MPLX (MPLX) 10-Q 2 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 29, 2021
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, 2021
Amendment Flag	false
Entity File Number	001-35714
Document Fiscal Year Focus	2021
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		1,019,863,638
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, 2021		Sep. 30, 2020		Sep. 30, 2021		Sep. 30, 2020
Operating Lease, Lease Income, Related Party [Line Items]		$ 164		$ 241		$ 574		$ 712
Income/(loss) from equity method investments(1)	[1]	92		83		228	[2]	(1,012)	[3]
Other Income		(2)		2		2		5
Related Party Transaction, Other Revenues from Transactions with Related Party		27		25		82		76
Revenues		2,559		2,247		7,293		5,320
Related Party Transaction, Purchases from Related Party		307		297		902		853
Depreciation and amortization	[4]	324		346		971		992
Goodwill and Intangible Asset Impairment		0		0		42		2,165
Taxes, Miscellaneous		27		33		93		94
Costs and Expenses		1,528		1,348		4,357		6,029
Interest And Other Financial Costs From Related Parties		2		0		4		4
Other Nonoperating Income (Expense)		21		17		67		49
Interest Expense, Debt, Excluding Amortization		197		207		590		624
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent, Total		1,031		899		2,936		(709)
Provision for income taxes		0		1		1		1
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total		811		675		2,275		(1,386)
Less: Net income (loss) attributable to noncontrolling interest		9		9		27		24
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total		811		674		2,274		(1,387)
Limited partners' interest in net income (loss) attributable to MPLX LP		754		635		2,137		(1,503)
Net income (loss) attributable to MPLX LP		802	[5]	665	[5]	2,247	[5]	(1,411)
Related Party and Third Party
General and Administrative Expense		94		96		267		289
Third Party [Member]
Operating Lease, Lease Income		88		102		286		296
Rental cost of sales
Labor and Related Expense		$ 33		$ 33		$ 97		$ 101
Limited Partners Common Units [Member]
Net income (loss) attributable to MPLX LP per limited partner unit:
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax		$ 0.74		$ 0.61		$ 2.07		$ (1.43)
Common - diluted (in USD per unit)		$ 0.74		$ 0.61		$ 2.07		$ (1.43)
Weighted average limited partner units outstanding:
Common - basic (in shares)		1,024		1,046		1,030		1,054
Common - diluted (in shares)		1,025		1,047		1,030		1,054
Service [Member]
Total revenues from contracts with customers		$ 600		$ 604		$ 1,767		$ 1,779
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		902		909		2,681		2,694
Service, Other [Member]
Total revenues from contracts with customers		82		41		235		102
Product [Member]
Total revenues from contracts with customers		448		165		1,034		454
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		26		37		99		100
Oil and Gas, Refining and Marketing [Member]
Cost of Goods and Services Sold		298		323		864		1,006
Natural Gas, Midstream [Member]
Cost of Goods and Services Sold		421		152		1,035		374
Series A Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		38		20		79		61
Net income (loss) attributable to MPLX LP		38	[5]	20	[5]	79	[5]	61
Series B Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		10		10		31		31
Net income (loss) attributable to MPLX LP		10	[5]	10	[5]	31	[5]	31
Marathon Petroleum Corporation [Member]
Sales-type Lease, Revenue		132		38		305		114
General and Administrative Expense		70		63		190		195
Restructuring Charges		0		36		0		36
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties
Labor and Related Expense		24		32		86		119
Marathon Petroleum Corporation [Member] | Product [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		$ 26		$ 37		$ 99		$ 100
[1]	The nine months ended September 30, 2021 and September 30, 2020 include $6 million and $1,264 million of impairment expense, respectively.
[2]	Includes impairment expense of $6 million.
[3]	Includes impairment expense of $1,264 million.
[4]	Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, respectively.
[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.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Equity Method Investment, Other than Temporary Impairment		$ 1,264	$ 6
Interest Costs Capitalized	$ 2	$ 8	$ 12	$ 31

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 811	$ 674	$ 2,274	$ (1,387)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax	0	0	(2)	(1)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total	811	674	2,272	(1,388)
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest [Abstract]
Less: Net income (loss) attributable to noncontrolling interest	9	9	27	24
Comprehensive Income (Loss), Net of Tax, Attributable to Parent, Total	$ 802	$ 665	$ 2,245	$ (1,412)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) shares in Millions, $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents	$ 39	$ 15
Receivables, net	617	452
Current Assets, Related Parties	645	677
Inventories	141	118
Total current assets	1,516	1,515
Equity method investments	4,001	4,036
Property, plant and equipment, net	20,158	21,218
Intangibles, net	864	959
Goodwill	7,657	7,657
Other noncurrent assets	61	48
Total assets	35,699	36,414
Current liabilities:
Accounts payable	177	152
Accrued Liabilities, Current	332	194
Due to Related Parties, Current	1,706	356
Accrued Property Plant and Equipment Current	66	84
Accrued interest payable	188	222
Operating Lease, Liability, Current	61	63
Other current liabilities	190	150
Total current liabilities	2,721	2,086
Long-term deferred revenue	366	314
Liabilities, Related Parties, Noncurrent	297	283
Long-term debt	18,253	19,375
Deferred income taxes	11	12
Operating Lease, Liability, Noncurrent	220	244
Debt, Current	1	764
Deferred credits and other liabilities	155	115
Total liabilities	22,023	22,429
Commitments and contingencies (see Note 19)
Equity
Total MPLX LP partners’ capital	12,447	12,772
Noncontrolling interests	243	245
Total equity	12,690	13,017
Total liabilities, preferred units and equity	35,699	36,414
Disposal Group, Including Discontinued Operation, Assets, Current	20	188
Disposal Group, Including Discontinued Operation, Liabilities, Current	0	101
Affiliated Entity
Assets
Due from Related Parties, Noncurrent	1,160	672
Series A Convertible Preferred Units
Current liabilities:
Series A preferred units	986	968
Third Party [Member]
Assets
Other current assets	54	65
Operating Lease, Right-of-Use Asset	282	309
Marathon Petroleum Corporation [Member]
Assets
Other current assets	2	1
Operating Lease, Right-of-Use Asset	230	231
Due from Related Parties, Noncurrent	$ 32	$ 32
Series B Preferred Stock [Member]
Current liabilities:
Units issued	0.6	0.6
Equity
Units Outstanding	0.6	0.6
Preferred Units, Preferred Partners' Capital Accounts	$ 601	$ 611
Limited Partners Common Units [Member] | Public [Member]
Current liabilities:
Limited Partners' Capital Account	$ 8,919	$ 9,384
Units issued	374	391
Equity
Units Outstanding	374	391
Limited Partners Common Units [Member] | Marathon Petroleum Corporation [Member]
Current liabilities:
Limited Partners' Capital Account	$ 2,944	$ 2,792
Units issued	647	647
Equity
Units Outstanding	647	647
LOOP and Explorer
Equity
Accumulated other comprehensive loss	$ (17)	$ (15)

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Sep. 30, 2021	Dec. 31, 2020
Limited Partners Common Units [Member] | Public [Member]
Units issued	374	391
Units Outstanding	374	391
Limited Partners Common Units [Member] | Marathon Petroleum Corporation [Member]
Units issued	647	647
Units Outstanding	647	647
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		3 Months Ended		9 Months Ended				12 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Operating activities:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 811	$ 674	$ 2,274		$ (1,387)
Adjustments to reconcile net income/(loss) to net cash provided by operating activities:
Amortization of deferred financing costs				53		44
Depreciation and amortization	[1]	324	346	971		992
Goodwill and Intangible Asset Impairment		0	0	42		2,165
Deferred income taxes				(1)		(1)
(Income)/loss from equity method investments(1)	[2]	(92)	(83)	(228)	[3]	1,012	[4]
Proceeds from Equity Method Investment, Distribution				361		350
Changes in:
Current receivables				(162)		69
Inventories				(22)		(8)
Fair value of derivatives				41		1
Current accounts payable and accrued liabilities				166		(27)
Current assets/current liabilities - related parties				94		36
Increase (Decrease) in Other Operating Assets and Liabilities, Net				2		(2)
Deferred revenue				65		85
All other, net				11		6
Net cash provided by operating activities				3,671		3,336
Net Cash Provided by (Used in) Investing Activities [Abstract]
Additions to property, plant and equipment				(374)		(982)
Disposal of assets				77		54
Payments to Acquire Equity Method Investments				(116)		(244)
Proceeds from Equity Method Investment, Distribution, Return of Capital				36		112
Net cash used in investing activities				(377)		(1,060)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Long-term debt - borrowings				3,000		5,990
Long-term debt - repayments				(4,946)		(5,372)
Payments of Debt Issuance Costs				0		23
Payments for Repurchase of Common Stock				(465)		0
Distributions to noncontrolling interests				(29)		(26)
Distributions to unitholders and general partner				(2,126)		(2,162)
Proceeds from Contributions from Parent				31		34
All other, net				(3)		(8)
Net cash used in financing activities				(3,270)		(2,263)
Net increase in cash, cash equivalents and restricted cash				24		13
Cash, cash equivalents and restricted cash at beginning of period				15		15		$ 15
Cash, cash equivalents and restricted cash at end of period		$ 39	$ 28	39		28		15
Loss on disposal of assets				4		1
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Related party debt - borrowings				6,571		4,870		6,264
Repayments of Related Party Debt				(5,201)		(5,464)		$ (6,858)
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders				(61)		(61)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders				$ (41)		$ (41)
[1]	Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, respectively.
[2]	The nine months ended September 30, 2021 and September 30, 2020 include $6 million and $1,264 million of impairment expense, respectively.
[3]	Includes impairment expense of $6 million.
[4]	Includes impairment expense of $1,264 million.

