above, we had no outstanding commodity contracts as of September 30, 2020 or December 31, 2019 . Changes in Level 3 Fair Value Measurements The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended September 30, 2020 Three Months Ended September 30, 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (51 ) $ — $ (65 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (12 ) — 9 Settlements — 2 — 2 Fair value at end of period — (61 ) — (54 ) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains/(losses) relating to liabilities still held at end of period $ — $ (11 ) $ — $ 9 Nine Months Ended September 30, 2020 Nine Months Ended September 30, 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (60 ) $ — $ (61 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (5 ) — 2 Settlements — 4 — 5 Fair value at end of period — (61 ) — (54 ) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains/(losses) relating to liabilities still held at end of period $ — $ (2 ) $ — $ 5 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income. Fair Values – Reported MPLX’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, lease receivables from related parties, accounts payable, payables to related parties and long-term debt. MPLX’s fair value assessment incorporates a variety of considerations, including (1) the duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. MPLX believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 14 ). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. The fair value of the SMR liability is estimated using a discounted cash flow approach based on the contractual cash flows and MPLX’s unsecured borrowing rate. The long-term debt and SMR liability fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of our debt, excluding finance leases, and SMR liability: September 30, 2020 December 31, 2019 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,721 $ 20,455 $ 21,054 $ 19,800 SMR liability $ 87 $ 76 $ 90 $ 80

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of September 30, 2020 , MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below. Embedded Derivative - MPLX has a natural gas purchase commitment embedded in a keep-whole processing agreement with a producer customer in the Southern Appalachian region expiring in December 2022. The customer has the unilateral option to extend the agreement for two consecutive five-year terms through December 2032. For accounting purposes, the natural gas purchase commitment and the term extending options have been aggregated into a single compound embedded derivative. The probability of the customer exercising its options is determined based on assumptions about the customer’s potential business strategy decision points that may exist at the time they would elect whether to renew the contract. The changes in fair value of this compound embedded derivative are based on the difference between the contractual and index pricing and the probability of the producer customer exercising its option to extend. The changes in fair value are recorded in earnings through “Purchased product costs” on the Consolidated Statements of Income. As of September 30, 2020 and December 31, 2019 , the estimated fair value of this contract was a liability of $61 million and $60 million , respectively. Certain derivative positions are subject to master netting agreements, therefore, MPLX has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of September 30, 2020 and December 31, 2019 , there were no derivative assets or liabilities that were offset on the Consolidated Balance Sheets. The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) September 30, 2020 December 31, 2019 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (4 ) $ — $ (5 ) Other noncurrent assets / Deferred credits and other liabilities — (57 ) — (55 ) Total $ — $ (61 ) $ — $ (60 ) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, see Note 13 . There were no material changes to MPLX’s policy regarding the accounting for Level 2 and Level 3 instruments as previously disclosed in MPLX’s Annual Report on Form 10-K for the year ended December 31, 2019 . MPLX does not designate any of its commodity derivative positions as hedges for accounting purposes. The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Purchased product costs Realized (loss)/gain $ (2 ) $ (2 ) $ (4 ) $ (5 ) Unrealized (loss)/gain (10 ) 11 (1 ) 7 Purchased product costs derivative (loss)/gain (12 ) 9 (5 ) 2 Total derivative (loss)/gain $ (12 ) $ 9 $ (5 ) $ 2

Debt

Debt	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings consist of the following: (In millions) September 30, 2020 December 31, 2019 MPLX LP: Bank revolving credit facility $ 95 $ — Term loan facility — 1,000 Floating rate senior notes 1,000 2,000 Fixed rate senior notes 19,506 16,887 Consolidated subsidiaries: MarkWest 23 23 ANDX 121 190 Financing lease obligations (1) 12 19 Total 20,757 20,119 Unamortized debt issuance costs (118 ) (106 ) Unamortized discount/premium (290 ) (300 ) Amounts due within one year (307 ) (9 ) Total long-term debt due after one year $ 20,042 $ 19,704 (1) See Note 20 for lease information. Credit Agreement Effective July 30, 2019, in connection with the closing of the Merger, MPLX amended and restated its revolving credit facility (as amended, the “MPLX Credit Agreement”) to, among other things, increase borrowing capacity to up to $3.5 billion and extend its term from July 2022 to July 2024. During the nine months ended September 30, 2020 , MPLX borrowed $2.995 billion under the MPLX Credit Agreement, at an average interest rate of 1.510 percent , and repaid $2.9 billion . At September 30, 2020 , MPLX had $95 million in outstanding borrowings and less than $1 million in letters of credit outstanding under the MPLX Credit Agreement, resulting in total availability of $3.405 billion , or 97.3 percent of the borrowing capacity . Term Loan Agreement On September 26, 2019, MPLX entered into a Term Loan Agreement which provides for a committed term loan facility for up to an aggregate of $1 billion . Borrowings under the Term Loan Agreement bear interest, at MPLX’s election, at either (i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement). On August 18, 2020 MPLX fully repaid the $1.0 billion of outstanding borrowings on the Term Loan Agreement, which resulted in the recognition of $1 million of unamortized issuance costs, which is included on the Consolidated Statements of Income as “Other financial costs”. Floating Rate Senior Notes On September 9, 2019, MPLX issued $2.0 billion aggregate principal amount of floating rate senior notes in a public offering, consisting of $1.0 billion aggregate principal amount of notes due September 2021 and $1.0 billion aggregate principal amount of notes due September 2022 (collectively, the “Floating Rate Senior Notes”). The Floating Rate Senior Notes were offered at a price to the public of 100 percent of par. The Floating Rate Senior Notes are callable, in whole or in part, at par plus accrued and unpaid interest at any time on or after September 10, 2020. Interest on the Floating Rate Senior Notes is payable quarterly in March, June, September and December, commencing on December 9, 2019. The interest rate applicable to the floating rate senior notes due September 2021 is LIBOR plus 0.9 percent per annum . The interest rate applicable to the floating rate senior notes due September 2022 is LIBOR plus 1.1 percent per annum . On September 14, 2020 MPLX redeemed, at par value, all of the $1.0 billion aggregate principal amount of notes due September 2021 which resulted in the recognition of $3 million of unamortized issuance costs, which is included on the Consolidated Statements of Income as “Other financial costs”. Fixed Rate Senior Notes MPLX’s senior notes, including those issued by consolidated subsidiaries, consist of various series of senior notes expiring between 2022 and 2058 with interest rates ranging from 1.750 percent to 6.250 percent . Interest on each series of notes is payable semi-annually in arrears on various dates depending on the series of the notes. On August 18, 2020 MPLX issued $3 billion aggregate principal amount of senior notes in a public offering, consisting of $1.5 billion aggregate principal amount of 1.750 percent senior notes due March 2026 and $1.5 billion aggregate principal amount of 2.650 percent senior notes due August 2030 (collectively, the “August 2020 New Senior Notes”). The August 2020 New Senior Notes were offered at a price to the public of 99.785 percent and 99.913 percent of par, respectively. Interest on each series of notes in the August 2020 New Senior Notes is payable semi-annually in arrears, commencing on March 1, 2021 for the senior notes due March 2026 and commencing on February 15, 2021 for the senior notes due August 2030. The net proceeds were used to repay the $1.0 billion of outstanding borrowings under the MPLX Term Loan Agreement, to repay the $1.0 billion floating rate notes due September 2021 and to redeem all of the $450 million aggregate principal amount of 6.375 percent senior notes due May 2024, $69 million of which was issued by ANDX. These notes were redeemed at 103.2 percent of the aggregate principal amount, which resulted in a payment of $14 million related to the note premium offset by the immediate recognition of $18 million of unamortized debt premium/discount and issuance costs, both of which are included on the Consolidated Statements of Income as “Other financial costs.” Proceeds were also used to reduce amounts outstanding under the MPLX Credit Agreement at the time and will be used to redeem the $300 million aggregate principal amount of 6.250 percent senior notes due October 15, 2022, including approximately $34 million in aggregate principal amount of senior notes issued by ANDX, in October of 2020. The 6.250 percent senior notes are included on the Consolidated Balance Sheets as “Long-term debt due within one year” at September 30, 2020.

Revenue

Revenue	9 Months Ended
	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Disaggregation of Revenue The following tables represent a disaggregation of revenue for each reportable segment for the three and nine months ended September 30, 2020 and 2019 : Three Months Ended September 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 87 $ 517 $ 604 Service revenue - related parties 902 7 909 Service revenue - product related — 41 41 Product sales 7 158 165 Product sales - related parties 2 35 37 Total revenues from contracts with customers $ 998 $ 758 1,756 Non-ASC 606 revenue (1) 491 Total revenues and other income $ 2,247 Three Months Ended September 30, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 95 $ 537 $ 632 Service revenue - related parties 881 18 899 Service revenue - product related — 26 26 Product sales 14 157 171 Product sales - related parties 8 24 32 Total revenues from contracts with customers $ 998 $ 762 1,760 Non-ASC 606 revenue (1) 520 Total revenues and other income $ 2,280 Nine Months Ended September 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 248 $ 1,531 $ 1,779 Service revenue - related parties 2,676 18 2,694 Service revenue - product related — 102 102 Product sales 39 415 454 Product sales - related parties 10 90 100 Total revenues from contracts with customers $ 2,973 $ 2,156 5,129 Non-ASC 606 loss (1) 191 Total revenues and other income $ 5,320 Nine Months Ended September 30, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 260 $ 1,605 $ 1,865 Service revenue - related parties 2,527 22 2,549 Service revenue - product related — 86 86 Product sales 40 536 576 Product sales - related parties 17 92 109 Total revenues from contracts with customers $ 2,844 $ 2,341 5,185 Non-ASC 606 revenue (1) 1,540 Total revenues and other income $ 6,725 (1) Non-ASC 606 Revenue includes rental income, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and MPLX’s rights to consideration for work completed exceeds the amount billed to the customer or for deficiency payments associated with minimum volume commitments which have not been billed to customers. Contract assets are generally classified as current and included in “Other current assets” on the Consolidated Balance Sheets. Contract liabilities, which we refer to as “Deferred revenue” and “Long-term deferred revenue,” typically relate to advance payments for aid in construction agreements and deferred customer credits associated with makeup rights and minimum volume commitments. Related to minimum volume commitments, breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. We classify contract liabilities as current or long-term based on the timing of when we expect to recognize revenue. “Receivables, net” primarily relate to our commodity sales. Portions of the “Receivables, net” balance are attributed to the sale of commodity product controlled by MPLX prior to sale while a significant portion of the balance relates to the sale of commodity product on behalf of our producer customers. Both types of transactions are commingled and excluded from the table below. MPLX remits the net sales price back to our producer customers upon completion of the sale. Each period end, certain amounts within accounts payable relate to our payments to producer customers. Such amounts are not deemed material at period end as a result of when we settle with each producer. The tables below reflect the changes in our contract balances for the nine -month periods ended September 30, 2020 and 2019: (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2020 Contract assets $ 39 $ (9 ) $ (1 ) $ 29 Deferred revenue 23 16 (6 ) 33 Deferred revenue - related parties 53 77 (60 ) 70 Long-term deferred revenue 90 25 — 115 Long-term deferred revenue - related parties $ 55 $ (7 ) $ — $ 48 (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2019 Contract assets $ 36 $ (6 ) $ (2 ) $ 28 Deferred revenue 13 11 (4 ) 20 Deferred revenue - related parties 65 34 (50 ) 49 Long-term deferred revenue 56 26 — 82 Long-term deferred revenue - related parties $ 52 $ 5 $ — $ 57 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations in the current periods. Remaining Performance Obligations The table below includes estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period. As of September 30, 2020 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $264 million and are reflected in the amounts below. This will be recognized as revenue as the obligations are satisfied, which is expected to occur over the next 30 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2020 $ 461 2021 1,727 2022 1,701 2023 1,591 2024 and thereafter 5,869 Total revenue on remaining performance obligations (1),(2),(3) $ 11,349 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above. We do not disclose information on the future performance obligations for any contract with an original expected duration of one year or less.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2020
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information Nine Months Ended September 30, (In millions) 2020 2019 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 631 $ 648 Income taxes paid 1 — Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories (1 ) 1 Fair value of common units redeemed for Wholesale Exchange $ 340 $ — The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Nine Months Ended September 30, (In millions) 2020 2019 (Decrease)/increase in capital accruals $ (197 ) $ (67 )

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX LP records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP LLC (“LOOP”) and Explorer Pipeline Company (“Explorer”) to their employees. MPLX LP is not a sponsor of these benefit plans. The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through September 30, 2020 . (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14 ) $ (1 ) $ (15 ) Other comprehensive loss - remeasurements (2) — (1 ) (1 ) Balance at September 30, 2020 (1) $ (14 ) $ (2 ) $ (16 ) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2018 through September 30, 2019 . (In millions) Pension Other Total Balance at December 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) Other comprehensive income - remeasurements (2) — 1 1 Balance at September 30, 2019 (1) $ (14 ) $ (1 ) $ (15 ) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.” (2)

