MPLX (MPLX) 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 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	Mar. 31, 2021
Amendment Flag	false
Entity File Number	001-35714
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	0001552000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		1,029,527,052
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
		Mar. 31, 2021		Mar. 31, 2020
Total revenues from contracts with customers		$ 1,862		$ 1,781
Operating Lease, Lease Income		99		96
Income/(loss) from equity method investments(1)		70	[1],[2]	(1,184)	[3],[4]
Other Operating Income		1		1
Related Party Transaction, Other Revenues from Transactions with Related Party		65		64
Revenues		2,339		992
Related Party Transaction, Purchases from Related Party		298		276
Depreciation and amortization	[5]	329		325
Goodwill and Intangible Asset Impairment		0		2,165
General and Administrative Expense		86		97
Taxes, Miscellaneous		32		31
Costs and Expenses		1,365		3,478
Interest And Other Financial Costs From Related Parties		0		3
Other Nonoperating Income (Expense)		27		16
Interest Expense, Debt, Excluding Amortization		198		211
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent, Total		974		(2,486)
Provision for income taxes		1		0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total		749		(2,716)
Less: Net income (loss) attributable to noncontrolling interest		9		8
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total		748		(2,716)
Limited partners' interest in net income (loss) attributable to MPLX LP		708		(2,755)
Net income (loss) attributable to MPLX LP	[6]	739		(2,724)
Rental cost of sales - related parties
Labor and Related Expense		32		35
Limited Partners Common Units [Member]
Net income (loss) attributable to MPLX LP	[6]	$ 708		$ (2,755)
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.68		$ (2.60)
Common - diluted (in USD per unit)		$ 0.68		$ (2.60)
Weighted average limited partner units outstanding:
Common - basic (in shares)		1,037		1,058
Common - diluted (in shares)		1,037		1,058
Service [Member]
Total revenues from contracts with customers		$ 589		$ 612
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		872		928
Service, Other [Member]
Total revenues from contracts with customers		77		39
Product [Member]
Total revenues from contracts with customers		282		169
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		42		33
Oil and Gas, Refining and Marketing [Member]
Cost of Goods and Services Sold		273		368
Natural Gas, Midstream [Member]
Cost of Goods and Services Sold		276		135
Series A Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		20		20
Net income (loss) attributable to MPLX LP	[6]	20		20
Series B Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		11		11
Net income (loss) attributable to MPLX LP	[6]	11		11
Marathon Petroleum Corporation [Member]
Operating Lease, Lease Income		242		234
Related Party Transaction, Other Revenues from Transactions with Related Party		49		48
Related Party Transaction, Purchases from Related Party		294		271
General and Administrative Expense		57		64
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties
Labor and Related Expense		39		46
Marathon Petroleum Corporation [Member] | Service [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		871		927
Marathon Petroleum Corporation [Member] | Product [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[7]	$ 42		$ 33
[1]	Includes the impact of any basis differential amortization or accretion.
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[5]	Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020.
[6]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[7]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2021 and March 31, 2020, these sales totaled $168 million and $173 million, respectively.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Equity Method Investment, Other than Temporary Impairment		$ 1,264
Interest Costs Capitalized	$ 5	$ 13

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 748	$ (2,716)
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	[1]	(2)	(1)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total		746	(2,717)
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest [Abstract]
Less: Net income (loss) attributable to noncontrolling interest		9	8
Comprehensive Income (Loss), Net of Tax, Attributable to Parent, Total		$ 737	$ (2,725)
[1]	Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) shares in Millions, $ in Millions	Mar. 31, 2021		Dec. 31, 2020
Assets
Cash and cash equivalents	$ 24		$ 15
Receivables, net	523		452
Current Assets, Related Parties	650		677
Inventories	128		118
Other current assets	49		65
Total current assets	1,374		1,515
Equity method investments	4,040		4,036
Property, plant and equipment, net	20,996		21,218
Intangibles, net	927		959
Goodwill	7,657		7,657
Operating Lease, Right-of-Use Asset	296		309
Due from Related Parties, Noncurrent	676		672
Other noncurrent assets	64		48
Total assets	36,030		36,414
Current liabilities:
Accounts payable	162		152
Accrued Liabilities, Current	233		194
Due to Related Parties, Current	338		356
Accrued Property Plant and Equipment Current	58		84
Accrued interest payable	189		222
Operating Lease, Liability, Current	63		63
Other current liabilities	151		150
Total current liabilities	1,196		2,086
Long-term deferred revenue	332		314
Liabilities, Related Parties, Noncurrent	279		283
Long-term debt	20,052		19,375
Deferred income taxes	11		12
Operating Lease, Liability, Noncurrent	230		244
Debt, Current	2		764
Deferred credits and other liabilities	112		115
Total liabilities	22,212		22,429
Commitments and contingencies (see Note 21)
Equity
Total MPLX LP partners’ capital	12,606		12,772
Noncontrolling interests	244		245
Total equity	12,850		13,017
Total liabilities, preferred units and equity	36,030		36,414
Disposal Group, Including Discontinued Operation, Assets, Current	0		188
Disposal Group, Including Discontinued Operation, Liabilities, Current	0		101
Series A Convertible Preferred Units
Current liabilities:
Series A preferred units	968		968
Marathon Petroleum Corporation [Member]
Assets
Other current assets	1		1
Operating Lease, Right-of-Use Asset	230		231
Due from Related Parties, Noncurrent	32		32
Current liabilities:
Due to Related Parties, Current	213		215
Operating Lease, Liability, Current	1		1
Operating Lease, Liability, Noncurrent	$ 229		$ 229
Series B Preferred Stock [Member]
Current liabilities:
Units issued	0.6		0.6
Equity
Total MPLX LP partners’ capital	$ 601		$ 611
Units Outstanding	0.6		0.6
Limited Partners Common Units [Member] | Public [Member]
Current liabilities:
Units issued	385		391
Equity
Total MPLX LP partners’ capital	$ 9,226		$ 9,384
Units Outstanding	385		391
Limited Partners Common Units [Member] | Marathon Petroleum Corporation [Member]
Current liabilities:
Units issued	647		647
Equity
Total MPLX LP partners’ capital	$ 2,796		$ 2,792
Units Outstanding	647		647
LOOP and Explorer
Equity
Accumulated other comprehensive loss	$ (17)	[1]	$ (15)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Mar. 31, 2021	Dec. 31, 2020
Limited Partners Common Units [Member] | Public [Member]
Units issued	385	391
Units Outstanding	385	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				12 Months Ended
		Mar. 31, 2021		Mar. 31, 2020		Dec. 31, 2020
Operating activities:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 748		$ (2,716)
Adjustments to reconcile net income/(loss) to net cash provided by operating activities:
Amortization of deferred financing costs		17		14
Depreciation and amortization	[1]	329		325
Goodwill and Intangible Asset Impairment		0		2,165
(Income)/loss from equity method investments(1)		(70)	[2],[3]	1,184	[4],[5]
Distributions from unconsolidated affiliates		119		119
Changes in:
Current receivables		(67)		71
Inventories		(11)		3
Fair value of derivatives		3		(15)
Current accounts payable and accrued liabilities		26		(142)
Current assets/current liabilities - related parties		(8)		(52)
Increase (Decrease) in Other Operating Assets and Liabilities, Net		(1)		(4)
Deferred revenue		24		27
All other, net		15		30
Net cash provided by operating activities		1,124		1,009
Net Cash Provided by (Used in) Investing Activities [Abstract]
Additions to property, plant and equipment		(126)		(379)
Disposal of assets		70		39
Investments in unconsolidated affiliates		(35)		(91)
Distributions from unconsolidated affiliates - return of capital		0		69
Payments for (Proceeds from) Other Investing Activities		(1)		0
Net cash used in investing activities		(90)		(362)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Long-term debt - borrowings		1,910		1,325
Long-term debt - repayments		(2,020)		(581)
Payments for Repurchase of Common Stock		(155)		0
Distributions to noncontrolling interests		(10)		(9)
Distributions to unitholders and general partner		(713)		(717)
Proceeds from Contributions from Parent		7		14
All other, net		(3)		(2)
Net cash used in financing activities		(1,025)		(605)
Net increase in cash, cash equivalents and restricted cash		9		42
Cash, cash equivalents and restricted cash at beginning of period		15		15		$ 15
Cash, cash equivalents and restricted cash at end of period		24		57		15
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Related party debt - borrowings		2,241		1,667		6,264
Repayments of Related Party Debt		(2,241)		(2,261)		$ (6,858)
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders		(20)		(20)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders		$ (21)		$ (21)
[1]	Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020.
[2]	Includes the impact of any basis differential amortization or accretion.
[3]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[5]	The 2020 period includes $1,264 million of impairment expense. See Note 4.