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021
Statement of Financial Position [Abstract]
Equity Method Investment, Other than Temporary Impairment	$ 6	$ 1,264	$ 1,264	$ 6

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($)	Total	LOOP and Explorer	Series B Preferred Stock [Member]	Public [Member]Limited Partners Common Units [Member]	Marathon Petroleum Corporation [Member]Limited Partners Common Units [Member]	AOCI Attributable to Parent [Member]	Noncontrolling Interest [Member]
Preferred Units, Preferred Partners' Capital Accounts			$ 611,000,000
Limited Partners' Capital Account				$ 10,800,000,000	$ 4,968,000,000
Noncontrolling interests	$ 249,000,000
Accumulated other comprehensive loss		$ (15,000,000)
Beginning Balance at Dec. 31, 2019	16,613,000,000
Distributions to:	2,736,000,000		(11,000,000)	(1,022,000,000)	(1,733,000,000)	$ 0	$ (8,000,000)
Contributions from:	(225,000,000)		0	0	(225,000,000)	0	0
Unitholders and general partner	(738,000,000)		(21,000,000)	(271,000,000)	(446,000,000)	0	0
Noncontrolling interests	(9,000,000)		0	0	0	0	(9,000,000)
Other	1,000,000		0	2,000,000	0	(1,000,000)	0
Ending Balance at Mar. 31, 2020	13,356,000,000
Preferred Units, Preferred Partners' Capital Accounts			601,000,000
Limited Partners' Capital Account				9,509,000,000	3,014,000,000
Noncontrolling interests	248,000,000
Accumulated other comprehensive loss		(16,000,000)
Distributions to:	(634,000,000)		(10,000,000)	229,000,000	388,000,000	0	(7,000,000)
Contributions from:	(6,000,000)		0	0	(6,000,000)	0	0
Unitholders and general partner	(728,000,000)		0	(270,000,000)	(458,000,000)	0	0
Noncontrolling interests	(8,000,000)		0	0	0	0	(8,000,000)
Other	2,000,000		0	1,000,000	1,000,000	0	0
Ending Balance at Jun. 30, 2020	13,262,000,000
Preferred Units, Preferred Partners' Capital Accounts			611,000,000
Limited Partners' Capital Account				9,469,000,000	2,951,000,000
Noncontrolling interests	247,000,000
Accumulated other comprehensive loss		(16,000,000)
Distributions to:	(654,000,000)		(10,000,000)	236,000,000	399,000,000	0	(9,000,000)
Contributions from:	(13,000,000)		0	0	(13,000,000)	0	0
Stockholders' Equity, Change in Reporting Entity	(90,000,000)		0	0	(90,000,000)	0	0
Unitholders and general partner	(736,000,000)		(20,000,000)	(271,000,000)	(445,000,000)	0	0
Noncontrolling interests	(9,000,000)		0	0	0	0	(9,000,000)
Other	1,000,000		0	2,000,000	(1,000,000)	0	0
Ending Balance at Sep. 30, 2020	13,095,000,000
Preferred Units, Preferred Partners' Capital Accounts			601,000,000
Limited Partners' Capital Account				9,436,000,000	2,827,000,000
Noncontrolling interests	247,000,000
Accumulated other comprehensive loss		(16,000,000)
Preferred Units, Preferred Partners' Capital Accounts			611,000,000
Limited Partners' Capital Account				9,384,000,000	2,792,000,000
Noncontrolling interests	245,000,000
Total MPLX LP partners’ capital	12,772,000,000
Accumulated other comprehensive loss		(15,000,000)
Beginning Balance at Dec. 31, 2020	13,017,000,000
Distributions to:	(729,000,000)		(11,000,000)	266,000,000	443,000,000	0	(9,000,000)
Stock Repurchased and Retired During Period, Value	(155,000,000)		0	(155,000,000)	0	0	0
Contributions from:	(7,000,000)		0	0	(7,000,000)	0	0
Unitholders and general partner	(735,000,000)		(21,000,000)	(269,000,000)	(445,000,000)	0	0
Noncontrolling interests	(10,000,000)		0	0	0	0	(10,000,000)
Other	(3,000,000)		0	0	(1,000,000)	(2,000,000)	0
Ending Balance at Mar. 31, 2021	12,850,000,000
Preferred Units, Preferred Partners' Capital Accounts			601,000,000
Limited Partners' Capital Account				9,226,000,000	2,796,000,000
Noncontrolling interests	244,000,000
Accumulated other comprehensive loss		(17,000,000)
Distributions to:	(693,000,000)		(10,000,000)	251,000,000	423,000,000	0	(9,000,000)
Stock Repurchased and Retired During Period, Value	(155,000,000)		0	(155,000,000)	0	0	0
Contributions from:	(122,000,000)		0	0	(122,000,000)	0	0
Unitholders and general partner	(707,000,000)		0	(262,000,000)	(445,000,000)	0	0
Noncontrolling interests	(10,000,000)		0	0	0	0	(10,000,000)
Other	2,000,000		0	1,000,000	1,000,000	0	0
Ending Balance at Jun. 30, 2021	12,795,000,000
Preferred Units, Preferred Partners' Capital Accounts			611,000,000
Limited Partners' Capital Account				9,061,000,000	2,897,000,000
Noncontrolling interests	243,000,000
Accumulated other comprehensive loss		(17,000,000)
Distributions to:	(773,000,000)		(10,000,000)	278,000,000	476,000,000	0	(9,000,000)
Stock Repurchased and Retired During Period, Value	(160,000,000)		0	(160,000,000)	0	0	0
Contributions from:	(16,000,000)		0	0	(16,000,000)	0	0
Unitholders and general partner	(726,000,000)		(20,000,000)	(261,000,000)	(445,000,000)	0	0
Noncontrolling interests	(9,000,000)		0	0	0	0	(9,000,000)
Other	1,000,000		0	1,000,000	0	$ 0	$ 0
Ending Balance at Sep. 30, 2021	12,690,000,000
Preferred Units, Preferred Partners' Capital Accounts			$ 601,000,000
Limited Partners' Capital Account				$ 8,919,000,000	$ 2,944,000,000
Noncontrolling interests	243,000,000
Total MPLX LP partners’ capital	$ 12,447,000,000
Accumulated other comprehensive loss		$ (17,000,000)

Description of the Business and

Description of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
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, marketing 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. Impairments – Through the first nine months of 2021, we continued to see improvements in the environment in which our business operates as COVID-19 impacts continue to subside. The increased availability of vaccinations and easing of COVID-19 restrictions has been followed by an increase in economic activity despite the recent resurgence in COVID-19. We are unable to predict the long-term impacts COVID-19 may have on our financial position and results. In the second quarter of 2021, we recognized impairment expense of $42 million within our G&P segment related to our continued emphasis on portfolio optimization with the anticipated divestiture of several non-core assets and the closure of other non-core assets. During the first quarter of 2020, 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 at that time resulting in impairments of the carrying value of certain assets. 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 environment at that time, which impacted their 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 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. (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 our Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the full year. 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.

Accounting Standards Accounting

Accounting Standards Accounting Standards	9 Months Ended
	Sep. 30, 2021
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Update and Change in Accounting Principle [Text Block]	Accounting StandardsRecently AdoptedWe did not adopt any ASUs during the first nine months of 2021 that are expected to have a material impact to our financial statements or financial statement disclosures

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Business Combination Disclosure [Text Block]	Acquisitions and Dispositions Sale of Javelina Assets and Liabilities On February 12, 2021, MarkWest Energy Operating Company, L.L.C., (“MarkWest Energy”) a wholly owned subsidiary of MPLX, completed the sale of all of MarkWest Energy’s equity interests in MarkWest Javelina Company L.L.C., MarkWest Javelina Pipeline Company L.L.C., and MarkWest Gas Services L.L.C. (collectively, “Javelina”) pursuant to the terms of an Equity Purchase Agreement entered into with a third party on December 23, 2020. The agreement included adjustments for working capital as well as an earnout provision based on the performance of the assets. No gain or loss was recorded on the sale. The estimated value of the earnout provision was recorded as a contingent asset shown within “Other noncurrent assets” on the Consolidated Balance Sheets as of September 30, 2021. Javelina’s assets and liabilities sold are shown on the Consolidated Balance Sheets as “Assets held for sale” and “Liabilities held for sale” for the year ended December 31, 2020. Prior to the sale, Javelina was reported within the G&P segment. Wholesale Exchange On July 31, 2020, MPLX entered into a Redemption Agreement (the “Redemption Agreement”) with Western Refining Southwest, Inc. (now known as Western Refining Southwest LLC) (“WRSW”), a wholly owned subsidiary of MPC, pursuant to which MPLX agreed to transfer to WRSW all of the outstanding membership interests in Western Refining Wholesale, LLC (“WRW”) in exchange for the redemption of MPLX common units held by WRSW. The transaction effected the transfer to MPC of the Western wholesale distribution business that MPLX acquired as a result of its acquisition of Andeavor Logistics LP (“ANDX”). 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, netted against 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. These financial statements include the results of WRW through July 31, 2020.

Investments and Noncontrolling

Investments and Noncontrolling Interest	9 Months Ended
	Sep. 30, 2021
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) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 457 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 253 254 LOOP LLC 41% 268 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 188 194 Minnesota Pipe Line Company, LLC 17% 184 188 Whistler Pipeline LLC (2) 38% 146 185 Explorer Pipeline Company 25% 68 72 W2W Holdings LLC (2)(3) 50% 66 72 Other (2) 115 103 Total L&S 1,745 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 682 698 Sherwood Midstream LLC (2) 50% 548 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 335 307 MarkWest Torñado GP, L.L.C. (2) 60% 223 188 Rendezvous Gas Services, L.L.C. (2) 78% 151 159 Sherwood Midstream Holdings LLC (2) 51% 138 148 Centrahoma Processing LLC 40% 132 145 Other (2) 47 49 Total G&P 2,256 2,251 Total $ 4,001 $ 4,036 (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) Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest in Wink to Webster Pipeline 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 LLC (“Sherwood Midstream”) has been deemed the primary beneficiary of Sherwood Midstream Holdings LLC (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, 2021, MPLX has a 24.55 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, 2021. 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, L.L.C. and its investment in Ohio Gathering Company, L.L.C. 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. Summarized financial information for MPLX’s equity method investments for the nine months ended September 30, 2021 and 2020 is as follows: Nine Months Ended September 30, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 565 $ 940 $ 1,505 Costs and expenses 350 427 777 Income from operations 215 513 728 Net income 172 449 621 Income from equity method investments (1) $ 120 $ 108 $ 228 (1) Includes impairment expense of $6 million. 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 (Loss)/income from operations (176) 528 352 Net (loss)/income (230) 477 247 (Loss)/income from equity method investments (1) $ (1,138) $ 126 $ (1,012) (1) Includes impairment expense of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of September 30, 2021 and December 31, 2020 is as follows: September 30, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 328 $ 444 $ 772 Noncurrent assets 7,426 4,907 12,333 Current liabilities 216 254 470 Noncurrent liabilities $ 2,448 $ 873 $ 3,321 December 31, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 530 $ 318 $ 848 Noncurrent assets 6,889 4,997 11,886 Current liabilities 323 187 510 Noncurrent liabilities $ 1,904 $ 830 $ 2,734

Related Party Agreements and Tr

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 2021
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 quarterly 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. 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 extends loans to MPLX on a revolving basis as requested by MPLX and as agreed to by MPC Investment. The borrowing capacity of the MPC Loan Agreement is $1.5 billion aggregate principal amount of all loans outstanding at any one time. The loan agreement is scheduled to expire, and borrowings under the loan agreement are scheduled to mature and 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 maturity. Borrowings under the MPC Loan Agreement 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, 2021 Year Ended December 31, 2020 Borrowings $ 6,571 $ 6,264 Average interest rate of borrowings 1.345 % 2.278 % Repayments $ 5,201 $ 6,858 Outstanding balance at end of period (1) $ 1,370 $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets. 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 includes revenue related to volume deficiency credits. Certain product sales to MPC 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, 2021, these sales totaled $203 million and $548 million, respectively. For the three and nine months ended September 30, 2020, these sales totaled $107 million and $332 million, respectively. 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. These agreements are classified as “Other income - related parties” on the Consolidated Statements of Income. Related Party Expenses MPC charges MPLX for executive management services and certain general and administrative services that MPC provides 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, as well as compensation expenses. 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. For the three and nine months ended September 30, 2021, “General and administrative expenses” incurred from MPC totaled $70 million and $190 million, respectively. For the three and nine months ended September 30, 2020, “General and administrative expenses” incurred from MPC totaled $63 million and $195 million, respectively. 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, 2021, these charges totaled $13 million and $40 million, respectively. For the three and nine months ended September 30, 2020, these charges totaled $29 million and $80 million, respectively. 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. In 2020, MPC approved and executed an involuntary workforce reduction plan, which 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. There were no such costs in the first nine months of 2021. 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 18 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 received under these agreements (excluding payments received under agreements classified as sales-type leases) 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.” Deficiency payments under agreements that have been classified as sales-type leases are recorded as a reduction against the corresponding lease receivable. In addition, capital projects MPLX undertakes at the request of MPC are reimbursed in cash and recognized as revenue over the remaining term of the applicable agreements or in some cases as a contribution from MPC. (In millions) September 30, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 547 $ 615 Receivables - Other 7 27 Prepaid - MPC 13 4 Other - MPC 2 1 Lease Receivables - MPC 76 30 Total 645 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 857 386 Unguaranteed residual asset - MPC 41 23 Total 1,160 672 Current liabilities - related parties Payables - MPC 1,577 215 Payables - Other 36 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 43 66 Deferred revenue - Project reimbursements - MPC 48 30 Deferred revenue - Project reimbursements - Other 1 1 Total 1,706 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 228 229 Long-term deferred revenue - Project reimbursements - MPC 62 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 297 $ 283 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 related parties for the nine months ended September 30, 2021 and 2020 were $19 million and $8 million, respectively. Purchases from related parties were immaterial for the nine months ended September 30, 2021 and 2020.