Equity-Based Compensation Plan

Equity-Based Compensation Plan	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Share-based Payment Arrangement [Text Block]	Equity-Based Compensation Phantom Units The following is a summary of phantom unit award activity of MPLX LP common units for the nine months ended September 30, 2020 : Number Weighted Outstanding at December 31, 2019 1,109,568 $ 35.97 Granted 215,677 19.31 Settled (558,398 ) 37.73 Forfeited (5,383 ) 35.87 Outstanding at September 30, 2020 761,464 $ 29.96 Performance Units – MPLX grants performance units to certain officers of the general partner and certain eligible MPC officers who make significant contributions to its business. These performance units pay out 75 percent in cash and 25 percent in MPLX LP common units and often contain both market and performance conditions based on various metrics. Market conditions are valued using a Monte Carlo valuation while performance conditions are reevaluated periodically and valued at the compensation cost associated with the performance outcome deemed most probable. The performance units granted in 2020 are hybrid awards having a three-year performance period of January 1, 2020 through December 31, 2022. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s distributable cash flow, and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.80 per unit for the 2020 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. During the first quarter of 2018, a performance award was granted; however, due to the nature of the award terms, the grant date for this award was not established until the first quarter of 2020 and we began recognizing units and expense related to this award at that time. The performance units granted in 2018 are hybrid awards having a three-year performance period of January 1, 2018 through December 31, 2020. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on an average of MPLX LP’s distributable cash flow and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.45 per unit for the 2018 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the nine months ended September 30, 2020 : Number of Outstanding at December 31, 2019 2,157,347 Granted 2,147,211 Settled (1,169,354 ) Forfeited (31,668 ) Outstanding at September 30, 2020 3,103,536

Leases Leases (Notes)

Leases Leases (Notes)	9 Months Ended
	Sep. 30, 2020
Disclosure Text Block [Abstract]
Leases of Lessor Disclosure [Text Block]	Leases During the first quarter of 2020, reimbursements for projects at certain MPLX refining logistics locations were agreed to between MPLX and MPC. These reimbursements relate to the storage services agreements between MPLX and MPC at these locations and required the embedded leases within these agreements to be reassessed under the leasing standard. As a result of the reassessment, one of our leases was reclassified from an operating lease to a sales-type lease. As a result, the underlying assets previously shown on the Consolidated Balance Sheets associated with the sales-type lease were derecognized and the net investment in the lease (i.e., the sum of the present value of the future lease payments and the unguaranteed residual value of the assets) was recorded as a lease receivable. See Note 5 for the location of lease receivables and unguaranteed residual assets on the Consolidated Balance Sheets. The difference between the net book value of the underlying assets and the net investment in the lease has been recorded as a Contribution from MPC in the Consolidated Statements of Equity given that the transaction related to refining logistics was a common control transaction. During the first quarter of 2020, MPLX derecognized approximately $171 million of property, plant and equipment, recorded a lease receivable of approximately $370 million , recorded an unguaranteed residual asset of approximately $10 million and a Contribution from MPC of $209 million . Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended September 30, 2020 Three Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 208 $ 70 $ 248 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A Nine Months Ended September 30, 2020 Nine Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 589 $ 199 $ 773 $ 196 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 113 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A (1) These amounts are presented net of executory costs. See Note 5 for additional information on where related party lease assets are recorded in the Consolidated Balance Sheets. Third party lease assets are less than $1 million as of September 30, 2020 and are included within the “Receivables, net” and “Other noncurrent assets” captions within the Consolidated Balance Sheets. The following is a schedule of future payments on the sales-type leases as of September 30, 2020 : (In millions) Related Party 2020 $ 39 2021 157 2022 157 2023 158 2024 158 2025 and thereafter 473 Total minimum future rentals 1,142 Less: present value discount 723 Lease receivable $ 419

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and Contingencies MPLX is the subject of, or a party to, a number of pending or threatened legal actions, contingencies and commitments involving a variety of matters, including laws and regulations relating to the environment. Some of these matters are discussed below. For matters for which MPLX has not recorded a liability, MPLX is unable to estimate a range of possible loss because the issues involved have not been fully developed through pleadings, discovery or court proceedings. However, the ultimate resolution of some of these contingencies could, individually or in the aggregate, be material. Environmental Matters – MPLX is subject to federal, state and local laws and regulations relating to the environment. These laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites. Penalties may be imposed for non-compliance. At September 30, 2020 and December 31, 2019 , accrued liabilities for remediation totaled $18 million and $19 million , respectively. It is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, that may be imposed. At September 30, 2020 and December 31, 2019 , there were no balances with MPC for indemnification of environmental costs. MPLX is involved in environmental enforcement matters arising in the ordinary course of business. While the outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on its consolidated results of operations, financial position or cash flows. MPLX is also a party to a number of other lawsuits and other proceedings arising in the ordinary course of business. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these other lawsuits and proceedings will not, individually or collectively, have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, MPLX has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require MPLX to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. MPLX is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. In connection with our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL, we have entered into a Contingent Equity Contribution Agreement whereby MPLX LP, along with the other joint venture owners in the Bakken Pipeline system, has agreed to make equity contributions to the joint venture upon certain events occurring to allow the entities that own and operate the Bakken Pipeline system to satisfy their senior note payment obligations. The senior notes were issued to repay amounts owed by the pipeline companies to fund the cost of construction of the Bakken Pipeline system. In March 2020, the U.S. District Court for the District of Columbia (the “D.D.C.”) ordered the U.S. Army Corps of Engineers (“Army Corps”), which granted permits and an easement for the Bakken Pipeline system, to conduct a full environmental impact statement (“EIS”), and further requested briefing on whether an easement necessary for the operation of the Bakken Pipeline system should be vacated while the EIS is being prepared. On July 6, 2020, the D.D.C. ordered vacatur of the easement to cross Lake Oahe during the pendency of an EIS and further ordered a shut down of the pipeline by August 5, 2020. The D.D.C. denied a motion to stay that order. Dakota Access and the Army Corps appealed the D.D.C.’s order to the U.S. Court of Appeals for the District of Columbia Circuit (the “Court of Appeals”). On July 14, 2020, the Court of Appeals issued an administrative stay while the court considered Dakota Access and the Army Corps’ emergency motion for stay pending appeal. On August 5, 2020, the Court of Appeals stayed the D.D.C.’s injunction that required the pipeline be shutdown and emptied of oil by August 5, 2020. The Court of Appeals denied a stay of the D.D.C.’s March order, which required the EIS, and further denied a stay of the D.D.C.’s July order, which vacated the easement. In the D.D.C., briefing is ongoing for a renewed request for an injunction, which is expected to be completed by the end of 2020. Oral argument on the merits of the case at the Court of Appeals occurred on November 4, 2020. The pipeline remains operational. If the pipeline is temporarily shutdown pending completion of the EIS, MPLX would have to contribute its 9.19 percent pro rata share of funds required to pay interest accruing on the notes and any portion of the principal that matures while the pipeline is shutdown. MPLX also expects to contribute its 9.19 percent pro rata share of any costs to remediate any deficiencies to reinstate the permit and/or return the pipeline into operation. If the vacatur of the easement permit results in a permanent shutdown of the pipeline, MPLX would have to contribute its 9.19 percent pro rata share of the cost to redeem the bonds (including the 1% redemption premium required pursuant to the indenture governing the notes) and any accrued and unpaid interest. As of September 30, 2020 , our maximum potential undiscounted payments under the Contingent Equity Contribution Agreement were approximately $230 million . Other Legal Proceedings – In early July, MPLX received a Notification of Trespass Determination from the Bureau of Indian Affairs (“BIA”) relating to a portion of the Tesoro High Plains Pipeline that crosses the Fort Berthold Reservation in North Dakota. The notification covered the rights of way for 23 tracts of land and demanded the immediate cessation of pipeline operations. The notification also assessed trespass damages of approximately $187 million . We appealed this determination, which triggered an automatic stay of the requested pipeline shutdown and payment. On October 29, the Assistant Secretary - Indian Affairs issued an order vacating the BIA’s trespass order and requiring the Regional Director for the BIA Great Plains Region to issue a new decision on or before December 15 covering all 34 tracts at issue. We continue to work towards a settlement of this matter with holders of the property rights at issue. Management does not believe the ultimate resolution of this matter will have a material adverse effect on our consolidated financial position, results of operations, or cash flows. Contractual Commitments and Contingencies – At September 30, 2020 , MPLX’s contractual commitments to acquire property, plant and equipment totaled $181 million . These commitments were primarily related to G&P plant expansion, terminal and pipeline projects. In addition, from time to time and in the ordinary course of business, MPLX and its affiliates provide guarantees of MPLX’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require MPLX to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of September 30, 2020

Description of the Business a_2

Description of the Business and Basis of Presentation Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 30, 2019, MPLX completed its acquisition by merger (the “Merger”) of Andeavor Logistics LP (“ANDX”). At the effective time of the Merger, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units. ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. See Note 3 for additional information regarding the Merger. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (WRW”) to Western Refining Southwest, Inc. (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – The outbreak of COVID-19 and its development into a pandemic in March 2020 resulted in significant economic disruption globally. Actions taken by various governmental authorities, individuals and companies around the world to prevent the spread of COVID-19 through social distancing have restricted travel, many business operations, public gatherings and the overall level of individual movement and in-person interaction across the globe. Although there have been some signs of economic improvement, these events significantly reduced global economic activity and resulted in a decline in the demand for the midstream services we provide beginning with the first quarter of 2020. Macroeconomic conditions and global geopolitical events have also resulted in significant price volatility related to those aforementioned products. The overall deterioration in the economy and the environment in which MPLX and its customers operate, as well as a sustained decrease in unit price, were considered triggering events resulting in impairments of the carrying value of certain assets during the first quarter of 2020. We recognized impairments related to goodwill, certain equity method investments and certain long-lived assets (including intangibles), within our G&P segment. Many of our producer customers refined and updated production forecasts in response to the current environment, which impacted their current and expected future demand for our services, including the future utilization of our assets. Additionally, certain of our contracts have commodity price exposure, including NGL prices, which have experienced increased volatility as noted above. The table below provides information related to the impairments recognized during the first quarter of 2020 as well as the corresponding footnote where additional information can be found. No additional events or circumstances arose during the second or third quarters of 2020 which would indicate the need for any additional impairment beyond those recognized during the first quarter. (In millions) Impairment Footnote Reference Goodwill $ 1,814 12 Equity method investments 1,264 4 Intangibles, net 177 12 Property, plant and equipment, net 174 11 Total impairments $ 3,429 Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2019 . The results of operations for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the full year. In relation to the Merger described above and in Note 3 , ANDX’s assets, liabilities and results of operations prior to the Merger are collectively included in what we refer to as the “Predecessor” from October 1, 2018, which was the date that MPC acquired Andeavor. MPLX’s acquisition of ANDX is considered a transfer between entities under common control due to MPC’s relationship with ANDX prior to the Merger. As an entity under common control with MPC, MPLX recorded the assets acquired and liabilities assumed on its consolidated balance sheets at MPC’s historical carrying value. For the acquiring entity, transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. Accordingly, the accompanying financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of ANDX beginning October 1, 2018. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .
Use of Estimates, Policy [Policy Text Block]	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation.
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.

Net Income Per Limited Partne_2

Net Income Per Limited Partner Unit Accounting Policy (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.