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]
Noncontrolling interests		$ 249,000,000
Total MPLX LP partners’ capital				$ 611,000,000	$ 10,800,000,000	$ 4,968,000,000
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
Noncontrolling interests		248,000,000
Total MPLX LP partners’ capital				601,000,000	9,509,000,000	3,014,000,000
Accumulated other comprehensive loss	[1]		(16,000,000)
Noncontrolling interests		245,000,000
Total MPLX LP partners’ capital		12,772,000,000		611,000,000	9,384,000,000	2,792,000,000
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
Noncontrolling interests		244,000,000
Total MPLX LP partners’ capital		$ 12,606,000,000		$ 601,000,000	$ 9,226,000,000	$ 2,796,000,000
Accumulated other comprehensive loss	[1]		$ (17,000,000)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 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 and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (“WRW”) to Western Refining Southwest, Inc. (now known as Western Refining Southwest LLC) (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – During the first quarter of 2021, we have seen improvement in the environment in which our business operates as COVID-19 impacts are beginning to subside. The increased availability of vaccinations, coupled with an easing of COVID-19 restrictions in certain areas, has been followed by an increase in economic activity, the opening of many businesses and schools and an increase in in-person interaction and associated travel. No additional events or circumstances arose during the first quarter of 2021 that would indicate potential impairment beyond those recognized in 2020 as noted below. 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. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three months ended March 31, 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. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.

Accounting Standards Accounting

Accounting Standards Accounting Standards	3 Months Ended
	Mar. 31, 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 three months of 2021 that are expected to have a material impact to our financial statements or financial statement disclosures

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 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 Sheet as of March 31, 2021. Javelina’s assets and liabilities sold are shown on the Consolidated Balance Sheet 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

Investments and Noncontrolling

Investments and Noncontrolling Interest	3 Months Ended
	Mar. 31, 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 March 31, March 31, December 31, (In millions, except ownership percentages) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 462 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 259 254 LOOP LLC 41% 257 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 193 194 Minnesota Pipe Line Company, LLC 17% 187 188 Whistler Pipeline LLC (2) 38% 187 185 Explorer Pipeline Company 25% 69 72 W2W Holdings LLC (2)(3) 50% 73 72 Other (2) 105 103 Total L&S 1,792 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 696 698 Sherwood Midstream LLC (2) 50% 552 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 313 307 Rendezvous Gas Services, L.L.C. (2) 78% 157 159 Sherwood Midstream Holdings LLC (2) 51% 145 148 Centrahoma Processing LLC 40% 137 145 Other (2) 248 237 Total G&P 2,248 2,251 Total $ 4,040 $ 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 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 March 31, 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 three months ended March 31, 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. No additional impairments have been recorded since that time. Summarized financial information for MPLX’s equity method investments for the three months ended March 31, 2021 and 2020 is as follows: Three Months Ended March 31, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 163 $ 284 $ 447 Costs and expenses 107 131 238 Income from operations 56 153 209 Net income 64 140 204 Income from equity method investments (1) $ 39 $ 31 $ 70 (1) Includes the impact of any basis differential amortization or accretion. Three Months Ended March 31, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ (217) $ 337 $ 120 Costs and expenses 104 132 236 (Loss)/income from operations (321) 205 (116) Net (loss)/income (337) 186 (151) (Loss)/income from equity method investments (1) $ (1,222) $ 38 $ (1,184) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of March 31, 2021 and December 31, 2020 is as follows: March 31, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 312 $ 321 $ 633 Noncurrent assets 7,266 4,967 12,233 Current liabilities 338 194 532 Noncurrent liabilities $ 2,053 $ 853 $ 2,906 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 As of March 31, 2021 and December 31, 2020, the underlying net assets of MPLX’s investees in the G&P segment exceeded the carrying value of its equity method investments by approximately $56 million and $57 million, respectively. As of March 31, 2021 and December 31, 2020, the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $331 million.