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Earnings Per Share [Text Block]	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. During the three and nine months ended September 30, 2021 and 2020, MPLX had participating securities consisting of common units, certain equity-based compensation awards, Series A preferred units and Series B preferred units and 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, 2021 and 2020 were less than 1 million. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2021 2020 2021 2020 Net income/(loss) attributable to MPLX LP $ 802 $ 665 $ 2,247 $ (1,411) Less: Distributions declared on Series A preferred units (1) 38 20 79 61 Distributions declared on Series B preferred units 10 10 31 31 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 1,305 715 2,717 2,158 Undistributed net loss attributable to MPLX LP $ (551) $ (80) $ (580) $ (3,661) (1) 2021 periods include the Special Distribution Amount. See Note 6 for distribution information. Three Months Ended September 30, 2021 (In millions, except per unit data) Limited Partners’ 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) $ 1,305 $ 38 $ 10 $ 1,353 Undistributed net loss attributable to MPLX LP (551) — — (551) Net income attributable to MPLX LP (2) $ 754 $ 38 $ 10 $ 802 Weighted average units outstanding: Basic 1,024 Diluted 1,025 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.74 Diluted $ 0.74 (1) Includes the Special Distribution Amount. (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. Three Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ 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. Nine Months Ended September 30, 2021 (In millions, except per unit data) Limited Partners’ 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) $ 2,717 $ 79 $ 31 $ 2,827 Undistributed net loss attributable to MPLX LP (580) — — (580) Net income attributable to MPLX LP (2) $ 2,137 $ 79 $ 31 $ 2,247 Weighted average units outstanding: Basic 1,030 Diluted 1,030 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.07 Diluted $ 2.07 (1) Includes the Special Distribution Amount. (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. Nine Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ 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.

Equity

Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Equity	Equity The changes in the number of common units during the nine months ended September 30, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 213,727 Units redeemed in unit repurchase program (17,563,855) Balance at September 30, 2021 1,021,427,850 Unit Repurchase Program On November 2, 2020, MPLX announced the board authorization of a unit repurchase program for the repurchase of up to $1 billion of MPLX’s outstanding common units held by the public. MPLX may utilize various methods to effect the repurchases, which could include open market repurchases, negotiated block transactions, tender offers, accelerated unit repurchases or open market solicitations for units, some of which may be effected through Rule 10b5-1 plans. The timing and amount of repurchases will depend upon several factors, including market and business conditions, and repurchases may be initiated, suspended or discontinued at any time. The repurchase authorization has no expiration date. During the nine months ended September 30, 2021, we repurchased 17,563,855 common units at an average cost per unit of $26.79 and paid $465 million of cash, with an additional $5 million of cash paid in early October in connection with the settlement of certain late September purchases. As of September 30, 2021, we had $497 million remaining under our repurchase authorization. Series B Preferred Units MPLX has outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of MPLX with a price to the public of $1,000 per unit (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. Series B preferred unitholders are entitled to receive a fixed distribution of $68.75 per unit, per annum, 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, in each case assuming a distribution is declared by the Board of Directors. Cash distributions – On November 2, 2021, MPLX declared a cash distribution for the third quarter of 2021, totaling $1,305 million, or $1.28 per common unit, consisting of a base distribution amount of $0.705 per common unit and a special distribution amount of $0.575 per common unit (the “Special Distribution Amount”). This distribution will be paid on November 19, 2021 to common unitholders of record on November 12, 2021. This rate will also be received by Series A preferred unitholders. Quarterly distributions for 2021 and 2020 are summarized below: (Per common unit) 2021 2020 March 31, $ 0.6875 $ 0.6875 June 30, 0.6875 0.6875 September 30, $ 1.2800 $ 0.6875 In accordance with the distribution rights discussed above, MPLX made a cash distribution totaling $21 million to Series B unitholders on August 16, 2021. The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three and nine months ended September 30, 2021 and 2020. Distributions, although earned, are not accrued until declared. 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) 2021 2020 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner (1) $ 1,305 $ 715 $ 2,717 $ 2,158 Series A preferred unit distributions (1) 38 20 79 61 Series B preferred unit distributions 10 10 31 31 Total cash distributions declared $ 1,353 $ 745 $ 2,827 $ 2,250

Series A Preferred Units

Series A Preferred Units	9 Months Ended
	Sep. 30, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units	Series A Preferred Units On November 2, 2021, MPLX declared a cash distribution of $1.28 per common unit for the third quarter of 2021. Holders of the Series A preferred units will receive the common unit rate in lieu of the lower $0.528125 base amount. Approximately 29.6 million Series A preferred units remaining outstanding as of September 30, 2021. The changes in the redeemable preferred balance from December 31, 2020 through September 30, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated (1) 79 Distributions received by Series A preferred unitholders (61) Balance at September 30, 2021 $ 986

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
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) gain/(loss) on 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, Segment Adjusted EBITDA, restructuring expenses, 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) 2021 2020 2021 2020 L&S Service revenue $ 983 $ 989 $ 2,928 $ 2,924 Rental income 172 249 597 737 Product related revenue 3 9 11 49 Sales-type lease revenue - related parties 132 38 305 114 Income from equity method investments 41 36 112 126 Other income 15 13 46 40 Total segment revenues and other income (1) 1,346 1,334 3,999 3,990 Segment Adjusted EBITDA (2) 904 893 2,747 2,604 Restructuring expenses — 27 — 27 Capital expenditures 85 118 220 410 Investments in unconsolidated affiliates 9 4 31 132 G&P Service revenue 519 524 1,520 1,549 Rental income 80 94 263 271 Product related revenue 553 234 1,357 607 Income/(loss) from equity method investments 51 47 116 (1,138) Other income 10 14 38 41 Total segment revenues and other income (1) 1,213 913 3,294 1,330 Segment Adjusted EBITDA (2) 485 442 1,368 1,252 Restructuring expenses — 9 — 9 Capital expenditures 69 131 135 375 Investments in unconsolidated affiliates $ 23 $ 18 $ 85 $ 112 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $405 million for the three and nine months ended September 30, 2021, respectively, and $139 million and $443 million for the three and nine months ended September 30, 2020, respectively. Third party revenues for the G&P segment were $1,170 million and $3,147 million for the three and nine months ended September 30, 2021, respectively, and $858 million and $1,181 million for the three and nine months ended September 30, 2020, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income/(loss).” (In millions) September 30, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 39 $ 15 L&S 20,729 20,938 G&P 14,931 15,461 Total assets $ 35,699 $ 36,414 The table below provides a reconciliation between “Net income/(loss)” and Segment Adjusted EBITDA. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2021 2020 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 904 $ 893 $ 2,747 $ 2,604 G&P Segment Adjusted EBITDA 485 442 1,368 1,252 Total reportable segments 1,389 1,335 4,115 3,856 Depreciation and amortization (1) (324) (346) (971) (992) Provision for income taxes — (1) (1) (1) Amortization of deferred financing costs (18) (15) (53) (44) Non-cash equity-based compensation (1) (4) (6) (12) Impairment expense — — (42) (2,165) Net interest and other financial costs (200) (223) (618) (647) Gain on extinguishment of debt (2) 14 10 14 Income/(loss) from equity method investments 92 83 228 (1,012) Distributions/adjustments related to equity method investments (129) (130) (371) (369) Unrealized derivative losses (2) (2) (10) (41) (1) Restructuring expenses — (36) — (36) Other (3) (3) (5) (5) Adjusted EBITDA attributable to noncontrolling interests 9 10 29 27 Net income/(loss) $ 811 $ 674 $ 2,274 $ (1,387) (1) Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, 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.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) September 30, 2021 December 31, 2020 NGLs $ 10 $ 5 Line fill 24 13 Spare parts, materials and supplies 107 100 Total inventories $ 141 $ 118

Property, Plant and Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: September 30, 2021 December 31, 2020 (In millions) Gross PP&E Accumulated Depreciation Net PP&E Gross PP&E Accumulated Depreciation Net PP&E L&S $ 12,296 $ 3,108 $ 9,188 $ 12,813 $ 2,997 $ 9,816 G&P 14,115 3,145 10,970 14,062 2,660 11,402 Total $ 26,411 $ 6,253 $ 20,158 $ 26,875 $ 5,657 $ 21,218 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.

Goodwill and Intangibles (Notes

Goodwill and Intangibles (Notes)	9 Months Ended
	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets Disclosure [Text Block]	Goodwill and Intangibles Goodwill During the first quarter of 2020, we performed an interim impairment analysis of the goodwill recorded 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. As a result of the interim impairment analysis, 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 reduced production growth forecasts from our producer customers. Changes in the carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2019 $ 7,722 $ 3,141 $ 10,863 Accumulated impairment losses — (1,327) (1,327) 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 December 31, 2020 7,657 — 7,657 Balance as of September 30, 2021 7,657 — 7,657 Gross goodwill as of September 30, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of September 30, 2021 $ 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. No additional impairments have been recorded since that time.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
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, 2021 and December 31, 2020 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, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (104) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (104) $ — $ (63) Level 3 instruments include an embedded derivative 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 $0.68 to $1.80 per gallon with a weighted average of $0.88 per gallon and (2) the probability of renewal of 100 percent for the first five five 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, 2021 Three Months Ended September 30, 2020 (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 $ — $ (102) $ — $ (51) Total losses (realized and unrealized) included in earnings (1) — (7) — (12) Settlements — 5 — 2 Fair value at end of period — (104) — (61) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (6) $ — $ (11) Nine Months Ended September 30, 2021 Nine Months Ended September 30, 2020 (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 $ — $ (63) $ — $ (60) Total losses (realized and unrealized) included in earnings (1) — (52) — (5) Settlements — 11 — 4 Fair value at end of period — (104) — (61) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (44) $ — $ (2) (1) Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” 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 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, approximate 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 debt is estimated based on recent market non-binding indicative quotes. The debt fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of our third-party debt, excluding finance leases: September 30, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Outstanding debt (1) $ 20,553 $ 18,350 $ 22,951 $ 20,244 (1) Amounts outstanding under the MPC Loan Agreement are not included in the table above, as the carrying value approximates fair value. This balance is reflected in “Current liabilities - related parties” on the Consolidated Balance Sheets.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of September 30, 2021, 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 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, 2021 and December 31, 2020, 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, 2021 December 31, 2020 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 $ — $ (15) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (89) — (56) Total $ — $ (104) $ — $ (63) (1) Includes the embedded derivative in the commodity contract discussed above. 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) 2021 2020 2021 2020 Purchased product costs Realized loss $ (5) $ (2) $ (11) $ (4) Unrealized loss (2) (10) (41) (1) Purchased product costs derivative loss (7) (12) (52) (5) Total derivative loss $ (7) $ (12) $ (52) $ (5)

Debt

Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings consist of the following: (In millions) September 30, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ — $ 175 Floating rate senior notes — 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations 9 11 Total 18,609 20,536 Unamortized debt issuance costs (105) (116) Unamortized discount/premium (250) (281) Amounts due within one year (1) (764) Total long-term debt due after one year $ 18,253 $ 19,375 Credit Agreement MPLX’s amended and restated credit agreement (as amended, the “MPLX Credit Agreement”), provides for borrowings up to $3.5 billion and a term that extends to July 2024. During the nine months ended September 30, 2021, MPLX borrowed $3.0 billion under the MPLX Credit Agreement, at an average interest rate of 1.345 percent, and repaid $3.175 billion. At September 30, 2021, MPLX had no outstanding borrowings and less than $1 million in letters of credit outstanding under the MPLX Credit Agreement, resulting in total availability of $3.5 billion, or 100 percent of the borrowing capacity . Floating Rate Senior Notes On September 3, 2021 MPLX redeemed, at par value, all of the $1.0 billion aggregate principal amount of floating rate senior notes due September 2022, which resulted in the immediate expense recognition of $2 million of unamortized debt issuance costs. These costs are included on the Consolidated Statements of Income as “Other financial costs.” This redemption was funded primarily by borrowings under the MPC Loan Agreement. 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 5.500 percent. Interest on each series of notes is payable semi-annually in arrears on various dates depending on the series of the notes.