Equity Method Investments (Tabl

Equity Method Investments (Tables)	9 Months Ended
	Sep. 30, 2020
Schedule of Equity Method Investments [Line Items]
Equity Method Investments [Table Text Block]	The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at September 30, September 30, December 31, (In millions, except ownership percentages) 2020 2020 2019 L&S MarEn Bakken Company LLC (1) 25% $ 469 $ 481 Illinois Extension Pipeline Company, L.L.C. 35% 261 265 LOOP LLC 41% 249 238 Andeavor Logistics Rio Pipeline LLC (2) 67% 195 202 Minnesota Pipe Line Company, LLC 17% 189 190 Whistler Pipeline LLC (2) 38% 184 134 Explorer Pipeline Company 25% 78 83 W2W Holdings LLC (2)(3) 50% 77 — Wink to Webster Pipeline LLC (2)(3) 15% — 126 Other (2) 100 55 Total L&S 1,802 1,774 G&P MarkWest Utica EMG, L.L.C. (2) 57% 720 1,984 Sherwood Midstream LLC (2) 50% 560 537 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 307 302 Rendezvous Gas Services, L.L.C. (2) 78% 164 170 Sherwood Midstream Holdings LLC (2) 51% 151 157 Centrahoma Processing LLC 40% 147 153 Other (2) 230 198 Total G&P 2,279 3,501 Total $ 4,081 $ 5,275 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL. (2) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (3) During the nine months ended September 30, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC.
Summarized Financial Information For Equity Method Investees Table [Table Text Block]	Summarized financial information for MPLX’s equity method investments for the nine months ended September 30, 2020 and 2019 is as follows: Nine Months Ended September 30, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ 132 $ 933 $ 1,065 Costs and expenses 308 405 713 Income from operations (176 ) 528 352 Net income (230 ) 477 247 (Loss)/income from equity method investments (1) $ (1,138 ) $ 126 $ (1,012 ) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million . Nine Months Ended September 30, 2019 (In millions) VIEs Non-VIEs Total Revenues and other income $ 479 $ 1,116 $ 1,595 Costs and expenses 251 434 685 Income from operations 228 682 910 Net income 192 605 797 Income from equity method investments (1) $ 89 $ 166 $ 255 (1) Includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for MPLX’s equity method investments as of September 30, 2020 and December 31, 2019 is as follows: September 30, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 774 $ 367 $ 1,141 Noncurrent assets 6,398 5,024 11,422 Current liabilities 304 194 498 Noncurrent liabilities $ 1,611 $ 857 $ 2,468 December 31, 2019 (In millions) VIEs Non-VIEs Total Current assets $ 534 $ 330 $ 864 Noncurrent assets 5,862 5,134 10,996 Current liabilities 192 245 437 Noncurrent liabilities $ 305 $ 822 $ 1,127

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Other Long-term Debt, Noncurrent	Activity on the MPC Loan Agreement was as follows: (In millions) Nine Months Ended September 30, 2020 Year Ended December 31, 2019 Borrowings $ 4,870 $ 8,540 Average interest rate of borrowings 2.380 % 3.441 % Repayments $ 5,464 $ 7,946 Outstanding balance at end of period (1) $ — $ 594 (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets. Prior to the Merger, ANDX was also party to a loan agreement with MPC (“ANDX-MPC Loan Agreement”). This facility was entered into on December 21, 2018, with a borrowing capacity of $500 million . In connection with the Merger, on July 31, 2019, MPLX repaid the entire outstanding balance and terminated the ANDX-MPC Loan Agreement. Activity on the ANDX-MPC Loan Agreement prior to the Merger was as follows: (In millions) Year Ended December 31, 2019 Borrowings $ 773 Average interest rate of borrowings 4.249 % Repayments $ 773 Outstanding balance at end of period $ —
Schedule of Sales to Related Parties	Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Service revenues - related parties MPC $ 908 $ 899 $ 2,692 $ 2,549 Other 1 — 2 — Total Service revenue - related parties 909 899 2,694 2,549 Rental income - related parties MPC 241 293 712 904 Product sales - related parties (1) MPC 37 32 100 109 Other income - related parties MPC 48 14 144 34 Other 15 17 46 50 Total Other income - related parties $ 63 $ 31 $ 190 $ 84 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2020 , these sales totaled $107 million and $332 million , respectively. For the three and nine months ended September 30, 2019 , these sales totaled $301 million and $819 million , respectively.
Schedule of Employee Services Expenses from Related Parties	Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Rental cost of sales - related parties MPC $ 32 $ 45 $ 119 $ 124 Purchases - related parties MPC 293 297 840 878 Other 4 6 13 16 Total Purchase - related parties 297 303 853 894 General and administrative expenses MPC 63 59 195 174 Restructuring expenses MPC $ 36 $ — $ 36 $ —
Schedule of Related Party Transactions [Table Text Block]	(In millions) September 30, 2020 December 31, 2019 Current assets - related parties Receivables - MPC $ 543 $ 621 Receivables - Other 7 22 Prepaid - MPC 11 9 Other - MPC 1 — Lease Receivables - MPC 29 4 Total 591 656 Noncurrent assets - related parties Long-term receivables - MPC 32 21 Right of use assets - MPC 231 232 Long-term lease receivables - MPC 390 43 Unguaranteed residual asset - MPC 22 7 Total 675 303 Current liabilities - related parties Payables - MPC 247 911 Payables - Other 42 37 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 54 42 Deferred revenue - Project reimbursements - MPC 19 16 Deferred revenue - Project reimbursements - Other 1 1 Total 364 1,008 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 230 230 Long-term deferred revenue - Project reimbursements - MPC 45 53 Long-term deferred revenue - Project reimbursements - Other 6 7 Total $ 281 $ 290

Net Income Per Limited Partne_3

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three and nine months ended September 30, 2020 and 2019 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three and nine months ended September 30, 2020 and 2019 were less than 1 million . Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Net income/(loss) attributable to MPLX LP $ 665 $ 629 $ (1,411 ) $ 1,614 Less: Distributions declared on Series A preferred units (1) 20 20 61 61 Distributions declared on Series B preferred units (1) 10 10 31 31 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1)(2) 715 704 2,158 1,919 Undistributed net loss attributable to MPLX LP $ (80 ) $ (105 ) $ (3,661 ) $ (397 ) (1) See Note 7 for distribution information. (2) The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates. Three Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 715 $ 20 $ 10 $ 745 Undistributed net loss attributable to MPLX LP (80 ) — — (80 ) Net income attributable to MPLX LP (1) $ 635 $ 20 $ 10 $ 665 Weighted average units outstanding: Basic 1,046 Diluted 1,047 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 715 $ 20 $ 10 $ 745 Undistributed net loss attributable to MPLX LP (80 ) — — (80 ) Net income attributable to MPLX LP (1) $ 635 $ 20 $ 10 $ 665 Weighted average units outstanding: Basic 1,046 Diluted 1,047 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended September 30, 2019 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 704 $ 20 $ 10 $ 734 Undistributed net loss attributable to MPLX LP (105 ) — — (105 ) Net income attributable to MPLX LP (1) $ 599 $ 20 $ 10 $ 629 Weighted average units outstanding: Basic (2) 974 Diluted (2) 975 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire three months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units. Nine Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 2,158 $ 61 $ 31 $ 2,250 Undistributed net loss attributable to MPLX LP (3,661 ) — — (3,661 ) Net (loss)/income attributable to MPLX LP (1) $ (1,503 ) $ 61 $ 31 $ (1,411 ) Weighted average units outstanding: Basic 1,054 Diluted 1,054 Net income attributable to MPLX LP per limited partner unit: Basic $ (1.43 ) Diluted $ (1.43 ) (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Nine Months Ended September 30, 2019 (In millions, except per unit data) Limited Partners’ Common Units Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,919 $ 61 $ 31 $ 2,011 Undistributed net loss attributable to MPLX LP (397 ) — — (397 ) Net income attributable to MPLX LP (1) $ 1,522 $ 61 $ 31 $ 1,614 Weighted average units outstanding: Basic (2) 855 Diluted (2) 855 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.78 Diluted $ 1.78 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire nine months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of common units outstanding during the nine months ended September 30, 2020 are summarized below: (In units) Common Balance at December 31, 2019 1,058,355,471 Unit-based compensation awards 395,091 Units redeemed in Wholesale Exchange (18,582,088 ) Balance at September 30, 2020 1,040,168,474 Wholesale Exchange and Merger In connection with the Wholesale Exchange as discussed in Note 3 , 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. In connection with the Merger and as discussed in Note 3 , each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units while ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. This resulted in the issuance of MPLX common units of approximately 102 million units to public unitholders and approximately 161 million units to MPC on July 30, 2019. Series B Preferred Units Prior to the Merger, ANDX had outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of ANDX at a price to the public of $1,000 per unit. Upon completion of the Merger, the ANDX preferred units converted to preferred units of MPLX representing substantially equivalent limited partnership interests in MPLX (the “Series B preferred units”). The Series B preferred units are pari passu with the Series A preferred units with respect to distribution rights and rights upon liquidation. Distributions on the Series B preferred units are payable semi-annually in arrears on the 15th day, or the first business day thereafter, of February and August of each year up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent . The changes in the Series B preferred unit balance from December 31, 2019 through September 30, 2020 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Equity of Predecessor” for the period prior to the Merger and within “Series B preferred units” for the period following the Merger. (In millions) Series B Preferred Units Balance at December 31, 2019 $ 611 Net income allocated 31 Distributions received by Series B preferred unitholders (41 ) Balance at September 30, 2020 $ 601
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three and nine months ended September 30, 2020 and 2019 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 715 $ 704 $ 2,158 $ 1,919 Series A preferred unit distributions 20 20 61 61 Series B preferred unit distributions 10 10 31 31 Total cash distributions declared $ 745 $ 734 $ 2,250 $ 2,011 The distribution on common units for the three and nine months ended September 30, 2019 includes the impact of the issuance of approximately 102 million units issued to public unitholders and approximately 161 million units issued to MPC in connection with the Merger. Due to the timing of the closing, distributions presented in the table above include second quarter distributions for MPLX common units and Series B preferred units issued to former ANDX unitholders in connection with the Merger. The distributions on common units exclude $12.5 million of waived distributions for the three months ended September 30, 2019 and $25 million

Series A Preferred Units (Table

Series A Preferred Units (Tables)	9 Months Ended
	Sep. 30, 2020
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2019 through September 30, 2020 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2019 $ 968 Net income allocated 61 Distributions received by Series A preferred unitholders (61 ) Balance at September 30, 2020 $ 968

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 L&S Service revenue $ 989 $ 976 $ 2,924 $ 2,787 Rental income 249 304 737 935 Product related revenue 9 22 49 57 Income from equity method investments 36 60 126 159 Other income 51 17 154 45 Total segment revenues and other income (1) 1,334 1,379 3,990 3,983 Segment Adjusted EBITDA (2) 893 766 2,604 1,895 Restructuring expenses 27 — 27 — Capital expenditures 118 272 410 700 Investments in unconsolidated affiliates 4 95 132 163 G&P Service revenue 524 555 1,549 1,627 Rental income 94 88 271 260 Product related revenue 234 207 607 714 Income/(loss) from equity method investments 47 35 (1,138 ) 96 Other income 14 16 41 45 Total segment revenues and other income (1) 913 901 1,330 2,742 Segment Adjusted EBITDA (2) 442 399 1,252 1,120 Restructuring expenses 9 — 9 — Capital expenditures 131 321 375 953 Investments in unconsolidated affiliates $ 18 $ 76 $ 112 $ 331 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $139 million and $443 million for the three and nine months ended September 30, 2020 , respectively, and $182 million and $498 million for the three and nine months ended September 30, 2019 , respectively. Third party revenues for the G&P segment were $858 million and $1,181 million for the three and nine months ended September 30, 2020 , respectively, and $843 million and $2,581 million for the three and nine months ended September 30, 2019 , respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to net income.
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	(In millions) September 30, 2020 December 31, 2019 Segment assets Cash and cash equivalents $ 28 $ 15 L&S 21,144 20,810 G&P 15,490 19,605 Total assets $ 36,662 $ 40,430
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The table below provides a reconciliation between net (loss)/income and Segment Adjusted EBITDA. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 893 $ 766 $ 2,604 $ 1,895 G&P Segment Adjusted EBITDA 442 399 1,252 1,120 Total reportable segments 1,335 1,165 3,856 3,015 Depreciation and amortization (1) (346 ) (302 ) (992 ) (916 ) Provision for income taxes (1 ) (4 ) (1 ) (2 ) Amortization of deferred financing costs (15 ) (10 ) (44 ) (29 ) Non-cash equity-based compensation (4 ) (5 ) (12 ) (17 ) Impairment expense — — (2,165 ) — Net interest and other financial costs (223 ) (223 ) (647 ) (657 ) Gain on extinguishment of debt 14 — 14 — Income/(loss) from equity method investments 83 95 (1,012 ) 255 Distributions/adjustments related to equity method investments (130 ) (145 ) (369 ) (399 ) Unrealized derivative (losses)/gains (2) (10 ) 11 (1 ) 7 Acquisition costs — (9 ) — (14 ) Restructuring expenses (36 ) — (36 ) — Other (3 ) (1 ) (5 ) (1 ) Adjusted EBITDA attributable to noncontrolling interests 10 9 27 23 Adjusted EBITDA attributable to Predecessor (3) — 108 — 770 Net income/(loss) $ 674 $ 689 $ (1,387 ) $ 2,035 (1) Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019 , respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (3) The adjusted EBITDA adjustments related to Predecessor are excluded from adjusted EBITDA attributable to MPLX LP prior to the Merger.