Related Party Agreements and Tr

Related Party Agreements and Transactions	3 Months Ended
	Mar. 31, 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) Three Months Ended March 31, 2021 Year Ended December 31, 2020 Borrowings $ 2,241 $ 6,264 Average interest rate of borrowings 1.371 % 2.278 % Repayments $ 2,241 $ 6,858 Outstanding balance at end of period (1) $ — $ — (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 consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2021 2020 Service revenues - related parties MPC $ 871 $ 927 Other 1 1 Total Service revenue - related parties 872 928 Rental income - related parties MPC 242 234 Product sales - related parties (1) MPC 42 33 Other income - related parties MPC 49 48 Other 16 16 Total Other income - related parties $ 65 $ 64 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2021 and March 31, 2020, these sales totaled $168 million and $173 million, respectively. Related Party Expenses MPC provides executive management services and certain general and administrative services to MPLX under the terms of our omnibus agreements (“Omnibus charges”). Omnibus charges included in “Rental cost of sales - related parties” primarily relate to services that support MPLX’s rental operations and maintenance of assets available for rent. Omnibus charges included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Omnibus charges included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. MPLX also obtains employee services from MPC under employee services agreements (“ESA charges”). ESA charges for personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties.” ESA charges for personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” ESA charges for personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses.” In addition to these agreements, MPLX purchases products from MPC, makes payments to MPC in its capacity as general contractor to MPLX, and has certain lease agreements with MPC. Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended March 31, (In millions) 2021 2020 Rental cost of sales - related parties MPC $ 39 $ 46 Purchases - related parties MPC 294 271 Other 4 5 Total Purchases - related parties 298 276 General and administrative expenses MPC $ 57 $ 64 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 months ended March 31, 2021 and March 31, 2020, these charges totaled $12 million and $36 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 and fourth quarters of 2020, MPLX reimbursed MPC for $37 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 quarter 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 20 for additional information) and deferred revenue on minimum volume commitments. If MPC fails to meet its minimum committed volumes, MPC will pay MPLX a deficiency payment based on the terms of the agreement. The deficiency amounts are recorded as “Current liabilities - related parties.” In many cases, MPC may then apply the amount of any such deficiency payments as a credit for volumes in excess of its minimum volume commitment in future periods under the terms of the applicable agreements. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes in excess of minimum quarterly volume commitments, where it is probable the customer will not use the credit in future periods or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Current liabilities - related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements or in some cases as an equity contribution from its sponsor. (In millions) March 31, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 586 $ 615 Receivables - Other 14 27 Prepaid - MPC 17 4 Other - MPC 1 1 Lease Receivables - MPC 32 30 Total 650 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 388 386 Unguaranteed residual asset - MPC 26 23 Total 676 672 Current liabilities - related parties Payables - MPC 213 215 Payables - Other 26 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 65 66 Deferred revenue - Project reimbursements - MPC 32 30 Deferred revenue - Project reimbursements - Other 1 1 Total 338 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 229 Long-term deferred revenue - Project reimbursements - MPC 43 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 279 $ 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 three months ended March 31, 2021 and 2020 were $9 million and $2 million, respectively. Purchases from related parties were immaterial for the three months ended March 31, 2021 and 2020.

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 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 months ended March 31, 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 months ended March 31, 2021 and 2020 were less than 1 million. Three Months Ended March 31, (In millions) 2021 2020 Net income/(loss) attributable to MPLX LP $ 739 $ (2,724) Less: Distributions declared on Series A preferred units (1) 20 20 Distributions declared on Series B preferred units (1) 11 11 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 707 728 Undistributed net gain/(loss) attributable to MPLX LP $ 1 $ (3,483) (1) See Note 7 for distribution information. Three Months Ended March 31, 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 $ 707 $ 20 $ 11 $ 738 Undistributed net gain attributable to MPLX LP 1 — — 1 Net income attributable to MPLX LP (1) $ 708 $ 20 $ 11 $ 739 Weighted average units outstanding: Basic 1,037 Diluted 1,037 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.68 Diluted $ 0.68 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended March 31, 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 $ 728 $ 20 $ 11 $ 759 Undistributed net loss attributable to MPLX LP (3,483) — — (3,483) Net (loss)/income attributable to MPLX LP (1) $ (2,755) $ 20 $ 11 $ (2,724) Weighted average units outstanding: Basic 1,058 Diluted 1,058 Net income attributable to MPLX LP per limited partner unit: Basic $ (2.60) Diluted $ (2.60) (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	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Equity	Equity The changes in the number of common units outstanding during the three months ended March 31, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 143,247 Units redeemed in unit repurchase program (6,272,981) Balance at March 31, 2021 1,032,648,244 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 three months ended March 31, 2021, 6,272,981 public common units were repurchased at an average cost per unit of $24.78. Total cash paid for units repurchased during 2021 was $155 million with $812 million remaining available under the program for future repurchases as of March 31, 2021. As of March 31, 2021, we had agreements to acquire 291,400 additional common units for $7 million, which settled in April 2021. Wholesale Exchange In connection with the Wholesale Exchange as discussed in Note 3, 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. 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.. The changes in the Series B preferred unit balance from December 31, 2020 through March 31, 2021 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Series B preferred units”. (In millions) Series B Preferred Units Balance at December 31, 2020 $ 611 Net income allocated 11 Distributions received by Series B preferred unitholders (21) Balance at March 31, 2021 $ 601 Cash distributions – In accordance with the MPLX partnership agreement, on April 27, 2021, MPLX declared a quarterly cash distribution for the first quarter of 2021, totaling $707 million, or $0.6875 per common unit. This rate will also be received by Series A preferred unitholders. These distributions will be paid on May 14, 2021 to common unitholders of record on May 7, 2021. MPLX also made a cash distribution payment totaling $21 million to Series B unitholders on February 16, 2021. Quarterly distributions for the first quarters of 2021 and 2020 are summarized below: (Per common unit) 2021 2020 March 31, $ 0.6875 $ 0.6875 The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three months ended March 31, 2021 and 2020. 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 March 31, (In millions) 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 707 $ 728 Series A preferred unit distributions 20 20 Series B preferred unit distributions 11 11 Total cash distributions declared $ 738 $ 759