Revenue

Revenue	9 Months Ended
	Sep. 30, 2021
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, 2021 and 2020: Three Months Ended September 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 87 $ 513 $ 600 Service revenue - related parties 896 6 902 Service revenue - product related — 82 82 Product sales 1 447 448 Product sales - related parties 2 24 26 Total revenues from contracts with customers $ 986 $ 1,072 2,058 Non-ASC 606 revenue (1) 501 Total revenues and other income $ 2,559 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 Nine Months Ended September 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 262 $ 1,505 $ 1,767 Service revenue - related parties 2,666 15 2,681 Service revenue - product related — 235 235 Product sales 3 1,031 1,034 Product sales - related parties 8 91 99 Total revenues from contracts with customers $ 2,939 $ 2,877 5,816 Non-ASC 606 revenue (1) 1,477 Total revenues and other income $ 7,293 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 (1) Non-ASC 606 Revenue includes rental income, sales-type lease revenue, 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. Contract assets are included in “Other current assets” and “Other noncurrent 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. The sales and related “Receivable, net” 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, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2021 Contract assets $ 40 $ (21) $ 1 $ 20 Long-term contract assets 2 — — 2 Deferred revenue 37 40 (27) 50 Deferred revenue - related parties 91 59 (77) 73 Long-term deferred revenue 119 11 — 130 Long-term deferred revenue - related parties 48 (12) — 36 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at 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 (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, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $288 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 22 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2021 $ 476 2022 1,795 2023 1,656 2024 1,524 2025 and thereafter 4,484 Total revenue on remaining performance obligations (1),(2),(3) $ 9,935 (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 and sales-type leases are excluded from this table, see further discussion about leases in Note 18. (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, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information Nine Months Ended September 30, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 627 $ 631 Income taxes paid 2 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 do 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) 2021 2020 Decrease in capital accruals $ (19) $ (197)

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
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, 2021 and December 31, 2020, accrued liabilities for remediation totaled $22 million and $17 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, 2021 and December 31, 2020, 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. We hold a 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. In 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 prepare an environmental impact statement (“EIS”) relating to an easement under Lake Oahe in North Dakota. The D.D.C. later ordered vacatur of the easement during the pendency of the EIS and further ordered a shut down of the pipeline by August 5, 2020. On August 5, 2020, the U.S. Court of Appeals for the District of Columbia (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. On January 26, 2021, the Court of Appeals upheld the D.D.C.’s order vacating the easement while the Army Corps prepares the EIS. The Court of Appeals reversed the D.D.C.’s order to the extent it directed that the pipeline be shutdown and emptied of oil. In May 2021, the D.D.C. denied a renewed request for an injunction to shut down the pipeline while the EIS is being prepared. In June 2021, the D.D.C. issued an order dismissing without prejudice the tribes’ claims against the Dakota Access Pipeline. The judge noted that the plaintiffs may move to reopen the case in the event of a violation of the court’s prior orders. Dakota Access has petitioned the U.S. Supreme Court for review of the Court of Appeal’s decision upholding the D.D.C.’s order vacating the easement. The pipeline remains operational. 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. If the pipeline were temporarily shut down, 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, 2021, our maximum potential undiscounted payments under the Contingent Equity Contribution Agreement were approximately $230 million. Other Legal Proceedings – In early July 2020, Tesoro High Plains Pipeline Company, LLC (“THPP”), a subsidiary of 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. THPP appealed this determination, which triggered an automatic stay of the requested pipeline shutdown and payment. On October 29, 2020, 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. On December 15, 2020, the Regional Director of the BIA issued a new trespass notice to THPP consistent with the Assistant Secretary - Indian Affairs order vacating the prior trespass order. The new order found that THPP was in trespass and assessed trespass damages of approximately $4 million (including interest), which has been paid. The order also required THPP to immediately cease and desist use of the portion of the pipeline that crosses the property at issue. THPP has complied with the Regional Director’s December 15, 2020 notice. On February 12, 2021, landowners filed suit in the U.S. District Court for the District of North Dakota (the “District of North Dakota”) against THPP, the Department of the Interior, the Assistant Secretary - Indian Affairs, the Interior Board of Indian Appeals and the BIA, requesting, among other things, that decisions by the Assistant Secretary - Indian Affairs and the Interior Board of Indian Appeals be vacated as to the award of damages to plaintiffs. In March 2021, THPP received a copy of an order purporting to vacate all orders related to THPP’s alleged trespass issued by the BIA between July 2, 2020 and January 14, 2021. The order directs the Regional Director of the BIA to reconsider the issue of THPP’s alleged trespass and issue a new order, if necessary, after all interested parties have had an opportunity to be heard. Subsequently, landowners voluntarily dismissed the suit filed in the District of North Dakota. On April 23, 2021, THPP filed a lawsuit in the District of North Dakota against the United States of America, the U.S. Department of the Interior and the BIA challenging the March order purporting to vacate all previous orders related to THPP’s alleged trespass. We continue to work towards a settlement of this matter with holders of the property rights at issue.

Description of the Business a_2

Description of the Business and Basis of Presentation Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
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, marketing 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. Impairments – Through the first nine months of 2021, we continued to see improvements in the environment in which our business operates as COVID-19 impacts continue to subside. The increased availability of vaccinations and easing of COVID-19 restrictions has been followed by an increase in economic activity despite the recent resurgence in COVID-19. We are unable to predict the long-term impacts COVID-19 may have on our financial position and results. In the second quarter of 2021, we recognized impairment expense of $42 million within our G&P segment related to our continued emphasis on portfolio optimization with the anticipated divestiture of several non-core assets and the closure of other non-core assets. During the first quarter of 2020, 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 at that time resulting in impairments of the carrying value of certain assets. 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 environment at that time, which impacted their 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 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. (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 our Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the full year. 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.
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.
Consolidation, Subsidiaries or Other Investments, Consolidated Entities, Policy [Policy Text Block]	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.
Nature of Operations	We are engaged in the transportation, storage, marketing 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.

Net Income Per Limited Partne_2

Net Income Per Limited Partner Unit Accounting Policy (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Earnings Per Share, Policy [Policy Text Block]	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.

Investments and Noncontrollin_2

Investments and Noncontrolling Interests (Tables)	9 Months Ended
	Sep. 30, 2021
Equity Method Investments and Joint Ventures [Abstract]
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) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 457 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 253 254 LOOP LLC 41% 268 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 188 194 Minnesota Pipe Line Company, LLC 17% 184 188 Whistler Pipeline LLC (2) 38% 146 185 Explorer Pipeline Company 25% 68 72 W2W Holdings LLC (2)(3) 50% 66 72 Other (2) 115 103 Total L&S 1,745 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 682 698 Sherwood Midstream LLC (2) 50% 548 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 335 307 MarkWest Torñado GP, L.L.C. (2) 60% 223 188 Rendezvous Gas Services, L.L.C. (2) 78% 151 159 Sherwood Midstream Holdings LLC (2) 51% 138 148 Centrahoma Processing LLC 40% 132 145 Other (2) 47 49 Total G&P 2,256 2,251 Total $ 4,001 $ 4,036 (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) Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest in Wink to Webster Pipeline 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, 2021 and 2020 is as follows: Nine Months Ended September 30, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 565 $ 940 $ 1,505 Costs and expenses 350 427 777 Income from operations 215 513 728 Net income 172 449 621 Income from equity method investments (1) $ 120 $ 108 $ 228 (1) Includes impairment expense of $6 million. 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 (Loss)/income from operations (176) 528 352 Net (loss)/income (230) 477 247 (Loss)/income from equity method investments (1) $ (1,138) $ 126 $ (1,012) (1) Includes impairment expense of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of September 30, 2021 and December 31, 2020 is as follows: September 30, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 328 $ 444 $ 772 Noncurrent assets 7,426 4,907 12,333 Current liabilities 216 254 470 Noncurrent liabilities $ 2,448 $ 873 $ 3,321 December 31, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 530 $ 318 $ 848 Noncurrent assets 6,889 4,997 11,886 Current liabilities 323 187 510 Noncurrent liabilities $ 1,904 $ 830 $ 2,734

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Schedule of Long-term Debt Instruments [Table Text Block]	Activity on the MPC Loan Agreement was as follows: (In millions) Nine Months Ended September 30, 2021 Year Ended December 31, 2020 Borrowings $ 6,571 $ 6,264 Average interest rate of borrowings 1.345 % 2.278 % Repayments $ 5,201 $ 6,858 Outstanding balance at end of period (1) $ 1,370 $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.
Schedule Of Entity Wide Information Allocated Related Party Employee Benefit Costs By Income Statement Line [Table Text Block]	incurred from MPC totaled $63 million and $195 million, respectively.
Schedule of Related Party Transactions [Table Text Block]	(In millions) September 30, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 547 $ 615 Receivables - Other 7 27 Prepaid - MPC 13 4 Other - MPC 2 1 Lease Receivables - MPC 76 30 Total 645 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 857 386 Unguaranteed residual asset - MPC 41 23 Total 1,160 672 Current liabilities - related parties Payables - MPC 1,577 215 Payables - Other 36 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 43 66 Deferred revenue - Project reimbursements - MPC 48 30 Deferred revenue - Project reimbursements - Other 1 1 Total 1,706 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 228 229 Long-term deferred revenue - Project reimbursements - MPC 62 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 297 $ 283

Net Income Per Limited Partne_3

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class [Table Text Block]	During the three and nine months ended September 30, 2021 and 2020, MPLX had participating securities consisting of common units, certain equity-based compensation awards, Series A preferred units and Series B preferred units and 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, 2021 and 2020 were less than 1 million. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2021 2020 2021 2020 Net income/(loss) attributable to MPLX LP $ 802 $ 665 $ 2,247 $ (1,411) Less: Distributions declared on Series A preferred units (1) 38 20 79 61 Distributions declared on Series B preferred units 10 10 31 31 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 1,305 715 2,717 2,158 Undistributed net loss attributable to MPLX LP $ (551) $ (80) $ (580) $ (3,661) (1) 2021 periods include the Special Distribution Amount. See Note 6 for distribution information.
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	Three Months Ended September 30, 2021 (In millions, except per unit data) Limited Partners’ 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) $ 1,305 $ 38 $ 10 $ 1,353 Undistributed net loss attributable to MPLX LP (551) — — (551) Net income attributable to MPLX LP (2) $ 754 $ 38 $ 10 $ 802 Weighted average units outstanding: Basic 1,024 Diluted 1,025 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.74 Diluted $ 0.74 (1) Includes the Special Distribution Amount. (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. Three Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ 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. Nine Months Ended September 30, 2021 (In millions, except per unit data) Limited Partners’ 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) $ 2,717 $ 79 $ 31 $ 2,827 Undistributed net loss attributable to MPLX LP (580) — — (580) Net income attributable to MPLX LP (2) $ 2,137 $ 79 $ 31 $ 2,247 Weighted average units outstanding: Basic 1,030 Diluted 1,030 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.07 Diluted $ 2.07 (1) Includes the Special Distribution Amount. (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. Nine Months Ended September 30, 2020 (In millions, except per unit data) Limited Partners’ 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.