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) September 30, 2020 December 31, 2019 NGLs $ 3 $ 5 Line fill 11 10 Spare parts, materials and supplies 103 95 Total inventories $ 117 $ 110

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) Estimated Useful Lives September 30, 2020 December 31, 2019 L&S Pipelines 2-51 years $ 6,001 $ 5,572 Refining Logistics 13-40 years 2,196 2,870 Terminals 4-40 years 1,534 1,109 Marine 15-20 years 961 906 Land, building and other 1-61 years 1,576 1,817 Construction-in progress 403 660 Total L&S property, plant and equipment 12,671 12,934 G&P Gathering and transportation 5-40 years 7,413 7,159 Processing and fractionation 10-40 years 5,944 5,545 Land, building and other 3-40 years 509 484 Construction-in-progress 382 745 Total G&P property, plant and equipment 14,248 13,933 Total property, plant and equipment 26,919 26,867 Less accumulated depreciation (1) 5,304 4,722 Property, plant and equipment, net $ 21,615 $ 22,145 (1) The September 30, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below.

Goodwill and Intangibles (Table

Goodwill and Intangibles (Tables)	9 Months Ended
	Sep. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Schedule of Finite-Lived Intangible Assets [Table Text Block]	Gross intangible assets with accumulated amortization as of September 30, 2020 and December 31, 2019 is shown below: September 30, 2020 December 31, 2019 (In millions) Useful Lives Gross Accumulated Amortization (1)(2) Net Gross Accumulated Amortization Net L&S 6 - 8 years $ 283 $ (72 ) $ 211 $ 283 $ (45 ) $ 238 G&P 6 - 25 years 1,288 (508 ) 780 1,288 (256 ) 1,032 $ 1,571 $ (580 ) $ 991 $ 1,571 $ (301 ) $ 1,270 (1) Amortization expense attributable to the G&P and L&S segments for the nine months ended September 30, 2020 was $75 million and $27 million , respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization.
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	Estimated future amortization expense related to the intangible assets at September 30, 2020 is as follows: (In millions) 2020 $ 32 2021 128 2022 128 2023 128 2024 124 Thereafter 451 Total $ 991

Goodwill and Intangibles Goodwi

Goodwill and Intangibles Goodwill and Intangibles (Tables)	9 Months Ended
	Sep. 30, 2020
Goodwill [Line Items]
Schedule of Goodwill [Table Text Block]	The changes in carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2018 $ 7,234 $ 2,912 $ 10,146 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2018 7,234 2,782 10,016 Impairment losses — (1,197 ) (1,197 ) Acquisitions 488 229 717 Balance as of December 31, 2019 7,722 1,814 9,536 Impairment losses — (1,814 ) (1,814 ) Wholesale Exchange (Note 3) (65 ) — (65 ) Balance as of September 30, 2020 7,657 — 7,657 Gross goodwill as of September 30, 2020 7,657 3,141 10,798 Accumulated impairment losses — (3,141 ) (3,141 ) Balance as of September 30, 2020 $ 7,657 $ — $ 7,657

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value on a recurring basis as of September 30, 2020 and December 31, 2019 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. September 30, 2020 December 31, 2019 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (61 ) $ — $ (60 ) Total carrying value on Consolidated Balance Sheets $ — $ (61 ) $ — $ (60 )
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.46 to $0.97 per gallon with a weighted average of $0.58 per gallon and (2) the probability of renewal of 100 percent for the first five-year term and second five-year term of the gas purchase commitment and related keep-whole processing agreement, respectively. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability, respectively. Beyond the embedded derivative discussed above, we had no outstanding commodity contracts as of September 30, 2020 or December 31, 2019 .
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]	The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended September 30, 2020 Three Months Ended September 30, 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (51 ) $ — $ (65 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (12 ) — 9 Settlements — 2 — 2 Fair value at end of period — (61 ) — (54 ) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains/(losses) relating to liabilities still held at end of period $ — $ (11 ) $ — $ 9 Nine Months Ended September 30, 2020 Nine Months Ended September 30, 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (60 ) $ — $ (61 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (5 ) — 2 Settlements — 4 — 5 Fair value at end of period — (61 ) — (54 ) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains/(losses) relating to liabilities still held at end of period $ — $ (2 ) $ — $ 5 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
Fair Value Carrying Value by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of our debt, excluding finance leases, and SMR liability: September 30, 2020 December 31, 2019 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,721 $ 20,455 $ 21,054 $ 19,800 SMR liability $ 87 $ 76 $ 90 $ 80

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of September 30, 2020 , MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below.
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) September 30, 2020 December 31, 2019 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (4 ) $ — $ (5 ) Other noncurrent assets / Deferred credits and other liabilities — (57 ) — (55 ) Total $ — $ (61 ) $ — $ (60 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2020 2019 2020 2019 Purchased product costs Realized (loss)/gain $ (2 ) $ (2 ) $ (4 ) $ (5 ) Unrealized (loss)/gain (10 ) 11 (1 ) 7 Purchased product costs derivative (loss)/gain (12 ) 9 (5 ) 2 Total derivative (loss)/gain $ (12 ) $ 9 $ (5 ) $ 2

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	MPLX’s outstanding borrowings consist of the following: (In millions) September 30, 2020 December 31, 2019 MPLX LP: Bank revolving credit facility $ 95 $ — Term loan facility — 1,000 Floating rate senior notes 1,000 2,000 Fixed rate senior notes 19,506 16,887 Consolidated subsidiaries: MarkWest 23 23 ANDX 121 190 Financing lease obligations (1) 12 19 Total 20,757 20,119 Unamortized debt issuance costs (118 ) (106 ) Unamortized discount/premium (290 ) (300 ) Amounts due within one year (307 ) (9 ) Total long-term debt due after one year $ 20,042 $ 19,704 (1) See Note 20 for lease information.

Revenue (Tables)

Revenue (Tables)	9 Months Ended
	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	The following tables represent a disaggregation of revenue for each reportable segment for the three and nine months ended September 30, 2020 and 2019 : Three Months Ended September 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 87 $ 517 $ 604 Service revenue - related parties 902 7 909 Service revenue - product related — 41 41 Product sales 7 158 165 Product sales - related parties 2 35 37 Total revenues from contracts with customers $ 998 $ 758 1,756 Non-ASC 606 revenue (1) 491 Total revenues and other income $ 2,247 Three Months Ended September 30, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 95 $ 537 $ 632 Service revenue - related parties 881 18 899 Service revenue - product related — 26 26 Product sales 14 157 171 Product sales - related parties 8 24 32 Total revenues from contracts with customers $ 998 $ 762 1,760 Non-ASC 606 revenue (1) 520 Total revenues and other income $ 2,280 Nine Months Ended September 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 248 $ 1,531 $ 1,779 Service revenue - related parties 2,676 18 2,694 Service revenue - product related — 102 102 Product sales 39 415 454 Product sales - related parties 10 90 100 Total revenues from contracts with customers $ 2,973 $ 2,156 5,129 Non-ASC 606 loss (1) 191 Total revenues and other income $ 5,320 Nine Months Ended September 30, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 260 $ 1,605 $ 1,865 Service revenue - related parties 2,527 22 2,549 Service revenue - product related — 86 86 Product sales 40 536 576 Product sales - related parties 17 92 109 Total revenues from contracts with customers $ 2,844 $ 2,341 5,185 Non-ASC 606 revenue (1) 1,540 Total revenues and other income $ 6,725 (1) Non-ASC 606 Revenue includes rental income, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block]	The tables below reflect the changes in our contract balances for the nine -month periods ended September 30, 2020 and 2019: (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2020 Contract assets $ 39 $ (9 ) $ (1 ) $ 29 Deferred revenue 23 16 (6 ) 33 Deferred revenue - related parties 53 77 (60 ) 70 Long-term deferred revenue 90 25 — 115 Long-term deferred revenue - related parties $ 55 $ (7 ) $ — $ 48 (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2019 Contract assets $ 36 $ (6 ) $ (2 ) $ 28 Deferred revenue 13 11 (4 ) 20 Deferred revenue - related parties 65 34 (50 ) 49 Long-term deferred revenue 56 26 — 82 Long-term deferred revenue - related parties $ 52 $ 5 $ — $ 57 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations in the current periods.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	As of September 30, 2020 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $264 million and are reflected in the amounts below. This will be recognized as revenue as the obligations are satisfied, which is expected to occur over the next 30 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2020 $ 461 2021 1,727 2022 1,701 2023 1,591 2024 and thereafter 5,869 Total revenue on remaining performance obligations (1),(2),(3) $ 11,349 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2020
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information [Table Text Block]	Nine Months Ended September 30, (In millions) 2020 2019 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 631 $ 648 Income taxes paid 1 — Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories (1 ) 1 Fair value of common units redeemed for Wholesale Exchange $ 340 $ —
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures [Table Text Block]	The following is the change of additions to property, plant and equipment related to capital accruals: Nine Months Ended September 30, (In millions) 2020 2019 (Decrease)/increase in capital accruals $ (197 ) $ (67 )

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through September 30, 2020 . (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14 ) $ (1 ) $ (15 ) Other comprehensive loss - remeasurements (2) — (1 ) (1 ) Balance at September 30, 2020 (1) $ (14 ) $ (2 ) $ (16 ) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2018 through September 30, 2019 . (In millions) Pension Other Total Balance at December 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) Other comprehensive income - remeasurements (2) — 1 1 Balance at September 30, 2019 (1) $ (14 ) $ (1 ) $ (15 ) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.” (2)

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	9 Months Ended
	Sep. 30, 2020
Phantom Units
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units for the nine months ended September 30, 2020 : Number Weighted Outstanding at December 31, 2019 1,109,568 $ 35.97 Granted 215,677 19.31 Settled (558,398 ) 37.73 Forfeited (5,383 ) 35.87 Outstanding at September 30, 2020 761,464 $ 29.96
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	Performance Units – MPLX grants performance units to certain officers of the general partner and certain eligible MPC officers who make significant contributions to its business. These performance units pay out 75 percent in cash and 25 percent in MPLX LP common units and often contain both market and performance conditions based on various metrics. Market conditions are valued using a Monte Carlo valuation while performance conditions are reevaluated periodically and valued at the compensation cost associated with the performance outcome deemed most probable. The performance units granted in 2020 are hybrid awards having a three-year performance period of January 1, 2020 through December 31, 2022. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s distributable cash flow, and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.80 per unit for the 2020 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. During the first quarter of 2018, a performance award was granted; however, due to the nature of the award terms, the grant date for this award was not established until the first quarter of 2020 and we began recognizing units and expense related to this award at that time. The performance units granted in 2018 are hybrid awards having a three-year performance period of January 1, 2018 through December 31, 2020. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on an average of MPLX LP’s distributable cash flow and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.45 per unit for the 2018 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the nine months ended September 30, 2020 : Number of Outstanding at December 31, 2019 2,157,347 Granted 2,147,211 Settled (1,169,354 ) Forfeited (31,668 ) Outstanding at September 30, 2020 3,103,536

Leases Lessor Disclosure (Table

Leases Lessor Disclosure (Tables)	9 Months Ended
	Sep. 30, 2020
Lessor Disclosure [Abstract]
Operating Lease, Lease Income [Table Text Block]	Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended September 30, 2020 Three Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 208 $ 70 $ 248 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A Nine Months Ended September 30, 2020 Nine Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 589 $ 199 $ 773 $ 196 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 113 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A (1) These amounts are presented net of executory costs.
Sales-type Lease, Lease Income [Table Text Block]	Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended September 30, 2020 Three Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 208 $ 70 $ 248 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A Nine Months Ended September 30, 2020 Nine Months Ended September 30, 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 589 $ 199 $ 773 $ 196 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 113 — 3 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 N/A (1) These amounts are presented net of executory costs.
Sales-type and Direct Financing Leases, Lease Receivable, Maturity [Table Text Block]	The following is a schedule of future payments on the sales-type leases as of September 30, 2020 : (In millions) Related Party 2020 $ 39 2021 157 2022 157 2023 158 2024 158 2025 and thereafter 473 Total minimum future rentals 1,142 Less: present value discount 723 Lease receivable $ 419

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions	Jul. 31, 2020shares	Jul. 30, 2019shares	Mar. 31, 2020USD ($)	Sep. 30, 2020USD ($)shares	Dec. 31, 2019USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss				$ 1,814	$ 1,197
Equity Method Investment, Other than Temporary Impairment			$ 1,264	$ 1,264
Impairment of Intangible Assets, Finite-lived			177
Number of reportable segments				2
Impairment of Long-Lived Assets Held-for-use			174
Impairment Charges			3,429
G&P
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss			$ 1,814	$ 1,814	$ 1,197
ANDX LP [Member] | Public
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.135
ANDX LP [Member] | Nonpublic [Member]
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.0328
Western Refining Wholesale & Western Refining Product Transport [Member]
Description Of Business And Basis Of Presentation [Line Items]
Partners' Capital Account, Units, Redeemed | shares	18,582,088			18,582,088

Accounting Standards Impact of

Accounting Standards Impact of Adoption (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounting Changes and Error Corrections [Abstract]
Receivables, net	$ 477	$ 593
Accounts Receivable, Allowance for Credit Loss	$ 4