Series A Preferred Units

Series A Preferred Units	3 Months Ended
	Mar. 31, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units	Series A Preferred Units On May 13, 2016, MPLX LP issued approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The Series A preferred units rank senior to all common units and pari passu with all Series B preferred units with respect to distributions and rights upon liquidation. The holders of the Series A preferred units are entitled to receive, when and if declared by the board, a quarterly distribution equal to the greater of $0.528125 per unit or the amount of distributions they would have received on an as converted basis. On April 27, 2021, MPLX declared a quarterly cash distribution of $0.6875 per common unit for the first 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. The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate. Approximately 29.6 million Series A preferred units remaining outstanding as of March 31, 2021. The changes in the redeemable preferred balance from December 31, 2020 through March 31, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated 20 Distributions received by Series A preferred unitholders (20) Balance at March 31, 2021 $ 968 The Series A preferred units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside MPLX’s control. Therefore, they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The Series A preferred units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value and declared distributions decrease the carrying value of the Series A preferred units. As the Series A preferred units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the Series A preferred units would become redeemable.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 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) extinguishment of debt; (v) non-cash equity-based compensation; (vi) impairment expense; (vii) net interest and other financial costs; (viii) income/(loss) from equity method investments; (ix) distributions and adjustments related to equity method investments; (x) unrealized derivative gains/(losses); (xi) acquisition costs; (xii) noncontrolling interest; and (xiii) other adjustments as deemed necessary. These items are either: (i) believed to be non-recurring in nature; (ii) not believed to be allocable or controlled by the segment; or (iii) are not tied to the operational performance of the segment. The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended March 31, (In millions) 2021 2020 L&S Service revenue $ 953 $ 1,004 Rental income 249 242 Product related revenue 4 19 Income from equity method investments 36 50 Other income 52 51 Total segment revenues and other income (1) 1,294 1,366 Segment Adjusted EBITDA (2) 896 872 Capital expenditures 59 184 Investments in unconsolidated affiliates 9 54 G&P Service revenue 508 536 Rental income 92 88 Product related revenue 397 222 Income/(loss) from equity method investments 34 (1,234) Other income 14 14 Total segment revenues and other income/(loss) (1) 1,045 (374) Segment Adjusted EBITDA (2) 456 422 Capital expenditures 30 134 Investments in unconsolidated affiliates $ 26 $ 37 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $129 million and $158 million for the three months ended March 31, 2021 and 2020, respectively. Third party revenues for the G&P segment were $989 million for the three months ended March 31, 2021 and third party losses were $425 million for the three months ended March 31, 2020. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (In millions) March 31, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 24 $ 15 L&S 20,787 20,938 G&P 15,219 15,461 Total assets $ 36,030 $ 36,414 The table below provides a reconciliation between “Net income/(loss)” and Segment Adjusted EBITDA. Three Months Ended March 31, (In millions) 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 896 $ 872 G&P Segment Adjusted EBITDA 456 422 Total reportable segments 1,352 1,294 Depreciation and amortization (1) (329) (325) Provision for income taxes (1) — Amortization of deferred financing costs (17) (14) Non-cash equity-based compensation (3) (5) Impairment expense — (2,165) Net interest and other financial costs (220) (216) Gain on extinguishment of debt 12 — Income/(loss) from equity method investments 70 (1,184) Distributions/adjustments related to equity method investments (121) (124) Unrealized derivative (losses)/gains (2) (3) 15 Other (2) (1) Adjusted EBITDA attributable to noncontrolling interests 10 9 Net income/(loss) $ 748 $ (2,716) (1) Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020. (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	3 Months Ended
	Mar. 31, 2021
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) March 31, 2021 December 31, 2020 NGLs $ 5 $ 5 Line fill 20 13 Spare parts, materials and supplies 103 100 Total inventories $ 128 $ 118

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 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: (In millions) March 31, 2021 December 31, 2020 L&S Pipelines $ 6,045 $ 6,026 Refining logistics 2,335 2,333 Terminals 1,649 1,643 Marine 965 965 Land, building and other 1,584 1,584 Construction-in-progress 277 262 Total L&S property, plant and equipment 12,855 12,813 G&P Gathering and transportation 7,559 7,547 Processing and fractionation 5,725 5,721 Land, building and other 510 507 Construction-in-progress 288 287 Total G&P property, plant and equipment 14,082 14,062 Total property, plant and equipment 26,937 26,875 Less accumulated depreciation 5,941 5,657 Property, plant and equipment, net $ 20,996 $ 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)	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets Disclosure [Text Block]	Goodwill and Intangibles Goodwill MPLX annually evaluates goodwill for impairment as of November 30, as well as whenever events or changes in circumstances indicate it is more likely than not that the fair value of a reporting unit with goodwill is less than its carrying amount. There were no impairments recorded as a result of our November 30, 2020 annual goodwill impairment evaluation. During the first quarter of 2020, we determined that an interim impairment analysis of the goodwill recorded was necessary based on consideration of a number of first quarter events and circumstances as discussed in Note 1. Our producer customers in our Eastern G&P region reduced production forecasts and drilling activity in response to the global economic downturn. Additionally, a decline in NGL prices impacted our future revenue forecast. After performing our evaluations related to the interim impairment of goodwill during the first quarter of 2020, we recorded an impairment of $1,814 million within the Eastern G&P reporting unit, which was recorded to “Impairment expense” on the Consolidated Statements of Income. The impairment was primarily driven by additional guidance related to the slowing of drilling activity, which 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 March 31, 2021 7,657 — 7,657 Gross goodwill as of March 31, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of March 31, 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. MPLX’s remaining intangible assets are comprised of customer contracts and relationships. Gross intangible assets with accumulated amortization as of March 31, 2021 and December 31, 2020 is shown below: March 31, 2021 December 31, 2020 (In millions) Useful Lives Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (2) Net L&S 6 - 8 years $ 283 $ (90) $ 193 $ 283 $ (81) $ 202 G&P 6 - 25 years 1,288 (554) 734 1,288 (531) 757 $ 1,571 $ (644) $ 927 $ 1,571 $ (612) $ 959 (1) Amortization expense attributable to the L&S and G&P segments for the three months ended March 31, 2021 was $9 million and $23 million, respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization. Estimated future amortization expense related to the intangible assets at March 31, 2021 is as follows: (In millions) 2021 $ 95 2022 127 2023 127 2024 127 2025 113 Thereafter 338 Total $ 927

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 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 March 31, 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. March 31, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (66) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (66) $ — $ (63) Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.48 to $1.36 per gallon with a weighted average of $0.62 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 March 31, 2021 Three Months Ended March 31, 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)/gains (realized and unrealized) included in earnings (1) — (6) — 14 Settlements — 3 — 1 Fair value at end of period — (66) — (45) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (5) $ — $ 13 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income. Fair Values – Reported MPLX’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, lease receivables from related parties, accounts payable, payables to related parties and long-term debt. MPLX’s fair value assessment incorporates a variety of considerations, including (1) the duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. MPLX believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 14). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. The long-term debt fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of our debt, excluding finance leases: March 31, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,988 $ 20,156 $ 22,951 $ 20,244

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of March 31, 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 March 31, 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) March 31, 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 $ — $ (10) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (56) — (56) Total $ — $ (66) $ — $ (63) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, see Note 13. There were no material changes to MPLX’s policy regarding the accounting for Level 2 and Level 3 instruments as previously disclosed in MPLX’s Annual Report on Form 10-K for the year ended December 31, 2020. 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 March 31, (In millions) 2021 2020 Purchased product costs Realized (loss)/gain $ (3) $ (1) Unrealized (loss)/gain (3) 15 Purchased product costs derivative (loss)/gain (6) 14 Total derivative (loss)/gain $ (6) $ 14