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of common units during the nine months ended September 30, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 213,727 Units redeemed in unit repurchase program (17,563,855) Balance at September 30, 2021 1,021,427,850 Unit Repurchase Program On November 2, 2020, MPLX announced the board authorization of a unit repurchase program for the repurchase of up to $1 billion of MPLX’s outstanding common units held by the public. MPLX may utilize various methods to effect the repurchases, which could include open market repurchases, negotiated block transactions, tender offers, accelerated unit repurchases or open market solicitations for units, some of which may be effected through Rule 10b5-1 plans. The timing and amount of repurchases will depend upon several factors, including market and business conditions, and repurchases may be initiated, suspended or discontinued at any time. The repurchase authorization has no expiration date. During the nine months ended September 30, 2021, we repurchased 17,563,855 common units at an average cost per unit of $26.79 and paid $465 million of cash, with an additional $5 million of cash paid in early October in connection with the settlement of certain late September purchases. As of September 30, 2021, we had $497 million remaining under our repurchase authorization. Series B Preferred Units MPLX has outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of MPLX with a price to the public of $1,000 per unit (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. Series B preferred unitholders are entitled to receive a fixed distribution of $68.75 per unit, per annum, 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, in each case assuming a distribution is declared by the Board of Directors.
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, 2021 and 2020. Distributions, although earned, are not accrued until declared. 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) 2021 2020 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner (1) $ 1,305 $ 715 $ 2,717 $ 2,158 Series A preferred unit distributions (1) 38 20 79 61 Series B preferred unit distributions 10 10 31 31 Total cash distributions declared $ 1,353 $ 745 $ 2,827 $ 2,250

Series A Preferred Units (Table

Series A Preferred Units (Tables)	9 Months Ended
	Sep. 30, 2021
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2020 through September 30, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated (1) 79 Distributions received by Series A preferred unitholders (61) Balance at September 30, 2021 $ 986

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, Segment Adjusted EBITDA, restructuring expenses, 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) 2021 2020 2021 2020 L&S Service revenue $ 983 $ 989 $ 2,928 $ 2,924 Rental income 172 249 597 737 Product related revenue 3 9 11 49 Sales-type lease revenue - related parties 132 38 305 114 Income from equity method investments 41 36 112 126 Other income 15 13 46 40 Total segment revenues and other income (1) 1,346 1,334 3,999 3,990 Segment Adjusted EBITDA (2) 904 893 2,747 2,604 Restructuring expenses — 27 — 27 Capital expenditures 85 118 220 410 Investments in unconsolidated affiliates 9 4 31 132 G&P Service revenue 519 524 1,520 1,549 Rental income 80 94 263 271 Product related revenue 553 234 1,357 607 Income/(loss) from equity method investments 51 47 116 (1,138) Other income 10 14 38 41 Total segment revenues and other income (1) 1,213 913 3,294 1,330 Segment Adjusted EBITDA (2) 485 442 1,368 1,252 Restructuring expenses — 9 — 9 Capital expenditures 69 131 135 375 Investments in unconsolidated affiliates $ 23 $ 18 $ 85 $ 112 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $405 million for the three and nine months ended September 30, 2021, respectively, and $139 million and $443 million for the three and nine months ended September 30, 2020, respectively. Third party revenues for the G&P segment were $1,170 million and $3,147 million for the three and nine months ended September 30, 2021, respectively, and $858 million and $1,181 million for the three and nine months ended September 30, 2020, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income/(loss).”
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	(In millions) September 30, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 39 $ 15 L&S 20,729 20,938 G&P 14,931 15,461 Total assets $ 35,699 $ 36,414
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The table below provides a reconciliation between “Net income/(loss)” and Segment Adjusted EBITDA. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2021 2020 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 904 $ 893 $ 2,747 $ 2,604 G&P Segment Adjusted EBITDA 485 442 1,368 1,252 Total reportable segments 1,389 1,335 4,115 3,856 Depreciation and amortization (1) (324) (346) (971) (992) Provision for income taxes — (1) (1) (1) Amortization of deferred financing costs (18) (15) (53) (44) Non-cash equity-based compensation (1) (4) (6) (12) Impairment expense — — (42) (2,165) Net interest and other financial costs (200) (223) (618) (647) Gain on extinguishment of debt (2) 14 10 14 Income/(loss) from equity method investments 92 83 228 (1,012) Distributions/adjustments related to equity method investments (129) (130) (371) (369) Unrealized derivative losses (2) (2) (10) (41) (1) Restructuring expenses — (36) — (36) Other (3) (3) (5) (5) Adjusted EBITDA attributable to noncontrolling interests 9 10 29 27 Net income/(loss) $ 811 $ 674 $ 2,274 $ (1,387) (1) Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, 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.

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) September 30, 2021 December 31, 2020 NGLs $ 10 $ 5 Line fill 24 13 Spare parts, materials and supplies 107 100 Total inventories $ 141 $ 118

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: September 30, 2021 December 31, 2020 (In millions) Gross PP&E Accumulated Depreciation Net PP&E Gross PP&E Accumulated Depreciation Net PP&E L&S $ 12,296 $ 3,108 $ 9,188 $ 12,813 $ 2,997 $ 9,816 G&P 14,115 3,145 10,970 14,062 2,660 11,402 Total $ 26,411 $ 6,253 $ 20,158 $ 26,875 $ 5,657 $ 21,218

Goodwill and Intangibles Goodwi

Goodwill and Intangibles Goodwill and Intangibles (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill [Line Items]
Schedule of Goodwill [Table Text Block]	Changes in the carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2019 $ 7,722 $ 3,141 $ 10,863 Accumulated impairment losses — (1,327) (1,327) 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 December 31, 2020 7,657 — 7,657 Balance as of September 30, 2021 7,657 — 7,657 Gross goodwill as of September 30, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of September 30, 2021 $ 7,657 $ — $ 7,657

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
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, 2021 and December 31, 2020 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, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (104) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (104) $ — $ (63)
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	Level 3 instruments include an embedded derivative 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 $0.68 to $1.80 per gallon with a weighted average of $0.88 per gallon and (2) the probability of renewal of 100 percent for the first five five
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, 2021 Three Months Ended September 30, 2020 (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 $ — $ (102) $ — $ (51) Total losses (realized and unrealized) included in earnings (1) — (7) — (12) Settlements — 5 — 2 Fair value at end of period — (104) — (61) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (6) $ — $ (11) Nine Months Ended September 30, 2021 Nine Months Ended September 30, 2020 (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 $ — $ (63) $ — $ (60) Total losses (realized and unrealized) included in earnings (1) — (52) — (5) Settlements — 11 — 4 Fair value at end of period — (104) — (61) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (44) $ — $ (2) (1) Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” 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 third-party debt, excluding finance leases: September 30, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Outstanding debt (1) $ 20,553 $ 18,350 $ 22,951 $ 20,244 (1) Amounts outstanding under the MPC Loan Agreement are not included in the table above, as the carrying value approximates fair value. This balance is reflected in “Current liabilities - related parties” on the Consolidated Balance Sheets.

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of September 30, 2021, 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, 2021 December 31, 2020 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 $ — $ (15) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (89) — (56) Total $ — $ (104) $ — $ (63) (1) Includes the embedded derivative in the commodity contract 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) 2021 2020 2021 2020 Purchased product costs Realized loss $ (5) $ (2) $ (11) $ (4) Unrealized loss (2) (10) (41) (1) Purchased product costs derivative loss (7) (12) (52) (5) Total derivative loss $ (7) $ (12) $ (52) $ (5)

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Schedule of Debt [Table Text Block]	MPLX’s outstanding borrowings consist of the following: (In millions) September 30, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ — $ 175 Floating rate senior notes — 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations 9 11 Total 18,609 20,536 Unamortized debt issuance costs (105) (116) Unamortized discount/premium (250) (281) Amounts due within one year (1) (764) Total long-term debt due after one year $ 18,253 $ 19,375

Revenue (Tables)

Revenue (Tables)	9 Months Ended
	Sep. 30, 2021
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, 2021 and 2020: Three Months Ended September 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 87 $ 513 $ 600 Service revenue - related parties 896 6 902 Service revenue - product related — 82 82 Product sales 1 447 448 Product sales - related parties 2 24 26 Total revenues from contracts with customers $ 986 $ 1,072 2,058 Non-ASC 606 revenue (1) 501 Total revenues and other income $ 2,559 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 Nine Months Ended September 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 262 $ 1,505 $ 1,767 Service revenue - related parties 2,666 15 2,681 Service revenue - product related — 235 235 Product sales 3 1,031 1,034 Product sales - related parties 8 91 99 Total revenues from contracts with customers $ 2,939 $ 2,877 5,816 Non-ASC 606 revenue (1) 1,477 Total revenues and other income $ 7,293 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
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, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at September 30, 2021 Contract assets $ 40 $ (21) $ 1 $ 20 Long-term contract assets 2 — — 2 Deferred revenue 37 40 (27) 50 Deferred revenue - related parties 91 59 (77) 73 Long-term deferred revenue 119 11 — 130 Long-term deferred revenue - related parties 48 (12) — 36 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at 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 (1) Balance represents ASC 606 portion of each respective line item.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	As of September 30, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $288 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 22 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2021 $ 476 2022 1,795 2023 1,656 2024 1,524 2025 and thereafter 4,484 Total revenue on remaining performance obligations (1),(2),(3) $ 9,935 (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 and sales-type leases are excluded from this table, see further discussion about leases in Note 18.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information [Table Text Block]	Nine Months Ended September 30, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 627 $ 631 Income taxes paid 2 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 Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that do 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) 2021 2020 Decrease in capital accruals $ (19) $ (197)