Acquisitions Andeavor Logistics

Acquisitions Andeavor Logistics (Details) - USD ($) $ in Millions		Jul. 30, 2019	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 09, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Cash and cash equivalents			$ 28				$ 28						$ 15
Receivables, net			477				477						593
Inventories			117				117						110
Other current assets			59				59						110
Assets, Current			1,272				1,272						1,484
Equity method investments			4,081				4,081						5,275
Property, plant and equipment, net			21,615				21,615						22,145
Intangibles, net			991				991						1,270
Other noncurrent assets			48				48						52
Assets			36,662				36,662						40,430
Accounts payable			140				140						242
Accrued Liabilities, Current			169				169						187
Accrued interest payable			207				207						210
Other current liabilities			158				158						127
Liabilities, Current			1,531				1,531						2,132
Long-term deferred revenue			291				291						217
Liabilities, Related Parties, Noncurrent			281				281						290
Long-term debt			20,042				20,042						19,704
Deferred credits and other liabilities			184				184						192
Liabilities			22,599				22,599						22,849
Series A preferred units			968				968						968
Total MPLX LP partners’ capital			12,848				12,848						16,364
Accumulated other comprehensive loss	[1]		(16)		$ (15)		(16)		$ (15)				(15)				$ (16)
Revenues			2,247		2,280		5,320		6,725
Goodwill			7,657				7,657					$ 7,700	9,536				10,016
Net Income (Loss) Attributable to Parent			665	[2]	629	[3]	(1,411)	[4]	1,614	[5]
Acquisition Costs, Period Cost			0		9		0		14
Noncontrolling interests			247				247						249
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			13,095		17,892		13,095		17,892		$ 13,262	$ 13,356	16,613		$ 17,611	$ 17,730	17,731
Liabilities and Equity			36,662				36,662						40,430
Net (loss)/income			674		689		(1,387)		2,035
Andeavor Logistics [Member]
Business Acquisition [Line Items]
Revenues					612				1,789
Acquisition Costs, Period Cost									14
Net (loss)/income					$ 191				$ 539
ANDX LP [Member] | Public
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.135
ANDX LP [Member] | Nonpublic [Member]
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.0328
Senior Notes [Member]
Business Acquisition [Line Items]
Long-term Debt, Gross														$ 2,000
L&S
Business Acquisition [Line Items]
Intangibles, net			211				211						238
Goodwill			7,657				$ 7,657						7,722				7,234
L&S | Minimum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							6 years
L&S | Maximum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							8 years
G&P
Business Acquisition [Line Items]
Intangibles, net			780				$ 780						1,032
Goodwill			$ 0				$ 0						$ 1,814				$ 2,782
G&P | Minimum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							6 years
G&P | Maximum [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life							25 years
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”
[2]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[3]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[4]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[5]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

Acquisitions Western Transporta

Acquisitions Western Transportation Reverse Drop (Details) - USD ($) $ in Millions	Jul. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Noncash or Part Noncash Divestitures [Line Items]
Goodwill		$ 7,657	$ 7,700	$ 9,536	$ 10,016
Western Refining Wholesale & Western Refining Product Transport [Member]
Noncash or Part Noncash Divestitures [Line Items]
Partners' Capital Account, Units, Contributed	18,582,088	18,582,088
Partner Capital, Units, contributed, fair value assigned	$ 340
Fair Value, Net Asset (Liability)	90
Excess of Fair Value over Book Value	250
Goodwill	$ 65

Equity Method Investments - Sum

Equity Method Investments - Summary of Equity Method Investment Financial Information (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019		Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Revenues		$ 2,247		$ 2,280	$ 5,320		$ 6,725
Equity method investments		4,081			4,081				$ 5,275
Costs and Expenses		1,348		1,354	6,029		4,002
Net (loss)/income		674		689	(1,387)		2,035
Income/(loss) from equity method investments(1)		83		95	(1,012)	[1],[2]	255
Assets, Current		1,272			1,272				1,484
Liabilities, Current		$ 1,531			1,531				2,132
Equity Method Investment, Other than Temporary Impairment			$ 1,264		$ 1,264
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%			25.00%
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%			35.00%
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%			41.00%
Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		38.00%			38.00%
W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%			50.00%
Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		15.00%			15.00%
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%			25.00%
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		57.00%			57.00%
Equity Method Investment, Other than Temporary Impairment			$ 1,251
Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%			50.00%
MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%			67.00%
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%			40.00%
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		9.19%			9.19%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		24.51%			24.51%
Direct Ownership Interest [Member] | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%			67.00%
Direct Ownership Interest [Member] | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		17.00%			17.00%
Direct Ownership Interest [Member] | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		78.00%			78.00%
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		51.00%			51.00%
G&P
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		$ (58)			$ (58)				1,000
Difference between carrying amount and underlying equity portion related to goodwill		31			31				498
G&P | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	720			720				1,984
G&P | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	560			560				537
G&P | MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	307			307				302
G&P | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	164			164				170
G&P | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	151			151				157
G&P | Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		147			147				153
G&P | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	230			230				198
L&S
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		331			331				329
Difference between carrying amount and underlying equity portion related to goodwill		167			167				167
L&S | MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	469			469				481
L&S | Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investments		261			261				265
L&S | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments		249			249				238
L&S | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	195			195				202
L&S | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		189			189				190
L&S | Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	184			184				134
L&S | W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3],[5]	77			77				0
L&S | Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3],[5]	0			0				126
L&S | Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investments		78			78				83
L&S | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	100			100				55
Operating Segments | G&P
Schedule of Equity Method Investments [Line Items]
Revenues	[6]	913		901	1,330		2,742
Equity method investments		2,279			2,279				3,501
Income/(loss) from equity method investments(1)		47		35	(1,138)		96
Operating Segments | L&S
Schedule of Equity Method Investments [Line Items]
Revenues	[6]	1,334		1,379	3,990		3,983
Equity method investments		1,802			1,802				1,774
Income/(loss) from equity method investments(1)		36		$ 60	126		159
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					1,065		1,595
Costs and Expenses					713		685
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					352		910
Net (loss)/income					247		797
Income/(loss) from equity method investments(1)					(1,012)	[7]	255	[8]
Assets, Current		1,141			1,141				864
Assets, Noncurrent		11,422			11,422				10,996
Liabilities, Current		498			498				437
Liabilities, Noncurrent		2,468			2,468				1,127
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					933		1,116
Costs and Expenses					405		434
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					528		682
Net (loss)/income					477		605
Income/(loss) from equity method investments(1)					126	[7]	166	[8]
Assets, Current		367			367				330
Assets, Noncurrent		5,024			5,024				5,134
Liabilities, Current		194			194				245
Liabilities, Noncurrent		857			857				822
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					132		479
Costs and Expenses					308		251
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					(176)		228
Net (loss)/income					(230)		192
Income/(loss) from equity method investments(1)					(1,138)	[7]	$ 89	[8]
Assets, Current		774			774				534
Assets, Noncurrent		6,398			6,398				5,862
Liabilities, Current		304			304				192
Liabilities, Noncurrent		$ 1,611			$ 1,611				$ 305
[1]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[2]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .
[3]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.
[4]	The investment in MarEn Bakken Company LLC includes our 9.19 percent
[5]	During the nine months ended September 30, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC.
[6]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $139 million and $443 million for the three and nine months ended September 30, 2020 , respectively, and $182 million and $498 million for the three and nine months ended September 30, 2019 , respectively. Third party revenues for the G&P segment were $858 million and $1,181 million for the three and nine months ended September 30, 2020 , respectively, and $843 million and $2,581 million for the three and nine months ended September 30, 2019 , respectively.
[7]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million .
[8]	Includes the impact of any basis differential amortization or accretion.

Related Party Agreements and _3

Related Party Agreements and Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Restructuring Charges	$ 36	$ 0	$ 36	$ 0
MPC Investment [Member]
Related Party Transaction [Line Items]
Restructuring Charges	$ 36

Related Party Agreements and _4

Related Party Agreements and Transactions MPC Loan Agreement (Details) - Related Party Revolving Credit Agreement [Member] - MPC Investment [Member] - USD ($) $ in Millions		Jul. 31, 2019	Apr. 27, 2018	Sep. 30, 2020	Dec. 31, 2019	Dec. 21, 2018
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 1,500
Debt Instrument, Description of Variable Rate Basis		LIBOR plus 1.25 percent	LIBOR plus 1.50 percent
Proceeds from Lines of Credit				$ 4,870	$ 8,540
Line of Credit Facility, Interest Rate During Period				2.38%	3.441%
Repayments of Lines of Credit				$ 5,464	$ 7,946
Long-term Line of Credit	[1]			$ 0	594
ANDX LP [Member]
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity						$ 500
Proceeds from Lines of Credit					$ 773
Line of Credit Facility, Interest Rate During Period					4.249%
Repayments of Lines of Credit					$ 773
Long-term Line of Credit	[1]				$ 0
[1]	Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.

Sales to Related Parties (Detai

Sales to Related Parties (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Rental income - related parties		$ 241	$ 293	$ 712	$ 904
Other income - related parties		63	31	190	84
MPC
Related Party Transaction [Line Items]
Rental income - related parties		241	293	712	904
Other income - related parties		48	14	144	34
Product sales to MPC that net to zero		107	301	332	819
Other Affiliates [Member]
Related Party Transaction [Line Items]
Other income - related parties		15	17	46	50
Product [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		37	32	100	109
Product [Member] | MPC
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[1]	37	32	100	109
Service [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		909	899	2,694	2,549
Service [Member] | MPC
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		908	899	2,692	2,549
Service [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		$ 1	$ 0	$ 2	$ 0
[1]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2020 , these sales totaled $107 million and $332 million , respectively. For the three and nine months ended September 30, 2019 , these sales totaled $301 million and $819 million , respectively.

Summary of Charges for Employee

Summary of Charges for Employee Services and Omnibus Agreements (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Restructuring Charges	$ 36	$ 0	$ 36	$ 0
Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	297	303	853	894
MPC | Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	32	45	119	124
MPC | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	293	297	840	878
MPC | General and administrative expenses
Related Party Transaction [Line Items]
Labor and Related Expense	63	59	195	174
Other Affiliates [Member] | Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	4	6	13	16
Construction-in-progress | MPC
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 29	$ 48	$ 80	$ 127

Other Assets and Liabilities fr

Other Assets and Liabilities from Related Parties (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Other current assets	$ 59	$ 110
Current Assets, Related Parties	591	656
Operating Lease, Right-of-Use Asset	323	365
Other Assets, Related Parties, Noncurrent	675	303
Operating Lease, Liability, Current	65	66
Current Liabilities, Related Parties	364	1,008
Operating Lease, Liability, Noncurrent	258	302
Liabilities, Related Parties, Noncurrent	281	290
MPC
Related Party Transaction [Line Items]
Receivables - related parties	543	621
Prepaid Insurance	11	9
Other current assets	1	0
Sales-type Lease, Interest Income	29	4
Sales-type Lease, Unguaranteed Residual Asset	22	7
Due from Related Parties, Noncurrent	32	21
Operating Lease, Right-of-Use Asset	231	232
Net Investment in Lease, Noncurrent	390	43
Due to Related Parties, Current	247	911
Operating Lease, Liability, Current	1	1
Operating Lease, Liability, Noncurrent	230	230
Other Affiliates [Member]
Related Party Transaction [Line Items]
Receivables - related parties	7	22
Due to Related Parties, Current	42	37
Minimum Committed Volume Contracts [Member] | MPC
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	54	42
Reimbursable Projects [Member] | MPC
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	19	16
Deferred Revenue, Noncurrent, Related Parties	45	53
Other Noncurrent Liabilities [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	1	1
Other Noncurrent Liabilities [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Deferred Revenue, Noncurrent, Related Parties	$ 6	$ 7

Net Income Per Limited Partne_4

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions		3 Months Ended								9 Months Ended
		Sep. 30, 2020		Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020		Sep. 30, 2019
Net Income Per Share [Line Items]
Potentially dilutive units		1				1				1		1
Net income attributable to MPLX LP		$ 665,000,000	[1]			$ 629,000,000	[2]			$ (1,411,000,000)	[3]	$ 1,614,000,000	[4]
Less: Distribution declared		745,000,000				734,000,000				2,250,000,000		2,011,000,000
Undistributed net income (loss) attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
Undistributed net loss attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
Capital Contribution By Related Party		13,000,000		$ 6,000,000	$ 225,000,000	295,000,000		$ 13,000,000	$ 15,000,000
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		635,000,000	[1]			599,000,000	[2]			(1,503,000,000)	[3]	1,522,000,000	[4]
Less: Distribution declared	[5]	715,000,000				704,000,000	[6]			2,158,000,000		1,919,000,000	[6]
Undistributed net income (loss) attributable to MPLX LP		(80,000,000)				(105,000,000)				(3,661,000,000)		(397,000,000)
MPC | Limited Partners Common Units
Net Income Per Share [Line Items]
Capital Contribution By Related Party		13,000,000		$ 6,000,000	$ 225,000,000	292,000,000		$ 0	$ 0
MPC | ANDX LP [Member]
Net Income Per Share [Line Items]
Capital Contribution By Related Party						12,500,000						25,000,000
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		20,000,000	[1]			20,000,000	[2]			61,000,000	[3]	61,000,000	[4]
Less: Distribution declared	[5]	20,000,000				20,000,000				61,000,000		61,000,000
Undistributed net income (loss) attributable to MPLX LP		0				0				0		0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP		10,000,000	[1]			10,000,000	[2]			31,000,000	[3]	31,000,000	[4]
Less: Distribution declared	[5]	10,000,000				10,000,000				31,000,000		31,000,000
Undistributed net income (loss) attributable to MPLX LP		$ 0				$ 0				$ 0		$ 0
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[3]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[4]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[5]	See Note 7 for distribution information.
[6]	The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates.