Debt

Debt	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings consist of the following: (In millions) March 31, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ 835 $ 175 Floating rate senior notes 1,000 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations (1) 10 11 Total 20,445 20,536 Unamortized debt issuance costs (112) (116) Unamortized discount/premium (279) (281) Amounts due within one year (2) (764) Total long-term debt due after one year $ 20,052 $ 19,375 (1) See Note 20 for lease information. Credit Agreement MPLX’s amended and restated revolving credit facility (as amended, the “MPLX Credit Agreement”), has a borrowing capacity of up to $3.5 billion and a term that extends to July 2024. During the three months ended March 31, 2021, MPLX borrowed $1.91 billion under the MPLX Credit Agreement, at an average interest rate of 1.347 percent, and repaid $1.25 billion. At March 31, 2021, MPLX had $835 million in outstanding borrowings and less than $1 million in letters of credit outstanding under the MPLX Credit Agreement, resulting in total availability of $2.67 billion, or 76.1 percent of the borrowing capacity . Floating Rate Senior Notes MPLX has $1.0 billion of aggregate principal amount of floating rate senior notes due September 2022. The notes were offered at a price to the public of 100 percent of par and are callable, in whole or in part, at par plus accrued and unpaid interest at any time on or after September 10, 2020. Interest is payable quarterly in March, June, September and December. The interest rate applicable to the notes is LIBOR plus 1.1 percent per annum. 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	3 Months Ended
	Mar. 31, 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 months ended March 31, 2021 and 2020: Three Months Ended March 31, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 84 $ 505 $ 589 Service revenue - related parties 869 3 872 Service revenue - product related — 77 77 Product sales 1 281 282 Product sales - related parties 3 39 42 Total revenues from contracts with customers $ 957 $ 905 1,862 Non-ASC 606 revenue (1) 477 Total revenues and other income $ 2,339 Three Months Ended March 31, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 84 $ 528 $ 612 Service revenue - related parties 920 8 928 Service revenue - product related — 39 39 Product sales 15 154 169 Product sales - related parties 4 29 33 Total revenues from contracts with customers $ 1,023 $ 758 1,781 Non-ASC 606 (loss)/revenue (1) (789) Total revenues and other income $ 992 (1) Non-ASC 606 Revenue includes rental income, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and MPLX’s rights to consideration for work completed exceeds the amount billed to the customer. 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 three-month periods ended March 31, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 40 $ (27) $ — $ 13 Long-term contract assets 2 — — 2 Deferred revenue 37 7 (1) 43 Deferred revenue - related parties 91 21 (20) 92 Long-term deferred revenue 119 4 — 123 Long-term deferred revenue - related parties 48 (4) — 44 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 39 $ (27) $ — $ 12 Deferred revenue 23 5 (3) 25 Deferred revenue - related parties 53 12 (16) 49 Long-term deferred revenue 90 6 — 96 Long-term deferred revenue - related parties $ 55 $ 1 $ — $ 56 (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 March 31, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $301 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 23 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2021 $ 1,435 2022 1,838 2023 1,674 2024 1,546 2025 and thereafter 4,580 Total revenue on remaining performance obligations (1),(2),(3) $ 11,073 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above. We do not disclose information on the future performance obligations for any contract with an original expected duration of one year or less.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information Three Months Ended March 31, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 231 $ 210 Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories $ — $ (1) The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Three Months Ended March 31, (In millions) 2021 2020 Decrease in capital accruals $ (37) $ (61)

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX LP records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP LLC (“LOOP”) and Explorer Pipeline Company (“Explorer”) to their employees. MPLX LP is not a sponsor of these benefit plans. The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2020 through March 31, 2021. (In millions) Pension Other Total Balance at December 31, 2020 (1) $ (13) $ (2) $ (15) Other comprehensive loss - remeasurements (2) (2) — (2) Balance at March 31, 2021 (1) $ (15) $ (2) $ (17) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through March 31, 2020. (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14) $ (1) $ (15) Other comprehensive loss - remeasurements (2) — (1) (1) Balance at March 31, 2020 (1) $ (14) $ (2) $ (16) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Plan

Equity-Based Compensation Plan	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Share-based Payment Arrangement [Text Block]	Equity-Based Compensation Phantom Units The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2021: Number Weighted Outstanding at December 31, 2020 644,023 $ 28.65 Granted 321,927 24.88 Settled (143,088) 29.73 Forfeited (1,270) 33.09 Outstanding at March 31, 2021 821,592 $ 26.98 Performance Units The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the three months ended March 31, 2021: Number of Outstanding at December 31, 2020 3,092,286 Granted 375,730 Settled (1,347,177) Forfeited (1,761) Outstanding at March 31, 2021 2,119,078

Leases Leases (Notes)