Leases Lessor Disclosure (Table

Leases Lessor Disclosure (Tables)	9 Months Ended
	Sep. 30, 2021
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, 2021 Three Months Ended September 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 139 $ 56 $ 208 $ 70 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type lease revenue- fixed minimum) 133 — 37 — Interest income (Revenue from variable lease payments) $ — $ — $ 1 $ — Nine Months Ended September 30, 2021 Nine Months Ended September 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 485 $ 192 $ 589 $ 199 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type lease revenue- fixed minimum) 305 — 113 — Interest income (Revenue from variable lease payments) $ — $ — $ 1 $ — (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. Capital expenditures related to assets subject to sales-type lease arrangements were $33 million for the three and nine months ended September 30, 2021. Third party lease assets are less than $1 million as of September 30, 2021 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, 2021: (In millions) Related Party 2021 $ 136 2022 543 2023 544 2024 538 2025 525 2026 and thereafter 1,018 Total minimum future rentals 3,304 Less: present value discount 2,371 Lease receivable $ 933
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, 2021 Three Months Ended September 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 139 $ 56 $ 208 $ 70 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type lease revenue- fixed minimum) 133 — 37 — Interest income (Revenue from variable lease payments) $ — $ — $ 1 $ — Nine Months Ended September 30, 2021 Nine Months Ended September 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 485 $ 192 $ 589 $ 199 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type lease revenue- fixed minimum) 305 — 113 — Interest income (Revenue from variable lease payments) $ — $ — $ 1 $ — (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. Capital expenditures related to assets subject to sales-type lease arrangements were $33 million for the three and nine months ended September 30, 2021. Third party lease assets are less than $1 million as of September 30, 2021 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, 2021: (In millions) Related Party 2021 $ 136 2022 543 2023 544 2024 538 2025 525 2026 and thereafter 1,018 Total minimum future rentals 3,304 Less: present value discount 2,371 Lease receivable $ 933

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)	Jun. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss							$ 1,814
Equity Method Investment, Other than Temporary Impairment		$ 6	$ 1,264	$ 1,264	$ 6
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
Goodwill and Intangible Asset Impairment	$ 0	$ 42	$ 0		$ 42	$ 2,165
G&P
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss				$ 1,814			$ 1,814

Javelina Sale (Details)

Javelina Sale (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Sale of Javelina Assets and Liabilities
Noncash or Part Noncash Divestitures [Line Items]
Gain (Loss) on Disposition of Assets	$ 0

Wholesale Exchange (Details)

Wholesale Exchange (Details) - USD ($) $ in Millions	Jul. 31, 2020	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill		$ 7,657	$ 7,657	$ 9,536
Western Refining Wholesale & Western Refining Product Transport [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Redeemed	18,582,088
Partner Capital, Units, contributed, fair value assigned	$ 340
Excess of Fair Value over Book Value	250
Fair Value, Net Asset (Liability)	90
Goodwill	$ 65

Investments and Noncontrollin_3

Investments and Noncontrolling Interests (Summary of Equity Method Investment Financial Information) (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 4,001				$ 4,001				$ 4,036
Revenues		2,559		$ 2,247		7,293		$ 5,320
Costs and Expenses		1,528		1,348		4,357		6,029
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		811		674		2,274		(1,387)
Income/(loss) from equity method investments(1)	[1]	92		83		228	[2]	(1,012)	[3]
Assets, Current		1,516				1,516				1,515
Liabilities, Current		$ 2,721				2,721				2,086
Equity Method Investment, Other than Temporary Impairment			$ 6	1,264	$ 1,264	$ 6
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%
Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%				67.00%
Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		17.00%				17.00%
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%
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%
Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		78.00%				78.00%
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%				40.00%
MarkWest Tornado GP, L.L.C.
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		60.00%				60.00%
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		24.55%				24.55%
Indirect Ownership Interest [Member] | Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		15.00%				15.00%
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 | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	$ 682				$ 682				698
G&P | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	548				548				557
G&P | MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	335				335				307
G&P | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	151				151				159
G&P | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	138				138				148
G&P | Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		132				132				145
G&P | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	47				47				49
G&P | MarkWest Tornado GP, L.L.C.
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	223				223				188
L&S | MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	457				457				465
L&S | Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investments		253				253				254
L&S | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments		268				268				252
L&S | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	188				188				194
L&S | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		184				184				188
L&S | Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	146				146				185
L&S | W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4],[6]	66				66				72
L&S | Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investments		68				68				72
L&S | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	115				115				103
Operating Segments | G&P
Schedule of Equity Method Investments [Line Items]
Equity method investments		2,256				2,256				2,251
Revenues	[7]	1,213		913		3,294		1,330
Income/(loss) from equity method investments(1)		51		47		116		(1,138)
Operating Segments | L&S
Schedule of Equity Method Investments [Line Items]
Equity method investments		1,745				1,745				1,785
Revenues	[7]	1,346		1,334		3,999		3,990
Income/(loss) from equity method investments(1)		41		$ 36		112		126
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues						1,505		1,065
Costs and Expenses						777		713
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest						728		352
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						621		247
Assets, Current		772				772				848
Assets, Noncurrent		12,333				12,333				11,886
Liabilities, Current		470				470				510
Liabilities, Noncurrent		3,321				3,321				2,734
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues						940		933
Costs and Expenses						427		405
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest						513		528
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						449		477
Income/(loss) from equity method investments(1)						108	[2]	126	[3]
Assets, Current		444				444				318
Assets, Noncurrent		4,907				4,907				4,997
Liabilities, Current		254				254				187
Liabilities, Noncurrent		873				873				830
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues						565		132
Costs and Expenses						350		308
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest						215		(176)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						172		(230)
Income/(loss) from equity method investments(1)						120	[2]	$ (1,138)	[3]
Assets, Current		328				328				530
Assets, Noncurrent		7,426				7,426				6,889
Liabilities, Current		216				216				323
Liabilities, Noncurrent		$ 2,448				$ 2,448				$ 1,904
[1]	The nine months ended September 30, 2021 and September 30, 2020 include $6 million and $1,264 million of impairment expense, respectively.
[2]	Includes impairment expense of $6 million.
[3]	Includes impairment expense of $1,264 million.
[4]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.
[5]	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.
[6]	Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest in Wink to Webster Pipeline LLC.
[7]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $405 million for the three and nine months ended September 30, 2021, respectively, and $139 million and $443 million for the three and nine months ended September 30, 2020, respectively. Third party revenues for the G&P segment were $1,170 million and $3,147 million for the three and nine months ended September 30, 2021, respectively, and $858 million and $1,181 million for the three and nine months ended September 30, 2020, respectively.

Related Party Agreements and _3

Related Party Agreements and Transactions - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Restructuring Charges	$ 0	$ 36	$ 0	$ 36	$ 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	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
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
Related party debt - borrowings			$ 6,571	$ 4,870	$ 6,264
Line of Credit Facility, Interest Rate During Period			1.345%		2.278%
Repayments of Lines of Credit			$ 5,201	$ 5,464	$ 6,858
Long-term Line of Credit	[1]		$ 1,370		$ 0
[1]	Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.

Related Party Disclosures (Deta

Related Party Disclosures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Revenue from Related Parties			$ 19	$ 8
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Sales Revenue, Goods, Related Party, Net Zero	$ 203	$ 107	548	332
Marathon Petroleum Corporation [Member] | Asset under Construction [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 13	$ 29	$ 40	$ 80

Summary of Charges for Employee

Summary of Charges for Employee Services and Omnibus Agreements (Details) - Marathon Petroleum Corporation [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
General and Administrative Expense	$ 70	$ 63	$ 190	$ 195
Asset under Construction [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 13	$ 29	$ 40	$ 80

Other Assets and Liabilities fr

Other Assets and Liabilities from Related Parties (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Current Assets, Related Parties	$ 645	$ 677
Due to Related Parties, Current	1,706	356
Operating Lease, Liability, Current	1	1
Liabilities, Related Parties, Noncurrent	297	283
Affiliated Entity
Related Party Transaction [Line Items]
Due from Related Parties, Noncurrent	1,160	672
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	547	615
Prepaid Insurance	13	4
Other current assets	2	1
Sales-type Lease, Interest Income	76	30
Due from Related Parties, Noncurrent	32	32
Due to Affiliate, Current	1,577	215
Operating Lease, Right-of-Use Asset	230	231
Net Investment in Lease, Noncurrent	857	386
Sales-type Lease, Unguaranteed Residual Asset	41	23
Operating Lease, Liability, Noncurrent	228	229
Other Affiliates [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	7	27
Due to Affiliate, Current	36	43
Minimum Committed Volume Contracts [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	43	66
Reimbursable Projects [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	48	30
Deferred Revenue, Noncurrent, Related Parties	62	47
Reimbursable Projects [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	1	1
Deferred Revenue, Noncurrent, Related Parties	$ 7	$ 7

Net Income Per Limited Partne_4

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2021		Sep. 30, 2020		Sep. 30, 2021		Sep. 30, 2020
Net Income Per Share [Line Items]
Potentially dilutive units		1		1		1		1
Net Income (Loss) Attributable to Parent		$ 802	[1]	$ 665	[1]	$ 2,247	[1]	$ (1,411)
Distribution Made to Limited Partner, Cash Distributions Declared		1,353		745		2,827		2,250
Undistributed net income (loss) attributable to MPLX LP		(551)		(80)		(580)		(3,661)
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[3]	1,305	[2],[4]	715		2,717	[2],[4]	2,158
Undistributed net income (loss) attributable to MPLX LP		(551)		(80)		(580)		(3,661)
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent		38	[1]	20	[1]	79	[1]	61
Distribution Made to Limited Partner, Cash Distributions Declared	[3]	38	[2],[4]	20		79	[2],[4]	61
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 (Loss) Attributable to Parent		10	[1]	10	[1]	31	[1]	31
Distribution Made to Limited Partner, Cash Distributions Declared	[3]	10		10		31		31
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Common Stock | Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent		$ 754	[1]	$ 635	[1]	$ 2,137	[1]	$ (1,503)
[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]	2021 periods include the Special Distribution Amount
[3]	2021 periods include the Special Distribution Amount. See Note 6 for distribution information.
[4]	Includes the Special Distribution Amount.