Net Income Per Limited Partne_5

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Potentially dilutive units		1		1		1		1
Distribution Made to Limited Partner, Cash Distributions Declared		$ 745		$ 734		$ 2,250		$ 2,011
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(80)		(105)		(3,661)		(397)
Net income attributable to MPLX LP		665	[1]	629	[2]	(1,411)	[3]	1,614	[4]
Limited Partners Common Units
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[5]	715		704	[6]	2,158		1,919	[6]
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		(80)		(105)		(3,661)		(397)
Net income attributable to MPLX LP		$ 635	[1]	$ 599	[2]	$ (1,503)	[3]	$ 1,522	[4]
Weighted average units outstanding:
Common - basic (in shares)		1,046		974	[7]	1,054		855	[8]
Common - diluted (in shares)		1,047		975	[7]	1,054		855	[8]
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
Diluted (in USD per unit)		$ 0.61		$ 0.61		$ (1.43)		$ 1.78
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[5]	$ 20		$ 20		$ 61		$ 61
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Net income attributable to MPLX LP		20	[1]	20	[2]	61	[3]	61	[4]
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[5]	10		10		31		31
Net income (loss) attributable to MPLX LP:
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Net income attributable to MPLX LP		$ 10	[1]	$ 10	[2]	$ 31	[3]	$ 31	[4]
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[3]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[4]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[5]	See Note 7 for distribution information.
[6]	The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates.
[7]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire three months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.
[8]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire nine months ended September 30, 2019. See Notes 3 and 7 for additional information about the treatment of these units.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($)		Oct. 27, 2020	Aug. 17, 2020	Jul. 31, 2020	Jul. 30, 2019	Jul. 29, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019		Dec. 31, 2019	Dec. 31, 2018
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							$ (654,000,000)	$ (634,000,000)	$ 2,736,000,000	$ (669,000,000)		$ (636,000,000)	$ (669,000,000)
Partners, Total Subsequent Distribution Amount							745,000,000			734,000,000				$ 2,250,000,000	$ 2,011,000,000
Contributions from:							13,000,000	6,000,000	225,000,000	295,000,000		13,000,000	15,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(736,000,000)	(728,000,000)	(738,000,000)	(715,000,000)		(764,000,000)	(776,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							13,095,000,000	13,262,000,000	13,356,000,000	17,892,000,000		17,611,000,000	17,730,000,000	13,095,000,000	17,892,000,000		$ 16,613,000,000	$ 17,731,000,000
Distribution Made to Limited Partner, Cash Distributions Declared							745,000,000			734,000,000				2,250,000,000	2,011,000,000
Preferred Class B [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							(10,000,000)	(10,000,000)	(11,000,000)	(7,000,000)		0	0	(31,000,000)
Contributions from:							0	0	0	0		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(20,000,000)	0	(21,000,000)	(21,000,000)		0	0	(41,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 601,000,000	611,000,000	$ 601,000,000	601,000,000		0	0	$ 601,000,000	601,000,000		$ 611,000,000	0
Limited Partners Common Units
Stockholders Equity [Line Items]
Balance at December 31, 2019									1,058,355,471					1,058,355,471
Unit-based compensation awards														395,091
Balance at September 30, 2020							1,040,168,474							1,040,168,474
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						$ 715,000,000			704,000,000	[2]			$ 2,158,000,000	1,919,000,000	[2]
Andeavor Logistics [Member]
Stockholders Equity [Line Items]
Preferred Units, Outstanding						600,000
ANDX LP [Member]
Stockholders Equity [Line Items]
Dividend rate, percentage						6.875%
Price per share						$ 1,000
Subsequent Event | Limited Partners Common Units
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared		$ 715,000,000
Limited Partners Common Units
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount							715,000,000			704,000,000				2,158,000,000	1,919,000,000
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						10,000,000			10,000,000				$ 31,000,000	31,000,000
Series B Preferred Stock [Member] | MPLX LP
Stockholders Equity [Line Items]
Dividend rate, percentage														4.652%
Preferred Stock, Dividend Payment Rate, Variable														LIBOR plus
Preferred Class B [Member]
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount							10,000,000			10,000,000				$ 31,000,000	31,000,000
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders			$ (21,000,000)
Preferred Class A [Member]
Stockholders Equity [Line Items]
Partners, Total Subsequent Distribution Amount							20,000,000			20,000,000				$ 61,000,000	61,000,000
MPC | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							$ 399,000,000	388,000,000	$ (1,733,000,000)	(380,000,000)		293,000,000	307,000,000
Units issued					161,000,000		647,000,000							647,000,000			666,000,000
Contributions from:							$ 13,000,000	6,000,000	225,000,000	292,000,000		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(445,000,000)	(458,000,000)	(446,000,000)	(432,000,000)		(332,000,000)	(327,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 2,827,000,000	2,951,000,000	3,014,000,000	5,767,000,000		(1,671,000,000)	(1,632,000,000)	$ 2,827,000,000	5,767,000,000		$ 4,968,000,000	(1,612,000,000)
Units Outstanding							647,000,000							647,000,000			666,000,000
MPC | ANDX LP [Member]
Stockholders Equity [Line Items]
Contributions from:										12,500,000					25,000,000
Public | Limited Partners Common Units
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity							$ 236,000,000	229,000,000	(1,022,000,000)	(222,000,000)		168,000,000	176,000,000
Units issued					102,000,000		393,000,000							393,000,000			392,000,000
Contributions from:							$ 0	0	0	0		0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders							(271,000,000)	(270,000,000)	(271,000,000)	(262,000,000)		(191,000,000)	(188,000,000)
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 9,436,000,000	$ 9,469,000,000	$ 9,509,000,000	$ 11,289,000,000		$ 8,305,000,000	$ 8,326,000,000	$ 9,436,000,000	$ 11,289,000,000		$ 10,800,000,000	$ 8,336,000,000
Units Outstanding							393,000,000							393,000,000			392,000,000
Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Contributed				(18,582,088)										(18,582,088)
Nonpublic [Member] | ANDX LP [Member]
Stockholders Equity [Line Items]
Common Units Conversion Ratio - ANDX to MPLX					1.0328
Public | ANDX LP [Member]
Stockholders Equity [Line Items]
Common Units Conversion Ratio - ANDX to MPLX					1.135
TexNew Mex units [Member] | MPLX LP
Stockholders Equity [Line Items]
Units Outstanding					80,000
Distribution Made to Limited Partner, Cash Distributions Declared							$ 5,000,000
[1]	See Note 7 for distribution information.
[2]	The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, shares in Millions		Nov. 13, 2020	Nov. 06, 2020	Oct. 27, 2020	Aug. 17, 2020	Jul. 29, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019		Dec. 31, 2019	Jul. 30, 2019	Dec. 31, 2018
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:							$ 13,000,000	$ 6,000,000	$ 225,000,000	$ 295,000,000		$ 13,000,000	$ 15,000,000
Distribution Made to Limited Partner, Cash Distributions Declared							745,000,000			734,000,000				$ 2,250,000,000	$ 2,011,000,000
Partners' Capital Account, Distributions							736,000,000	728,000,000	738,000,000	715,000,000		764,000,000	776,000,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							13,095,000,000	$ 13,262,000,000	$ 13,356,000,000	17,892,000,000		$ 17,611,000,000	$ 17,730,000,000	13,095,000,000	17,892,000,000		$ 16,613,000,000		$ 17,731,000,000
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date					Aug. 17, 2020
Partners' Capital Account, Distributions					$ 21,000,000
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Declaration Date				Oct. 27, 2020
Cash distributions declared per limited partner common unit				$ 0.6875
Date of record			Nov. 6, 2020
Subsequent Event | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date		Nov. 13, 2020
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						$ 715,000,000			$ 704,000,000	[2]			2,158,000,000	1,919,000,000	[2]
Cash distributions declared per limited partner common unit							$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6775		$ 0.6675	$ 0.6575
Limited Partners Common Units | Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared				$ 715,000,000
Series A Preferred Stock [Member] | Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]						$ 10,000,000			$ 10,000,000				31,000,000	31,000,000
Series A Preferred Stock [Member] | Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Cash distributions declared per limited partner common unit							$ 68.75
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:							$ 0	$ 0	$ 0	0		$ 0	$ 0
Partners' Capital Account, Distributions							20,000,000	0	21,000,000	21,000,000		0	0	41,000,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 601,000,000	611,000,000	601,000,000	601,000,000		0	0	$ 601,000,000	601,000,000		$ 611,000,000		0
MPC | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited Partners' Capital Account, Units Issued							647							647			666	161
Contributions from:							$ 13,000,000	6,000,000	225,000,000	292,000,000		0	0
Partners' Capital Account, Distributions							445,000,000	458,000,000	446,000,000	432,000,000		332,000,000	327,000,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							2,827,000,000	2,951,000,000	3,014,000,000	5,767,000,000		(1,671,000,000)	(1,632,000,000)	$ 2,827,000,000	5,767,000,000		$ 4,968,000,000		(1,612,000,000)
MPC | ANDX LP [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contributions from:										12,500,000					25,000,000
ANDX LP [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Preferred Stock, Dividend Rate, Percentage						6.875%
MPLX LP | Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Preferred Stock, Dividend Payment Rate, Variable														LIBOR plus
Preferred Stock, Dividend Rate, Percentage														4.652%
TexNew Mex units [Member] | MPLX LP
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared							$ 5,000,000
Public | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited Partners' Capital Account, Units Issued							393							393			392	102
Contributions from:							$ 0	0	0	0		0	0
Partners' Capital Account, Distributions							271,000,000	270,000,000	271,000,000	262,000,000		191,000,000	188,000,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest							$ 9,436,000,000	$ 9,469,000,000	$ 9,509,000,000	$ 11,289,000,000		$ 8,305,000,000	$ 8,326,000,000	$ 9,436,000,000	$ 11,289,000,000		$ 10,800,000,000		$ 8,336,000,000
[1]	See Note 7 for distribution information.
[2]	The three and nine months ended September 30, 2019 amounts are net of $12.5 million and $25 million , respectively, of waived distributions with respect to units held by MPC and its affiliates.