Leases Leases (Notes)	3 Months Ended
	Mar. 31, 2021
Disclosure Text Block [Abstract]
Leases of Lessor Disclosure [Text Block]	LeasesDuring the first quarter of 2020, reimbursements for projects at certain MPLX refining logistics locations were agreed to between MPLX and MPC. These reimbursements relate to the storage services agreements between MPLX and MPC at these locations and required the embedded leases within these agreements to be reassessed under the leasing standard. As a result of the reassessment, one of our leases was reclassified from an operating lease to a sales-type lease. As a result, the underlying assets previously shown on the Consolidated Balance Sheets associated with the sales-type lease were derecognized and the net investment in the lease (i.e., the sum of the present value of the future lease payments and the unguaranteed residual value of the assets) was recorded as a lease receivable. See Note 5 for the location of lease receivables and unguaranteed residual assets on the Consolidated Balance Sheets. The difference between the net book value of the underlying assets and the net investment in the lease has been recorded as a Contribution from MPC in the Consolidated Statements of Equity given that the transaction related to refining logistics was a common control transaction. During the first quarter of 2020, MPLX derecognized approximately $171 million of property, plant and equipment, recorded a lease receivable of approximately $370 million, recorded an unguaranteed residual asset of approximately $10 million and a Contribution from MPC of $209 million. Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended March 31, 2021 Three Months Ended March 31, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 202 $ 68 $ 186 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 38 N/A Interest income (Revenue from variable lease payments) $ — $ — $ — N/A (1) These amounts are presented net of executory costs.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 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 March 31, 2021 and December 31, 2020, accrued liabilities for remediation totaled $18 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 March 31, 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. In connection with our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL, we have entered into a Contingent Equity Contribution Agreement whereby MPLX LP, along with the other joint venture owners in the Bakken Pipeline system, has agreed to make equity contributions to the joint venture upon certain events occurring to allow the entities that own and operate the Bakken Pipeline system to satisfy their senior note payment obligations. The senior notes were issued to repay amounts owed by the pipeline companies to fund the cost of construction of the Bakken Pipeline system. In March 2020, the U.S. District Court for the District of Columbia (the “D.D.C.”) ordered the U.S. Army Corps of Engineers (“Army Corps”), which granted permits and an easement for the Bakken Pipeline system, to conduct a full environmental impact statement (“EIS”), and further requested briefing on whether an easement necessary for the operation of the Bakken Pipeline system should be vacated while the EIS is being prepared. On July 6, 2020, the D.D.C. ordered vacatur of the easement to cross Lake Oahe during the pendency of an EIS and further ordered a shut down of the pipeline by August 5, 2020. The D.D.C. denied a motion to stay that order. Dakota Access and the Army Corps appealed the D.D.C.’s orders to the U.S. Court of Appeals for the District of Columbia Circuit (the “Court of Appeals”). On July 14, 2020, the Court of Appeals issued an administrative stay while the court considered Dakota Access and the Army Corps’ emergency motion for stay pending appeal. On August 5, 2020, the Court of Appeals stayed the D.D.C.’s injunction that required the pipeline be shutdown and emptied of oil by August 5, 2020. The Court of Appeals denied a stay of the D.D.C.’s March order, which required the EIS, and further denied a stay of the D.D.C.’s July order, which vacated the easement. 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 the D.D.C., briefing has been completed for a renewed request for an injunction. The pipeline remains operational. If the pipeline is temporarily shut down pending completion of the EIS, MPLX would have to contribute its 9.19 percent pro rata share of funds required to pay interest accruing on the notes and any portion of the principal that matures while the pipeline is shutdown. MPLX also expects to contribute its 9.19 percent pro rata share of any costs to remediate any deficiencies to reinstate the permit and/or return the pipeline into operation. If the vacatur of the easement permit results in a permanent shutdown of the pipeline, MPLX would have to contribute its 9.19 percent pro rata share of the cost to redeem the bonds (including the 1% redemption premium required pursuant to the indenture governing the notes) and any accrued and unpaid interest. As of March 31, 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, 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. Contractual Commitments and Contingencies – At March 31, 2021, MPLX’s contractual commitments to acquire property, plant and equipment totaled $101 million. These commitments were primarily related to G&P plant expansion, terminal and pipeline projects. In addition, from time to time and in the ordinary course of business, MPLX and its affiliates provide guarantees of MPLX’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require MPLX to construct new natural gas processing plants, natural gas gathering pipelines and

Description of the Business a_2

Description of the Business and Basis of Presentation Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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 and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (“WRW”) to Western Refining Southwest, Inc. (now known as Western Refining Southwest LLC) (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – During the first quarter of 2021, we have seen improvement in the environment in which our business operates as COVID-19 impacts are beginning to subside. The increased availability of vaccinations, coupled with an easing of COVID-19 restrictions in certain areas, has been followed by an increase in economic activity, the opening of many businesses and schools and an increase in in-person interaction and associated travel. No additional events or circumstances arose during the first quarter of 2021 that would indicate potential impairment beyond those recognized in 2020 as noted below. 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. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three months ended March 31, 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. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.
Use of Estimates, Policy [Policy Text Block]	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements.
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.
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.
Nature of Operations	We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio.MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments.

Net Income Per Limited Partne_2

Net Income Per Limited Partner Unit Accounting Policy (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.

Investments and Noncontrollin_2

Investments and Noncontrolling Interests (Tables)	3 Months Ended
	Mar. 31, 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 March 31, March 31, December 31, (In millions, except ownership percentages) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 462 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 259 254 LOOP LLC 41% 257 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 193 194 Minnesota Pipe Line Company, LLC 17% 187 188 Whistler Pipeline LLC (2) 38% 187 185 Explorer Pipeline Company 25% 69 72 W2W Holdings LLC (2)(3) 50% 73 72 Other (2) 105 103 Total L&S 1,792 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 696 698 Sherwood Midstream LLC (2) 50% 552 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 313 307 Rendezvous Gas Services, L.L.C. (2) 78% 157 159 Sherwood Midstream Holdings LLC (2) 51% 145 148 Centrahoma Processing LLC 40% 137 145 Other (2) 248 237 Total G&P 2,248 2,251 Total $ 4,040 $ 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 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 three months ended March 31, 2021 and 2020 is as follows: Three Months Ended March 31, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 163 $ 284 $ 447 Costs and expenses 107 131 238 Income from operations 56 153 209 Net income 64 140 204 Income from equity method investments (1) $ 39 $ 31 $ 70 (1) Includes the impact of any basis differential amortization or accretion. Three Months Ended March 31, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ (217) $ 337 $ 120 Costs and expenses 104 132 236 (Loss)/income from operations (321) 205 (116) Net (loss)/income (337) 186 (151) (Loss)/income from equity method investments (1) $ (1,222) $ 38 $ (1,184) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of March 31, 2021 and December 31, 2020 is as follows: March 31, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 312 $ 321 $ 633 Noncurrent assets 7,266 4,967 12,233 Current liabilities 338 194 532 Noncurrent liabilities $ 2,053 $ 853 $ 2,906 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)	3 Months Ended
	Mar. 31, 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) Three Months Ended March 31, 2021 Year Ended December 31, 2020 Borrowings $ 2,241 $ 6,264 Average interest rate of borrowings 1.371 % 2.278 % Repayments $ 2,241 $ 6,858 Outstanding balance at end of period (1) $ — $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.
Schedule of Entity Wide Information Sales to Related Parties [Table Text Block]	Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2021 2020 Service revenues - related parties MPC $ 871 $ 927 Other 1 1 Total Service revenue - related parties 872 928 Rental income - related parties MPC 242 234 Product sales - related parties (1) MPC 42 33 Other income - related parties MPC 49 48 Other 16 16 Total Other income - related parties $ 65 $ 64 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2021 and March 31, 2020, these sales totaled $168 million and $173 million, respectively.
Schedule Of Entity Wide Information Allocated Related Party Employee Benefit Costs By Income Statement Line [Table Text Block]	Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended March 31, (In millions) 2021 2020 Rental cost of sales - related parties MPC $ 39 $ 46 Purchases - related parties MPC 294 271 Other 4 5 Total Purchases - related parties 298 276 General and administrative expenses MPC $ 57 $ 64
Schedule of Related Party Transactions [Table Text Block]	(In millions) March 31, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 586 $ 615 Receivables - Other 14 27 Prepaid - MPC 17 4 Other - MPC 1 1 Lease Receivables - MPC 32 30 Total 650 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 388 386 Unguaranteed residual asset - MPC 26 23 Total 676 672 Current liabilities - related parties Payables - MPC 213 215 Payables - Other 26 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 65 66 Deferred revenue - Project reimbursements - MPC 32 30 Deferred revenue - Project reimbursements - Other 1 1 Total 338 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 229 Long-term deferred revenue - Project reimbursements - MPC 43 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 279 $ 283