Net Income Per Limited Partne_5

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Details) - Limited Partners Common Units [Member] - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Weighted average units outstanding:
Common - basic (in shares)	1,024	1,046	1,030	1,054
Common - diluted (in shares)	1,025	1,047	1,030	1,054
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax	$ 0.74	$ 0.61	$ 2.07	$ (1.43)
Common - diluted (in USD per unit)	$ 0.74	$ 0.61	$ 2.07	$ (1.43)

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ / shares in Units, $ in Millions		Oct. 26, 2021	Oct. 04, 2021	Aug. 16, 2021	Jul. 27, 2021	Apr. 27, 2021	Oct. 27, 2020	Jul. 28, 2020	Apr. 28, 2020	Jul. 29, 2019	Sep. 30, 2021		Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021		Sep. 30, 2020
Stockholders Equity [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders											$ (726)		$ (707)	$ (735)	$ (736)	$ (728)	$ (738)
Distribution Made to Limited Partner, Cash Distributions Declared											1,353				745			$ 2,827		$ 2,250
Stock Repurchase Program, Authorized Amount											1,000							$ 1,000
Stock Repurchased and Retired During Period, Shares																		17,563,855
Treasury Stock Acquired, Average Cost Per Share																		$ 26.79
Stock Repurchased and Retired During Period, Value											160		155	$ 155
Stock Repurchase Program, Remaining Authorized Repurchase Amount											$ 497							$ 497
Cash distributions declared per limited partner common unit					$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875
Payments for Repurchase of Common Stock																		$ 465		0
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2020			1,021,427,850											1,038,777,978				1,038,777,978
Unit-based compensation awards																		213,727
Balance at September 30, 2021											1,021,427,850							1,021,427,850
Distribution Made to Limited Partner, Cash Distributions Declared	[2]										$ 1,305	[1],[3]			715			$ 2,717	[1],[3]	2,158
Series B Preferred Stock [Member]
Stockholders Equity [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders											(20)		0	$ (21)	(20)	$ 0	$ (21)
Stock Repurchased and Retired During Period, Value											0		$ 0	$ 0
Subsequent Event
Stockholders Equity [Line Items]
Cash distributions declared per limited partner common unit		$ 1.2800
Payments for Repurchase of Common Stock			$ 5
Series B Preferred Stock [Member]
Stockholders Equity [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders				$ (21)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[2]										$ 10				$ 10			$ 31		$ 31
Series B Preferred Stock [Member] | ANDX LP [Member]
Stockholders Equity [Line Items]
Preferred Units, Outstanding										600,000
Dividend rate, percentage										6.875%								4.652%
Price per share										$ 1,000
Preferred Stock, Dividend Payment Rate, Variable																		LIBOR plus
Series B Preferred Stock [Member] | ANDX LP [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Cash distributions declared per limited partner common unit																		$ 68.75
[1]	2021 periods include the Special Distribution Amount
[2]	2021 periods include the Special Distribution Amount. See Note 6 for distribution information.
[3]	Includes the Special Distribution Amount.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions

Nov. 12, 2021

Nov. 05, 2021

Oct. 26, 2021

Aug. 16, 2021

Jul. 27, 2021

Apr. 27, 2021

Oct. 27, 2020

Jul. 28, 2020

Apr. 28, 2020

Jul. 29, 2019

Sep. 30, 2021

Jun. 30, 2021

Mar. 31, 2021

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Sep. 30, 2021

Sep. 30, 2020

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution Made to Limited Partner, Cash Distributions Declared

$ 1,353

$ 745

$ 2,827

$ 2,250

Cash distributions declared per limited partner common unit

$ 0.6875

$ 0.6875

$ 0.6875

$ 0.6875

$ 0.6875

Partners' Capital Account, Distributions

726

$ 707

$ 735

736

$ 728

$ 738

Series B Preferred Stock [Member]

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution date

Aug. 16, 2021

Partners' Capital Account, Distributions

$ 21

Subsequent Event

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution Made to Limited Partner, Declaration Date

Nov. 2, 2021

Cash distributions declared per limited partner common unit

$ 1.2800

Date of record

Nov. 12, 2021

Subsequent Event | Base quarterly distribution

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Cash distributions declared per limited partner common unit

0.705

Subsequent Event | Special Distribution

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Cash distributions declared per limited partner common unit

$ 0.575

Subsequent Event | Limited Partners Common Units [Member]

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution date

Nov. 19, 2021

Limited Partners Common Units [Member]

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution Made to Limited Partner, Cash Distributions Declared

[2]

1,305

[1],[3]

715

2,717

[1],[3]

2,158

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

[2]

10

10

31

31

Series A Preferred Stock [Member] | Series A Preferred Stock [Member]

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Distribution Made to Limited Partner, Cash Distributions Declared

[2]

$ 38

[1],[3]

$ 20

$ 79

[1],[3]

$ 61

ANDX LP [Member] | 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 Units, Outstanding

600,000

Preferred Stock, Dividend Rate, Percentage

6.875%

4.652%

ANDX LP [Member] | Series A Preferred Stock [Member] | Series B Preferred Stock [Member]

Incentive Distribution Made to Managing Member or General Partner [Line Items]

Cash distributions declared per limited partner common unit

$ 68.75

[1]

2021 periods include the Special Distribution Amount

[2]

2021 periods include the Special Distribution Amount. See Note 6 for distribution information.

[3]

Includes the Special Distribution Amount.

Series A Preferred Units (Narra

Series A Preferred Units (Narrative) (Details) - $ / shares shares in Millions	Oct. 26, 2021	Jul. 27, 2021	Apr. 27, 2021	Oct. 27, 2020	Jul. 28, 2020	Apr. 28, 2020	May 13, 2016	Sep. 30, 2021
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Distributions Declared, Per Unit		$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Dividend rate, per-dollar-amount							$ 0.528125
Partners' Capital Account, Units								29.6
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Declaration Date	Nov. 2, 2021
Distribution Made to Limited Partner, Distributions Declared, Per Unit	$ 1.2800

Series A Preferred Units (Rollf

Series A Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020
Redeemable Noncontrolling Interest [Line Items]
Distributions to unitholders and general partner		$ (2,126)	$ (2,162)
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2020		968
Net income allocated (1)	[1]	79
Distributions to unitholders and general partner		(61)
Balance at September 30, 2021		$ 986
[1]	Includes the Special Distribution Amount

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 2021
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				12 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	$ 324	$ 346	$ 971		$ 992
Income/(loss) from equity method investments(1)	[2]	92	83	228	[3]	(1,012)	[4]
Total segment revenues and other income		(2,559)	(2,247)	(7,293)		(5,320)
Payments to Acquire Equity Method Investments				(116)		(244)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[5]	(2)	(10)	(41)		(1)
Marathon Petroleum Corporation [Member]
Segment Reporting Information [Line Items]
Restructuring Charges		0	36	0		36		$ 36
Sales-type Lease, Revenue		132	38	305		114
L&S
Segment Reporting Information [Line Items]
Depreciation and amortization		131	164	414		440
Total revenues from contracts with customers		986	998	2,939		2,973
Payments to Acquire Equity Method Investments		(9)	(4)	(31)		(132)
L&S | Marathon Petroleum Corporation [Member]
Segment Reporting Information [Line Items]
Sales-type Lease, Revenue		132	38	305		114
G&P
Segment Reporting Information [Line Items]
Depreciation and amortization		193	182	557		552
Total revenues from contracts with customers		1,072	758	2,877		2,156
Payments to Acquire Equity Method Investments		(23)	(18)	(85)		(112)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA		1,389	1,335	4,115		3,856
Operating Segments | L&S
Segment Reporting Information [Line Items]
Restructuring Charges		0	27	0		27
Operating Lease, Lease Income		172	249	597		737
Income/(loss) from equity method investments(1)		41	36	112		126
Other income		15	13	46		40
Total segment revenues and other income	[6]	(1,346)	(1,334)	(3,999)		(3,990)
Adjusted EBITDA	[7]	904	893	2,747		2,604
Capital Expenditure		85	118	220		410
Operating Segments | G&P
Segment Reporting Information [Line Items]
Restructuring Charges		0	9	0		9
Operating Lease, Lease Income		80	94	263		271
Income/(loss) from equity method investments(1)		51	47	116		(1,138)
Other income		10	14	38		41
Total segment revenues and other income	[6]	(1,213)	(913)	(3,294)		(1,330)
Adjusted EBITDA	[7]	485	442	1,368		1,252
Capital Expenditure		69	131	135		375
Service [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		600	604	1,767		1,779
Service [Member] | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		87	87	262		248
Service [Member] | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		513	517	1,505		1,531
Service [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		983	989	2,928		2,924
Service [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		519	524	1,520		1,549
Product [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		448	165	1,034		454
Product [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		3	9	11		49
Product [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		553	234	1,357		607
Third Party [Member] | L&S
Segment Reporting Information [Line Items]
Total segment revenues and other income		(138)	(139)	(405)		(443)
Third Party [Member] | G&P
Segment Reporting Information [Line Items]
Total segment revenues and other income		$ (1,170)	$ (858)	$ (3,147)		$ (1,181)
[1]	Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, respectively.
[2]	The nine months ended September 30, 2021 and September 30, 2020 include $6 million and $1,264 million of impairment expense, respectively.
[3]	Includes impairment expense of $6 million.
[4]	Includes impairment expense of $1,264 million.
[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.
[6]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $405 million for the three and nine months ended September 30, 2021, respectively, and $139 million and $443 million for the three and nine months ended September 30, 2020, respectively. Third party revenues for the G&P segment were $1,170 million and $3,147 million for the three and nine months ended September 30, 2021, respectively, and $858 million and $1,181 million for the three and nine months ended September 30, 2020, respectively.
[7]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income/(loss).”

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 39	$ 15
Assets	35,699	36,414
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	20,729	20,938
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,931	$ 15,461

Segment Information - Reconcili

Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended				12 Months Ended
		Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization	[1]	$ 324		$ 346	$ 971		$ 992
Income Tax Expense (Benefit)		0		(1)	(1)		(1)
Amortization of Debt Issuance Costs					(53)		(44)
Share-based Payment Arrangement, Expense		(1)		(4)	(6)		(12)
Goodwill and Intangible Asset Impairment		0	$ (42)	0	(42)		(2,165)
Interest and Other Financial Costs		(200)		(223)	(618)		(647)
Extinguishment of Debt, Gain (Loss), Net of Tax		(2)		14	10		14
Income/(loss) from equity method investments(1)	[2]	92		83	228	[3]	(1,012)	[4]
Proceeds from Equity Method Investment, Distribution					(361)		(350)
Other Cost and Expense, Operating		3		3	5		5
Net income		811		674	2,274		(1,387)
Marathon Petroleum Corporation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		0		(36)	0		(36)		$ (36)
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		1,389		1,335	4,115		3,856
Amortization of Debt Issuance Costs		(18)		(15)	(53)		(44)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution		(129)		(130)	(371)		(369)
Adjusted EBITDA attributable to noncontrolling interests		9		10	29		27
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		131		164	414		440
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		0		(27)	0		(27)
Adjusted EBITDA	[5]	904		893	2,747		2,604
Income/(loss) from equity method investments(1)		41		36	112		126
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		193		182	557		552
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		0		(9)	0		(9)
Adjusted EBITDA	[5]	485		442	1,368		1,252
Income/(loss) from equity method investments(1)		51		47	116		(1,138)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[6]	$ (2)		$ (10)	$ (41)		$ (1)
[1]	Depreciation and amortization attributable to L&S was $131 million and $414 million for the three and nine months ended September 30, 2021, respectively, and $164 million and $440 million for the three and nine months ended September 30, 2020, respectively. Depreciation and amortization attributable to G&P was $193 million and $557 million for the three and nine months ended September 30, 2021, respectively, and $182 million and $552 million for the three and nine months ended September 30, 2020, respectively.
[2]	The nine months ended September 30, 2021 and September 30, 2020 include $6 million and $1,264 million of impairment expense, respectively.
[3]	Includes impairment expense of $6 million.
[4]	Includes impairment expense of $1,264 million.
[5]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income/(loss).”
[6]	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, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
NGLs	$ 10	$ 5
Line fill	24	13
Spare parts, materials and supplies	107	100
Total inventories	$ 141	$ 118

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 174
Property, plant and equipment, gross		$ 26,411	$ 26,875
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		6,253	5,657
Property, Plant and Equipment, Net, Total		20,158	21,218
L&S
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,296	12,813
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		3,108	2,997
Property, Plant and Equipment, Net, Total		9,188	9,816
G&P
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		14,115	14,062
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		3,145	2,660
Property, Plant and Equipment, Net, Total		$ 10,970	$ 11,402

Goodwill and Intangibles Good_2

Goodwill and Intangibles Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Sep. 30, 2021	Jul. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill, Gross			$ 10,798		$ 10,863
Goodwill, Impaired, Accumulated Impairment Loss			(3,141)		(1,327)
Goodwill		$ (7,657)	(7,657)		(9,536)
Goodwill, Impairment Loss		(1,814)
Goodwill, Written off Related to Sale of Business Unit		(65)
Impairment of Intangible Assets, Finite-lived	$ 177
L&S
Goodwill [Line Items]
Goodwill, Gross			7,657		7,722
Goodwill, Impaired, Accumulated Impairment Loss			0		0
Goodwill		(7,657)	(7,657)		(7,722)
Goodwill, Impairment Loss		0
G&P
Goodwill [Line Items]
Goodwill, Gross			3,141		3,141
Goodwill, Impaired, Accumulated Impairment Loss			(3,141)		(1,327)
Goodwill		0	$ 0		$ (1,814)
Goodwill, Impairment Loss	$ (1,814)	(1,814)
Western Refining Wholesale & Western Refining Product Transport [Member]
Goodwill [Line Items]
Goodwill				$ (65)
Western Refining Wholesale & Western Refining Product Transport [Member] | L&S
Goodwill [Line Items]
Goodwill, Written off Related to Sale of Business Unit		(65)
Western Refining Wholesale & Western Refining Product Transport [Member] | G&P
Goodwill [Line Items]
Goodwill, Written off Related to Sale of Business Unit		$ 0

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Commodity contracts | Not Designated as Hedging Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	(104)	(63)
Embedded derivatives in commodity contracts | Fair Value, Recurring [Member] | 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		$ 104	$ 63
[1]	Includes the embedded derivative in the commodity contract discussed above.