Series A Preferred Units (Narra

Series A Preferred Units (Narrative) (Details) - $ / shares shares in Millions	Oct. 27, 2020	May 13, 2016	Sep. 30, 2020
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units		30.8
Dividend rate, percentage		6.50%
Price per share		$ 32.50
Dividend rate, per-dollar-amount		$ 0.528125
Description		The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate.
Partners' Capital Account, Units			29.6
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Declaration Date	Oct. 27, 2020
Distribution Made to Limited Partner, Distributions Declared, Per Unit	$ 0.6875

Series A Preferred Units (Rollf

Series A Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2019	$ 968
Distributions to unitholders and general partner	(2,162)	$ (1,731)
Balance at September 30, 2020	968
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2019	968
Net income allocated	61
Distributions to unitholders and general partner	(61)
Balance at September 30, 2020	$ 968

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Number of reportable segments	2

Segment Information - Segment A

Segment Information - Segment Adjusted EBITDA (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Segment Reporting Information [Line Items]
Restructuring Charges		$ 36	$ 0		$ 36		$ 0
Depreciation and amortization	[1]	346	302		992		916
Total revenues from contracts with customers		1,756	1,760		5,129		5,185
Rental income		102	99		296		291
Income/(loss) from equity method investments(1)		83	95		(1,012)	[2],[3]	255
Other income		2	2		5		6
Total segment revenues and other income		(2,247)	(2,280)		(5,320)		(6,725)
Investments in unconsolidated affiliates					(244)		(494)
L&S
Segment Reporting Information [Line Items]
Depreciation and amortization		164	113		440		373
Total revenues from contracts with customers		998	998		2,973		2,844
Investments in unconsolidated affiliates		(4)	(95)		(132)		(163)
G&P
Segment Reporting Information [Line Items]
Depreciation and amortization		182	189		552		543
Total revenues from contracts with customers		758	762		2,156		2,341
Investments in unconsolidated affiliates		(18)	(76)		(112)		(331)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA		1,335	1,165		3,856		3,015
Operating Segments | L&S
Segment Reporting Information [Line Items]
Restructuring Charges		27	0		27		0
Rental income		249	304		737		935
Income/(loss) from equity method investments(1)		36	60		126		159
Other income		51	17		154		45
Total segment revenues and other income	[4]	(1,334)	(1,379)		(3,990)		(3,983)
Adjusted EBITDA	[5]	893	766		2,604		1,895
Capital Expenditure		118	272		410		700
Operating Segments | G&P
Segment Reporting Information [Line Items]
Restructuring Charges		9	0		9		0
Rental income		94	88		271		260
Income/(loss) from equity method investments(1)		47	35		(1,138)		96
Other income		14	16		41		45
Total segment revenues and other income	[4]	(913)	(901)		(1,330)		(2,742)
Adjusted EBITDA	[5]	442	399		1,252		1,120
Capital Expenditure		131	321		375		953
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA attributable to Predecessor		0	108	[6]	0		770	[6]
Service [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		604	632		1,779		1,865
Service [Member] | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		87	95		248		260
Service [Member] | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		517	537		1,531		1,605
Service [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		989	976		2,924		2,787
Service [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		524	555		1,549		1,627
Product [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		165	171		454		576
Product [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		9	22		49		57
Product [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		234	207		607		714
Third Party [Member] | L&S
Segment Reporting Information [Line Items]
Total segment revenues and other income		(139)	(182)		(443)		(498)
Third Party [Member] | G&P
Segment Reporting Information [Line Items]
Total segment revenues and other income		$ (858)	$ (843)		$ (1,181)		$ (2,581)
[1]	Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[3]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .
[4]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $139 million and $443 million for the three and nine months ended September 30, 2020 , respectively, and $182 million and $498 million for the three and nine months ended September 30, 2019 , respectively. Third party revenues for the G&P segment were $858 million and $1,181 million for the three and nine months ended September 30, 2020 , respectively, and $843 million and $2,581 million for the three and nine months ended September 30, 2019 , respectively.
[5]	See below for the reconciliation from Segment Adjusted EBITDA to net income.
[6]	The adjusted EBITDA adjustments related to Predecessor are excluded from adjusted EBITDA attributable to MPLX LP prior to the Merger.

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 28	$ 15
Assets	36,662	40,430
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	21,144	20,810
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 15,490	$ 19,605

Segment Information - Reconcili

Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		$ (36)	$ 0	$ (36)		$ 0
Depreciation and amortization	[1]	346	302	992		916
Income Tax Expense (Benefit)		(1)	(4)	(1)		(2)
Amortization of Debt Issuance Costs				(44)		(29)
Share-based Payment Arrangement, Expense		(4)	(5)	(12)		(17)
Goodwill and Intangible Asset Impairment		0	0	(2,165)		0
Interest and Other Financial Costs		(223)	(223)	(647)		(657)
Extinguishment of Debt, Gain (Loss), Net of Tax		14	0	14		0
Income/(loss) from equity method investments(1)		83	95	(1,012)	[2],[3]	255
Proceeds from Equity Method Investment, Distribution, Return of Capital				(350)		(379)
Acquisition Costs, Period Cost		0	(9)	0		(14)
Other Cost and Expense, Operating		3	1	5		1
Net income		674	689	(1,387)		2,035
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		1,335	1,165	3,856		3,015
Amortization of Debt Issuance Costs		(15)	(10)	(44)		(29)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(130)	(145)	(369)		(399)
Adjusted EBITDA attributable to noncontrolling interests		10	9	27		23
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		164	113	440		373
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		(27)	0	(27)		0
Adjusted EBITDA	[4]	893	766	2,604		1,895
Income/(loss) from equity method investments(1)		36	60	126		159
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		182	189	552		543
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		(9)	0	(9)		0
Adjusted EBITDA	[4]	442	399	1,252		1,120
Income/(loss) from equity method investments(1)		47	35	(1,138)		96
Not Designated as Hedging Instrument [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[5]	$ (10)	$ 11	$ (1)		$ 7
[1]	Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020 , respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019 , respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020 , respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[3]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .
[4]	See below for the reconciliation from Segment Adjusted EBITDA to net income.
[5]	MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Inventory Disclosure [Abstract]
NGLs	$ 3	$ 5
Line fill	11	10
Spare parts, materials and supplies	103	95
Total inventories	$ 117	$ 110

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020		Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 174
Property, plant and equipment, gross		$ 26,919		$ 26,867
Less accumulated depreciation(1)		5,304	[1]	4,722
Property, plant and equipment, net		21,615		22,145
L&S
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,671		12,934
L&S | Terminals
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		6,001		5,572
L&S | Marine
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		961		906
L&S | Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,534		1,109
L&S | Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		2,196		2,870
L&S | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,576		1,817
L&S | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		403		660
G&P
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		14,248		13,933
G&P | Pipelines
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		7,413		7,159
G&P | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		5,944		5,545
G&P | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		509		484
G&P | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 382		$ 745
Maximum [Member] | L&S | Terminals
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		51 years
Maximum [Member] | L&S | Marine
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		20 years
Maximum [Member] | L&S | Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Maximum [Member] | L&S | Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Maximum [Member] | L&S | Land, building and other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		61 years
Maximum [Member] | G&P | Pipelines
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Maximum [Member] | G&P | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Maximum [Member] | G&P | Land, building and other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Minimum [Member] | L&S | Terminals
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		2 years
Minimum [Member] | L&S | Marine
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		15 years
Minimum [Member] | L&S | Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		4 years
Minimum [Member] | L&S | Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		13 years
Minimum [Member] | L&S | Land, building and other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		1 year
Minimum [Member] | G&P | Pipelines
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Minimum [Member] | G&P | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		10 years
Minimum [Member] | G&P | Land, building and other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		3 years
[1]	The September 30, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below.

Goodwill and Intangibles Good_2

Goodwill and Intangibles Goodwill (Details)	Nov. 30, 2019	Mar. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2019USD ($)	Jul. 31, 2020USD ($)	Dec. 31, 2018USD ($)
Goodwill [Line Items]
Goodwill, Gross			$ 10,798,000,000			$ 10,146,000,000
Goodwill, Impaired, Accumulated Impairment Loss			(3,141,000,000)			(130,000,000)
Goodwill		$ (7,700,000,000)	(7,657,000,000)	$ (9,536,000,000)		(10,016,000,000)
Goodwill, Impairment Loss			(1,814,000,000)	(1,197,000,000)
Goodwill, Acquired During Period				717,000,000
Total Number of Reporting Units		6
Reporting Unit, Zero or Negative Carrying Amount, Amount of Allocated Goodwill				1
Number of Reporting Units	6	4
Crude Gathering [Member]
Goodwill [Line Items]
Goodwill		$ (1,100,000,000)
Minimum [Member]
Goodwill [Line Items]
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount		8.50%
Maximum [Member]
Goodwill [Line Items]
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount		270.00%
L&S
Goodwill [Line Items]
Goodwill, Gross			7,657,000,000			7,234,000,000
Goodwill, Impaired, Accumulated Impairment Loss			0			0
Goodwill			(7,657,000,000)	(7,722,000,000)		(7,234,000,000)
Goodwill, Impairment Loss			0	0
Goodwill, Acquired During Period				488,000,000
G&P
Goodwill [Line Items]
Goodwill, Gross			3,141,000,000			2,912,000,000
Goodwill, Impaired, Accumulated Impairment Loss			(3,141,000,000)			(130,000,000)
Goodwill			0	(1,814,000,000)		$ (2,782,000,000)
Goodwill, Impairment Loss		$ (1,814,000,000)	$ (1,814,000,000)	(1,197,000,000)
Goodwill, Acquired During Period				$ 229,000,000
Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Goodwill [Member] | Minimum [Member]
Goodwill [Line Items]
Fair Value Inputs		9.50%
Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Goodwill [Member] | Maximum [Member]
Goodwill [Line Items]
Fair Value Inputs		11.50%
Western Refining Wholesale & Western Refining Product Transport [Member]
Goodwill [Line Items]
Goodwill					$ (65,000,000)
Western Refining Wholesale & Western Refining Product Transport [Member] | L&S
Goodwill [Line Items]
Goodwill					(65,000,000)
Western Refining Wholesale & Western Refining Product Transport [Member] | G&P
Goodwill [Line Items]
Goodwill					$ 0

Goodwill and Intangibles Intang

Goodwill and Intangibles Intangibles (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020		Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived	$ 177
Finite-Lived Intangible Assets, Gross		$ 1,571		$ 1,571
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		32
Finite-Lived Intangible Asset, Expected Amortization, Year Two		128
Finite-Lived Intangible Asset, Expected Amortization, Year Three		128
Finite-Lived Intangible Asset, Expected Amortization, Year Four		128
Finite-Lived Intangible Asset, Expected Amortization, Year Five		124
Finite-Lived Intangible Asset, Expected Amortization, after Year Five		451
Finite-Lived Intangible Assets, Net		991
Finite-Lived Intangible Assets, Accumulated Amortization		(580)	[1]	(301)
Intangibles, net		991		1,270
L&S
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		27
Finite-Lived Intangible Assets, Gross		283		283
Finite-Lived Intangible Assets, Accumulated Amortization		(72)	[1]	(45)
Intangibles, net		$ 211		238
L&S | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		6 years
L&S | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		8 years
G&P
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		$ 75
Finite-Lived Intangible Assets, Gross		1,288		1,288
Finite-Lived Intangible Assets, Accumulated Amortization		(508)	[1],[2]	(256)
Intangibles, net		$ 780		$ 1,032
G&P | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		6 years
G&P | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		25 years
[1]	Amortization expense attributable to the G&P and L&S segments for the nine months ended September 30, 2020 was $75 million and $27 million , respectively.
[2]	Impairment charge of $177 million is included within the G&P accumulated amortization.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 0	$ 0
Derivative liability	61	60
Embedded derivatives in commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	$ 61	$ 60

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details)	9 Months Ended
	Sep. 30, 2020USD ($)$ / gal
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, Average Forward Price | $ / gal	0.58
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Embedded Derivative Renewal Term	5 years
Embedded Derivative Second Renewal Term	5 years
Fair Value, Inputs, Level 3 [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Probability of Renewal Second Term	100.00%
Fair Value Inputs Probability of Renewal	100.00%
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	$ 0.46
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	$ 0.97

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Commodity Derivative Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		$ 0	$ 0	$ 0	$ 0
Total gains (losses) (realized and unrealized) included in earnings	[1]	0	0	0	0
Settlements		0	0	0	0
Fair value at end of period		0	0	0	0
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		0	0	0	0
Embedded Derivatives in Commodity Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		(51)	(65)	(60)	(61)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(12)	9	(5)	2
Settlements		2	2	4	5
Fair value at end of period		(61)	(54)	(61)	(54)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		$ (11)	$ 9	$ (2)	$ 5
[1]	Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 20,455	$ 19,800
SMR liability	76	80
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	21,721	21,054
SMR liability	$ 87	$ 90

Derivative Financial Instrume_3

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Natural Gas [Member] - Embedded derivatives in commodity contracts $ in Millions	9 Months Ended
	Sep. 30, 2020USD ($)	Dec. 31, 2019USD ($)
Derivative [Line Items]
Number of Renewals	2
Embedded Derivative Renewal Term	5 years
Embedded Derivative Fair Value of Embedded Derivative Liability Including Inception Value Allocable to Host Contract	$ 61	$ 60

Derivative Financial Instrume_4

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	61	60
Other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	4	5
Other noncurrent assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 57	$ 55
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative [Line Items]
Total gain (loss)		$ (12)	$ 9	$ (5)	$ 2
Purchased product costs
Derivative [Line Items]
Realized gain (loss)		(2)	(2)	(4)	(5)
Total gain (loss)		(12)	9	(5)	2
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[1]	(10)	11	(1)	7
Not Designated as Hedging Instrument [Member] | Purchased product costs
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		$ (10)	$ 11	$ (1)	$ 7
[1]	MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Oct. 15, 2020	Aug. 18, 2020	Dec. 31, 2019	Sep. 09, 2019
Debt Instrument [Line Items]
Amortization of deferred financing costs		$ 44	$ 29
Total		20,757				$ 20,119
Unamortized debt issuance costs		(118)				(106)
Unamortized discount/premium		(290)				(300)
Amounts due within one year		(307)				(9)
Total long-term debt due after one year		20,042				19,704
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross							$ 2,000
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 3,000
Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		23				23
Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		121				190
Variable Rate Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross		1,000				2,000
Fixed Rate Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross		19,506				16,887
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Financing lease obligations(1)	[1]	12				19
Floating Rate Senior Notes Due September 2021 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross							1,000
Floating Rate Senior Notes Due September 2022 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross							$ 1,000
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		95				0
MPLX Term Loan [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		$ 0			$ 1,000	$ 1,000
Senior Notes Due October 2022 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.25%
Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		1.75%
Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.25%
Subsequent Event | Senior Notes Due October 2022 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 300
Subsequent Event | Senior Notes Due October 2022 [Member] | Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 34
[1]	See Note 20 for lease information.