Net Income Per Limited Partne_3

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class [Table Text Block]	During the three months ended March 31, 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 months ended March 31, 2021 and 2020 were less than 1 million. Three Months Ended March 31, (In millions) 2021 2020 Net income/(loss) attributable to MPLX LP $ 739 $ (2,724) Less: Distributions declared on Series A preferred units (1) 20 20 Distributions declared on Series B preferred units (1) 11 11 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 707 728 Undistributed net gain/(loss) attributable to MPLX LP $ 1 $ (3,483) (1) See Note 7 for distribution information.
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	Three Months Ended March 31, 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 $ 707 $ 20 $ 11 $ 738 Undistributed net gain attributable to MPLX LP 1 — — 1 Net income attributable to MPLX LP (1) $ 708 $ 20 $ 11 $ 739 Weighted average units outstanding: Basic 1,037 Diluted 1,037 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.68 Diluted $ 0.68 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended March 31, 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 $ 728 $ 20 $ 11 $ 759 Undistributed net loss attributable to MPLX LP (3,483) — — (3,483) Net (loss)/income attributable to MPLX LP (1) $ (2,755) $ 20 $ 11 $ (2,724) Weighted average units outstanding: Basic 1,058 Diluted 1,058 Net income attributable to MPLX LP per limited partner unit: Basic $ (2.60) Diluted $ (2.60) (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)	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of common units outstanding during the three months ended March 31, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 143,247 Units redeemed in unit repurchase program (6,272,981) Balance at March 31, 2021 1,032,648,244 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 three months ended March 31, 2021, 6,272,981 public common units were repurchased at an average cost per unit of $24.78. Total cash paid for units repurchased during 2021 was $155 million with $812 million remaining available under the program for future repurchases as of March 31, 2021. As of March 31, 2021, we had agreements to acquire 291,400 additional common units for $7 million, which settled in April 2021. Wholesale Exchange In connection with the Wholesale Exchange as discussed in Note 3, 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. 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.. The changes in the Series B preferred unit balance from December 31, 2020 through March 31, 2021 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Series B preferred units”. (In millions) Series B Preferred Units Balance at December 31, 2020 $ 611 Net income allocated 11 Distributions received by Series B preferred unitholders (21) Balance at March 31, 2021 $ 601
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three months ended March 31, 2021 and 2020. 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 March 31, (In millions) 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 707 $ 728 Series A preferred unit distributions 20 20 Series B preferred unit distributions 11 11 Total cash distributions declared $ 738 $ 759

Series A Preferred Units (Table

Series A Preferred Units (Tables)	3 Months Ended
	Mar. 31, 2021
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2020 through March 31, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated 20 Distributions received by Series A preferred unitholders (20) Balance at March 31, 2021 $ 968

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended March 31, (In millions) 2021 2020 L&S Service revenue $ 953 $ 1,004 Rental income 249 242 Product related revenue 4 19 Income from equity method investments 36 50 Other income 52 51 Total segment revenues and other income (1) 1,294 1,366 Segment Adjusted EBITDA (2) 896 872 Capital expenditures 59 184 Investments in unconsolidated affiliates 9 54 G&P Service revenue 508 536 Rental income 92 88 Product related revenue 397 222 Income/(loss) from equity method investments 34 (1,234) Other income 14 14 Total segment revenues and other income/(loss) (1) 1,045 (374) Segment Adjusted EBITDA (2) 456 422 Capital expenditures 30 134 Investments in unconsolidated affiliates $ 26 $ 37 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $129 million and $158 million for the three months ended March 31, 2021 and 2020, respectively. Third party revenues for the G&P segment were $989 million for the three months ended March 31, 2021 and third party losses were $425 million for the three months ended March 31, 2020. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	(In millions) March 31, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 24 $ 15 L&S 20,787 20,938 G&P 15,219 15,461 Total assets $ 36,030 $ 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 March 31, (In millions) 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 896 $ 872 G&P Segment Adjusted EBITDA 456 422 Total reportable segments 1,352 1,294 Depreciation and amortization (1) (329) (325) Provision for income taxes (1) — Amortization of deferred financing costs (17) (14) Non-cash equity-based compensation (3) (5) Impairment expense — (2,165) Net interest and other financial costs (220) (216) Gain on extinguishment of debt 12 — Income/(loss) from equity method investments 70 (1,184) Distributions/adjustments related to equity method investments (121) (124) Unrealized derivative (losses)/gains (2) (3) 15 Other (2) (1) Adjusted EBITDA attributable to noncontrolling interests 10 9 Net income/(loss) $ 748 $ (2,716) (1) Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020. (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)	3 Months Ended
	Mar. 31, 2021
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) March 31, 2021 December 31, 2020 NGLs $ 5 $ 5 Line fill 20 13 Spare parts, materials and supplies 103 100 Total inventories $ 128 $ 118

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2021
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) March 31, 2021 December 31, 2020 L&S Pipelines $ 6,045 $ 6,026 Refining logistics 2,335 2,333 Terminals 1,649 1,643 Marine 965 965 Land, building and other 1,584 1,584 Construction-in-progress 277 262 Total L&S property, plant and equipment 12,855 12,813 G&P Gathering and transportation 7,559 7,547 Processing and fractionation 5,725 5,721 Land, building and other 510 507 Construction-in-progress 288 287 Total G&P property, plant and equipment 14,082 14,062 Total property, plant and equipment 26,937 26,875 Less accumulated depreciation 5,941 5,657 Property, plant and equipment, net $ 20,996 $ 21,218

Goodwill and Intangibles (Table

Goodwill and Intangibles (Tables)	3 Months Ended
	Mar. 31, 2021
Finite-Lived Intangible Assets [Line Items]
Schedule of Finite-Lived Intangible Assets [Table Text Block]	Gross intangible assets with accumulated amortization as of March 31, 2021 and December 31, 2020 is shown below: March 31, 2021 December 31, 2020 (In millions) Useful Lives Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (2) Net L&S 6 - 8 years $ 283 $ (90) $ 193 $ 283 $ (81) $ 202 G&P 6 - 25 years 1,288 (554) 734 1,288 (531) 757 $ 1,571 $ (644) $ 927 $ 1,571 $ (612) $ 959 (1) Amortization expense attributable to the L&S and G&P segments for the three months ended March 31, 2021 was $9 million and $23 million, respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization.
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	Estimated future amortization expense related to the intangible assets at March 31, 2021 is as follows: (In millions) 2021 $ 95 2022 127 2023 127 2024 127 2025 113 Thereafter 338 Total $ 927