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details)		9 Months Ended
		Sep. 30, 2021USD ($)$ / gal	Dec. 31, 2020USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, Average Forward Price | $ / gal		0.88
Commodity contracts | Not Designated as Hedging Instrument [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	$ (104,000,000)	(63,000,000)
Embedded derivatives in commodity contracts | Natural Gas [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%
Fair Value, Inputs, Level 3 [Member] | Embedded derivatives in commodity contracts | Fair Value, Recurring [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative Asset		$ 0	$ 0
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		0.68
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		$ 1.80
[1]	Includes the embedded derivative in the commodity contract discussed above.

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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
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
Commodity Derivative Contracts (net) | Not Designated as Hedging Instrument [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[2]	0		0		$ 0
Derivative Liability, Fair Value, Gross Liability	[2]	(104)		(104)		$ (63)
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		(102)	(51)	(63)	(60)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(7)	(12)	(52)	(5)
Settlements		5	2	11	4
Fair value at end of period		(104)	(61)	(104)	(61)
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		$ (6)	$ (11)	$ (44)	$ (2)
[1]	Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” on the Consolidated Statements of Income.
[2]	Includes the embedded derivative in the commodity contract discussed above.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt(1)	$ 18,350	$ 20,244
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding debt(1)	$ 20,553	$ 22,951

Derivative Financial Instrume_3

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)	Dec. 31, 2020USD ($)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Derivative [Line Items]
Derivative Liability	$ (104)	$ (63)
Natural Gas [Member]
Derivative [Line Items]
Number of Renewals	2
Embedded Derivative Renewal Term	5 years
Embedded Derivative Second Renewal Term	5 years

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, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	104	63
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]	15	7
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]	$ 89	$ 56
[1]	Includes the embedded derivative in the commodity contract discussed above.

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Total gain (loss)		$ (7)	$ (12)	$ (52)	$ (5)
Purchased product costs
Derivative [Line Items]
Realized gain (loss)		(5)	(2)	(11)	(4)
Total gain (loss)		(7)	(12)	(52)	(5)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[1]	$ (2)	$ (10)	$ (41)	$ (1)
[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	Sep. 30, 2021	Sep. 03, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total	$ 18,609		$ 20,536
Unamortized debt issuance costs	(105)		(116)
Unamortized discount/premium	(250)		(281)
Amounts due within one year	(1)		(764)
Total long-term debt due after one year	18,253		19,375
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0	$ 1,000	1,000
Fixed Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	18,532		19,240
Fixed Rate Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	23		23
Fixed Rate Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	45		87
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Financing lease obligations	9		11
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Long-term Line of Credit	$ 0		$ 175
Minimum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.75%
Maximum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 03, 2021	Jan. 15, 2021	Dec. 31, 2020	Jul. 30, 2019
Debt Instrument [Line Items]
Debt and Lease Obligation	$ 18,609			$ 20,536
Unamortized Debt Issuance Expense	105			116
Debt Instrument, Unamortized Discount	250			281
Debt, Current	1			764
Long-term debt	18,253			19,375
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	0			175
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit	$ 3,000
Debt Instrument, Interest Rate, Effective Percentage	1.345%
Repayments of Long-term Lines of Credit	$ 3,175
Letters of Credit Outstanding	1
Line of Credit Facility, Remaining Borrowing Capacity	$ 3,500
Line of Credit Facility, Remaining Borrowing Capacity, Percentage	100.00%
Fixed Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 18,532			19,240
Fixed Rate Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	45			87
Fixed Rate Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	23			23
Fixed Rate Senior Notes [Member] | Senior Notes Due January 2025
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt	$ (8)
Long-term Debt, Gross			$ 750
Debt Instrument, Interest Rate, Stated Percentage	525.00%
Percent of Par			102.625%
Payment for Debt Extinguishment or Debt Prepayment Cost	$ 20
Write off of Deferred Debt Issuance Cost	12
Fixed Rate Senior Notes [Member] | Senior Notes Due January 2025 | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 42
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0	$ 1,000		1,000
Variable Rate Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt	(2)
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Finance Lease, Liability	$ 9			$ 11
Minimum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.75%
Maximum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Noninterest Income		$ 2,058	$ 1,756	$ 5,816	$ 5,129
Revenue Not from Contract with Customer, Other	[1]	501	491	1,477	191
Revenues		2,559	2,247	7,293	5,320
L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		986	998	2,939	2,973
G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		1,072	758	2,877	2,156
Service [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		600	604	1,767	1,779
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		902	909	2,681	2,694
Service [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		87	87	262	248
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		896	902	2,666	2,676
Service [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		513	517	1,505	1,531
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		6	7	15	18
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		82	41	235	102
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		82	41	235	102
Product [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		448	165	1,034	454
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		26	37	99	100
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		448	165	1,034	454
Product [Member] | L&S
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		2	2	8	10
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		1	7	3	39
Product [Member] | G&P
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		24	35	91	90
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		$ 447	$ 158	$ 1,031	$ 415
[1]	Non-ASC 606 Revenue includes rental income, sales-type lease revenue, 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, 2021	Sep. 30, 2020	Dec. 31, 2020	[1]
Long-term deferred revenue, beginning balance		$ 314
Long-term deferred revenue, ending balance		366
Liability, change in timeframe, performance obligation satisfied, revenue recognized		0	$ 0
Contract Liability, Noncurrent, Period Increase (Decrease)		0
ASC 606
Contract assets, beginning balance	[1]	40	39
Contract assets, additions/(deletions)		(21)	(9)
Contract assets, revenue recognized	[2]	1	(1)
Contract assets, ending balance		20	29
Contract With Customer Non Current Asset Reclassified To Receivable [Line Items]		0
Contract with Customer, Asset, before Allowance for Credit Loss, Noncurrent		2		$ 2
Deferred revenue, beginning balance	[1]	37	23
Deferred revenue, additions/(deletions)		40	16
Deferred revenue, revenue recognized	[2]	(27)	(6)
Deferred revenue, ending balance		50	33
Deferred revenue - related parties, beginning balance	[1]	91	53
Deferred revenue - related party, additions/(deletions)		59	77
Deferred revenue - related parties, revenue recognized	[2]	(77)	(60)
Deferred revenue - related parties, ending balance		73	70
Long-term deferred revenue, beginning balance	[1]	119	90
Long-term deferred revenue, additions/(deletions)		11	25
Long-term deferred revenue, revenue recognized	[2]	0	0
Long-term deferred revenue, ending balance		130	115
Long-term deferred revenue - related parties, beginning balance	[1]	48	55
Long-term deferred revenue - related party, additions/(deletions)		(12)	(7)
Long-term deferred revenue - related parties, revenue recognized	[2]	0	0
Long-term deferred revenue - related parties, ending balance		36	$ 48
Contract Liability, Noncurrent, Revenue Recognized		0
Contract with Customer, Liability, Noncurrent		6		$ 6
Contract with Customer, Asset Increase (Decrease), Noncurrent		$ 0
[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, 2021USD ($)
Revenue from Contract with Customer [Abstract]
Contract with customer, liability	$ 288
Revenue, Remaining Performance Obligation, Amount	9,935	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 476
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]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,795
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]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,656
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]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,524
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]: 2025-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 4,484
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2043-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	22 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 and sales-type leases are excluded from this table, see further discussion about leases in Note 18.
[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 (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jul. 31, 2020
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 627	$ 631
Income Taxes Paid, Net	2	1
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 (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Supplemental Cash Flow Elements [Abstract]
Decrease in capital accruals	$ (19)	$ (197)

Leases Leases Narrative (Detail

Leases Leases Narrative (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Reclassification, Other
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount		$ 33	$ 421			$ 33
Marathon Petroleum Corporation [Member]
Operating Lease, Lease Income	[1]	139		$ 208		485	$ 589
Sales-type Lease, Unguaranteed Residual Asset		41				41		$ 23
Sales-type Lease, Selling Profit (Loss)		0		0		0	0
Sales-type Lease, Lease Receivable		933				933
Marathon Petroleum Corporation [Member] | Reclassification, Other
Sales-type Lease, Unguaranteed Residual Asset			14
Sales-type Lease, Selling Profit (Loss)			112
Sales-type Lease, Lease Receivable			$ 519
Refining Logistics [Member] | Reclassification, Other
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount					$ 171
Refining Logistics [Member] | Marathon Petroleum Corporation [Member] | Reclassification, Other
Sales-type Lease, Unguaranteed Residual Asset					10
Sales-type Lease, Selling Profit (Loss)					209
Sales-type Lease, Lease Receivable					$ 370
Third Party [Member]
Operating Lease, Lease Income	[1]	56		70		192	199
Sales-type Lease, Selling Profit (Loss)		0		$ 0		0	$ 0
Sales-type Lease, Lease Receivable		$ 1				$ 1
[1]	These amounts are presented net of executory costs

Leases Lessor Maturity Table (D

Leases Lessor Maturity Table (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable	$ 933
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Current Year	136
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year One	543
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Three Years	544
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Three	538
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Four	525
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, after Year Five	1,018
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received	3,304
Sales-type and Direct Financing Leases, Lease Receivable, Undiscounted Excess Amount	2,371
Marathon Petroleum Corporation [Member] | Reclassification, Other
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable		$ 519
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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	$ 139	$ 208	$ 485	$ 589
Sales-type Lease, Selling Profit (Loss)		0	0	0	0
Sales-type Lease, Interest Income		133	37	305	113
Sales-type Lease, Variable Lease Income		0	1	0	1
Third Party [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	56	70	192	199
Sales-type Lease, Selling Profit (Loss)		0	0	0	0
Sales-type Lease, Interest Income		0	0	0	0
Sales-type Lease, Variable Lease Income		$ 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	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2021
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation	$ 17		$ 22
Due to Related Parties, Current	356		1,706
Loss Contingency, Damages Sought, Value		$ 187
Loss Contingency, Damages Paid, Value	4
Environmental Loss Contingency [Member] | Marathon Petroleum Corporation [Member]
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