Debt Debt - Summary of Outstand

Debt Debt - Summary of Outstanding Borrowings - Interest Rates and Table Due Dates (Details)	9 Months Ended
	Sep. 30, 2020
MPLX LP | Line of Credit [Member] | MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Jul. 30, 2024
MPLX LP | Senior Notes [Member] | Senior Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.25%
MPLX LP | Senior Notes [Member] | Floating Rate Senior Notes Due September 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 9, 2021
MPLX LP | Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 9, 2022
Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.75%
Minimum [Member] | MPLX LP | Senior Notes [Member] | Senior Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Oct. 15, 2022
Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.25%
Maximum [Member] | MPLX LP | Senior Notes [Member] | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Apr. 15, 2058

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions		Sep. 26, 2019	Sep. 09, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Oct. 15, 2020	Aug. 31, 2020	Aug. 28, 2020	Aug. 18, 2020	Dec. 31, 2019	Jul. 30, 2019
Debt Instrument [Line Items]
Payments of Debt Issuance Costs						$ 23	$ 20
Repayments of Long-term Debt						5,372	7,423
Payment for Debt Extinguishment or Debt Prepayment Cost				$ (14)	$ 0	(14)	$ 0
Debt and Lease Obligation				20,757		20,757						$ 20,119
Unamortized Debt Issuance Expense				118		118						106
Debt Instrument, Unamortized Discount				290		290						300
Debt, Current				307		307						9
Long-term debt				20,042		20,042						19,704
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity													$ 3,500
Long-term Line of Credit				$ 95		95						0
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP
Debt Instrument [Line Items]
Proceeds from Lines of Credit						$ 2,995
Debt Instrument, Interest Rate, Effective Percentage				1.51%		1.51%
Repayments of Long-term Lines of Credit						$ 2,900
Letters of Credit Outstanding				$ 1		1
Line of Credit Facility, Remaining Borrowing Capacity				$ 3,405		$ 3,405
Line of Credit Facility, Remaining Borrowing Capacity, Percentage				97.30%		97.30%
MPLX Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,000
Debt Instrument, Description of Variable Rate Basis		(i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement).
Long-term Line of Credit				$ 0		$ 0					$ 1,000	1,000
Repayments of Long-term Debt						1,000
Payment for Debt Extinguishment or Debt Prepayment Cost						(1)
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 2,000
Percent of Par			100.00%
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross											$ 3,000
Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				121		121						190
Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				23		23						23
Senior Notes [Member] | Floating Rate Senior Notes Due September 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 1,000
Debt Instrument, Description of Variable Rate Basis			LIBOR plus 0.9 percent per annum
Payment for Debt Extinguishment or Debt Prepayment Cost						(3)
Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 1,000
Debt Instrument, Description of Variable Rate Basis			LIBOR plus 1.1 percent per annum
Senior Notes [Member] | Senior Notes Due March 2026 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 1,500		$ 1,500
Debt Instrument, Interest Rate, Stated Percentage				1.75%		1.75%
Percent of Par											99.785%
Senior Notes [Member] | Senior Notes Due August 2030 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 1,500		$ 1,500
Debt Instrument, Interest Rate, Stated Percentage				2.65%		2.65%
Percent of Par											99.913%
Senior Notes [Member] | Senior Notes Due October 2022 [Member] | MPLX LP
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				6.25%		6.25%
Senior Notes [Member] | Senior Notes Due May 2024 [Member]
Debt Instrument [Line Items]
Payment for Debt Extinguishment or Debt Prepayment Cost						$ (18)
Senior Notes [Member] | Senior Notes Due May 2024 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross									$ 450
Percent of Par										103.20%
Senior Notes [Member] | Senior Notes Due May 2024 [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross										$ 69
Debt Instrument, Interest Rate, Stated Percentage										6.375%
Fixed Rate Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 19,506		19,506						16,887
Variable Rate Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross				1,000		1,000						2,000
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Finance Lease, Liability	[1]			$ 12		$ 12						$ 19
Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				1.75%		1.75%
Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				6.25%		6.25%
Subsequent Event | Senior Notes [Member] | Senior Notes Due October 2022 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term Debt, Gross								$ 300
Subsequent Event | Senior Notes [Member] | Senior Notes Due October 2022 [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross								$ 34
Debt Premium [Member] | Senior Notes [Member] | Senior Notes Due May 2024 [Member]
Debt Instrument [Line Items]
Payment for Debt Extinguishment or Debt Prepayment Cost						$ (14)
[1]	See Note 20 for lease information.

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		$ 1,756	$ 1,760	$ 5,129		$ 5,185
Rental income		102	99	296		291
Rental income - related parties		241	293	712		904
Income/(loss) from equity method investments(1)		83	95	(1,012)	[1],[2]	255
Other income		2	2	5		6
Other income - related parties		63	31	190		84
Operating Income (Loss)	[3]	491	520	191		1,540
Total segment revenues and other income		2,247	2,280	5,320		6,725
L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		998	998	2,973		2,844
G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		758	762	2,156		2,341
Service [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		604	632	1,779		1,865
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		909	899	2,694		2,549
Service [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		87	95	248		260
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		902	881	2,676		2,527
Service [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		517	537	1,531		1,605
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		7	18	18		22
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		41	26	102		86
Service, Other [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		0	0	0		0
Service, Other [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		41	26	102		86
Product [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		165	171	454		576
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		37	32	100		109
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		165	171	454		576
Product [Member] | L&S
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		2	8	10		17
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		7	14	39		40
Product [Member] | G&P
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		35	24	90		92
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		$ 158	$ 157	$ 415		$ 536
[1]	The 2020 period includes $1,264 million of impairment expense. See Note 4 .
[2]	The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4 .
[3]	Non-ASC 606 Revenue includes rental income, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.

Revenue Contract Balances (Deta

Revenue Contract Balances (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Long-term deferred revenue, beginning balance		$ 217
Long-term deferred revenue, ending balance		291
Liability, change in timeframe, performance obligation satisfied, revenue recognized		0	$ 0
ASC 606
Contract assets, beginning balance	[1]	39	36
Contract assets, additions/(deletions)		(9)	(6)
Contract assets, revenue recognized	[2]	(1)	(2)
Contract assets, ending balance		29	28
Deferred revenue, beginning balance	[1]	23	13
Deferred revenue, additions/(deletions)		16	11
Deferred revenue, revenue recognized	[2]	(6)	(4)
Deferred revenue, ending balance		33	20
Deferred revenue - related parties, beginning balance	[1]	53	65
Deferred revenue - related party, additions/(deletions)		77	34
Deferred revenue - related parties, revenue recognized	[2]	(60)	(50)
Deferred revenue - related parties, ending balance		70	49
Long-term deferred revenue, beginning balance	[1]	90	56
Long-term deferred revenue, additions/(deletions)		25	26
Long-term deferred revenue, revenue recognized	[2]	0	0
Long-term deferred revenue, ending balance		115	82
Long-term deferred revenue - related parties, beginning balance	[1]	55	52
Long-term deferred revenue - related party, additions/(deletions)		(7)	5
Long-term deferred revenue - related parties, revenue recognized	[2]	0	0
Long-term deferred revenue - related parties, ending balance		$ 48	$ 57
[1]	Balance represents ASC 606 portion of each respective line item.
[2]	No significant revenue was recognized related to past performance obligations in the current periods.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	Sep. 30, 2020USD ($)
Revenue from Contract with Customer [Abstract]
Contract with customer, liability	$ 264
Revenue, Remaining Performance Obligation, Amount	11,349	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 461
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,727
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,701
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,591
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 5,869
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	30 years
[1]	All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded.
[2]	Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table.
[3]	Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jul. 31, 2020
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 631	$ 648
Income Taxes Paid, Net	1	0
Non-cash investing and financing activities
Net transfers of property, plant and equipment (to)/from materials and supplies inventories	$ (1)	$ 1
Western Refining Wholesale & Western Refining Product Transport [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Partner Capital, Units, contributed, fair value assigned			$ 340

Supplemental Cash Flow Inform_4

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Supplemental Cash Flow Elements [Abstract]
(Decrease)/increase in capital accruals	$ (197)	$ (67)

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	$ (16)	$ (15)	$ (15)	$ (16)
Other Comprehensive Income (Loss), Net of Tax	[2]	(1)	1
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(14)	(14)	(14)	(14)
Other Comprehensive Income (Loss), Net of Tax	[2]	0	0
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(2)	(1)	$ (1)	$ (2)
Other Comprehensive Income (Loss), Net of Tax	[2]	$ (1)	$ 1
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”
[2]	Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation Pla_2

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units	9 Months Ended
	Sep. 30, 2020$ / sharesshares
Number of Units
Outstanding at December 31, 2019 | shares	1,109,568
Granted | shares	215,677
Settled | shares	558,398
Forfeited | shares	5,383
Outstanding at September 30, 2020 | shares	761,464
Weighted Average Fair Value
Outstanding at December 31, 2019 | $ / shares	$ 35.97
Granted | $ / shares	19.31
Settled | $ / shares	37.73
Forfeited | $ / shares	35.87
Outstanding at September 30, 2020 | $ / shares	$ 29.96

Equity-Based Compensation Pla_3

Equity-Based Compensation Plan - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2020$ / shares
Officer | Performance Shares [Member] | MPLX LP 2012 Incentive Compensation Plan [Member]
Equity Transactions And Share Based Compensation [Line Items]
Percentage paid out in cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%
2020 [Member] | Performance Units Market Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 0.80
2018 [Member] | Performance Units Market Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	0.45
Maximum [Member] | 2020 [Member] | Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	2
Maximum [Member] | 2018 [Member] | Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 2

Equity-Based Compensation Pla_4

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	9 Months Ended
	Sep. 30, 2020shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2019	2,157,347
Granted	2,147,211
Settled	1,169,354
Forfeited	(31,668)
Outstanding at September 30, 2020	3,103,536

Leases Leases Narrative (Detail

Leases Leases Narrative (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount			$ 171
Sales-type Lease, Lease Receivable			370
Sales-type Lease, Selling Profit (Loss)			209
MPC
Operating Lease, Lease Income	[1]	$ 208		$ 248	$ 589	$ 773
Sales-type Lease, Lease Receivable		419			419
Sales-type Lease, Unguaranteed Residual Asset		22			22		$ 7
Refining Logistics [Member] | MPC
Sales-type Lease, Unguaranteed Residual Asset			$ 10
Third Party [Member]
Operating Lease, Lease Income	[1]	70		$ 63	199	$ 196
Sales-type Lease, Lease Receivable		1			1
Sales-type Lease, Selling Profit (Loss)		$ 0			$ 0
[1]	These amounts are presented net of executory costs.

Leases Lessor Maturity Table (D

Leases Lessor Maturity Table (Details) - USD ($) $ in Millions	Sep. 30, 2020	Mar. 31, 2020
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable		$ 370
MPC
Lessor, Lease, Description [Line Items]
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Current Year	$ 39
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year One	157
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Three Years	157
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Three	158
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Four	158
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, after Year Five	473
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received	1,142
Sales-type and Direct Financing Leases, Lease Receivable, Undiscounted Excess Amount	723
Sales-type Lease, Lease Receivable	419
Third Party [Member]
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable	$ 1

Leases Lessor Lease Revenues (D

Leases Lessor Lease Revenues (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessor, Lease, Description [Line Items]
Sales-type Lease, Selling Profit (Loss)			$ 209
MPC
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	$ 208		$ 248	$ 589	$ 773
Sales-type Lease, Interest Income		37		3	113	3
Sales-type Lease, Variable Lease Income		1		1	1	1
Third Party [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	70		63	199	196
Sales-type Lease, Selling Profit (Loss)		0			0
Sales-type Lease, Interest Income		0			0
Sales-type Lease, Variable Lease Income		0			0
MPC
Lessor, Lease, Description [Line Items]
Sales-type Lease, Selling Profit (Loss)		$ 0		$ 0	$ 0	$ 0
[1]	These amounts are presented net of executory costs.

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation	$ 18	$ 19
Loss Contingency, Damages Sought, Value	187
Contractual commitments to acquire property, plant and equipment	181
MPC
Commitments And Contingencies [Line Items]
Due to Related Parties, Current	247	911
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Due to Related Parties, Current	$ 0	$ 0
Whistler Pipeline LLC [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage	38.00%
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage	9.19%
Guarantee Type, Other [Member]
Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 230