Goodwill and Intangibles Goodwi

Goodwill and Intangibles Goodwill and Intangibles (Tables)	3 Months Ended
	Mar. 31, 2021
Goodwill [Line Items]
Schedule of Goodwill [Table Text Block]	hanges 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 March 31, 2021 7,657 — 7,657 Gross goodwill as of March 31, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of March 31, 2021 $ 7,657 $ — $ 7,657

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 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. March 31, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (66) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (66) $ — $ (63)
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.48 to $1.36 per gallon with a weighted average of $0.62 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 March 31, 2021 Three Months Ended March 31, 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)/gains (realized and unrealized) included in earnings (1) — (6) — 14 Settlements — 3 — 1 Fair value at end of period — (66) — (45) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (5) $ — $ 13 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
Fair Value Carrying Value by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of our debt, excluding finance leases: March 31, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,988 $ 20,156 $ 22,951 $ 20,244

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of March 31, 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) March 31, 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 $ — $ (10) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (56) — (56) Total $ — $ (66) $ — $ (63)
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 March 31, (In millions) 2021 2020 Purchased product costs Realized (loss)/gain $ (3) $ (1) Unrealized (loss)/gain (3) 15 Purchased product costs derivative (loss)/gain (6) 14 Total derivative (loss)/gain $ (6) $ 14

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Debt [Table Text Block]	MPLX’s outstanding borrowings consist of the following: (In millions) March 31, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ 835 $ 175 Floating rate senior notes 1,000 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations (1) 10 11 Total 20,445 20,536 Unamortized debt issuance costs (112) (116) Unamortized discount/premium (279) (281) Amounts due within one year (2) (764) Total long-term debt due after one year $ 20,052 $ 19,375 (1) See Note 20 for lease information.

Revenue (Tables)

Revenue (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2021 and 2020: Three Months Ended March 31, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 84 $ 505 $ 589 Service revenue - related parties 869 3 872 Service revenue - product related — 77 77 Product sales 1 281 282 Product sales - related parties 3 39 42 Total revenues from contracts with customers $ 957 $ 905 1,862 Non-ASC 606 revenue (1) 477 Total revenues and other income $ 2,339 Three Months Ended March 31, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 84 $ 528 $ 612 Service revenue - related parties 920 8 928 Service revenue - product related — 39 39 Product sales 15 154 169 Product sales - related parties 4 29 33 Total revenues from contracts with customers $ 1,023 $ 758 1,781 Non-ASC 606 (loss)/revenue (1) (789) Total revenues and other income $ 992
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block]	The tables below reflect the changes in our contract balances for the three-month periods ended March 31, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 40 $ (27) $ — $ 13 Long-term contract assets 2 — — 2 Deferred revenue 37 7 (1) 43 Deferred revenue - related parties 91 21 (20) 92 Long-term deferred revenue 119 4 — 123 Long-term deferred revenue - related parties 48 (4) — 44 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 39 $ (27) $ — $ 12 Deferred revenue 23 5 (3) 25 Deferred revenue - related parties 53 12 (16) 49 Long-term deferred revenue 90 6 — 96 Long-term deferred revenue - related parties $ 55 $ 1 $ — $ 56 (1) Balance represents ASC 606 portion of each respective line item.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	As of March 31, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $301 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 23 years. Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2021 $ 1,435 2022 1,838 2023 1,674 2024 1,546 2025 and thereafter 4,580 Total revenue on remaining performance obligations (1),(2),(3) $ 11,073 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2021
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information [Table Text Block]	Three Months Ended March 31, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 231 $ 210 Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories $ — $ (1)
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 did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Three Months Ended March 31, (In millions) 2021 2020 Decrease in capital accruals $ (37) $ (61)

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2020 through March 31, 2021. (In millions) Pension Other Total Balance at December 31, 2020 (1) $ (13) $ (2) $ (15) Other comprehensive loss - remeasurements (2) (2) — (2) Balance at March 31, 2021 (1) $ (15) $ (2) $ (17) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through March 31, 2020. (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14) $ (1) $ (15) Other comprehensive loss - remeasurements (2) — (1) (1) Balance at March 31, 2020 (1) $ (14) $ (2) $ (16) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	3 Months Ended
	Mar. 31, 2021
Phantom Units
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2021: Number Weighted Outstanding at December 31, 2020 644,023 $ 28.65 Granted 321,927 24.88 Settled (143,088) 29.73 Forfeited (1,270) 33.09 Outstanding at March 31, 2021 821,592 $ 26.98
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	Performance Units The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the three months ended March 31, 2021: Number of Outstanding at December 31, 2020 3,092,286 Granted 375,730 Settled (1,347,177) Forfeited (1,761) Outstanding at March 31, 2021 2,119,078

Leases Lessor Disclosure (Table

Leases Lessor Disclosure (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2021 Three Months Ended March 31, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 202 $ 68 $ 186 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 38 N/A Interest income (Revenue from variable lease payments) $ — $ — $ — N/A (1) These amounts are presented net of executory costs.
Sales-type Lease, Lease Income [Table Text Block]	Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended March 31, 2021 Three Months Ended March 31, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 202 $ 68 $ 186 $ 63 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 37 — 38 N/A Interest income (Revenue from variable lease payments) $ — $ — $ — N/A (1) These amounts are presented net of executory costs.

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions	Jul. 31, 2020shares	Mar. 31, 2021	Mar. 31, 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			$ 1,264
Impairment of Intangible Assets, Finite-lived			177
Number of reportable segments		2
Impairment of Long-Lived Assets Held-for-use			174
Impairment Charges			3,429
G&P
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss			$ 1,814	$ 1,814
Western Refining Wholesale & Western Refining Product Transport [Member]
Description Of Business And Basis Of Presentation [Line Items]
Partners' Capital Account, Units, Redeemed | shares	18,582,088

Javelina Sale (Details)

Javelina Sale (Details) $ in Millions	3 Months Ended
	Mar. 31, 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	Mar. 31, 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
		Mar. 31, 2021		Mar. 31, 2020		Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Revenues		$ 2,339		$ 992
Equity method investments		4,040				$ 4,036
Costs and Expenses		1,365		3,478
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		748		(2,716)
Income/(loss) from equity method investments(1)		70	[1],[2]	(1,184)	[3],[4]
Assets, Current		1,374				1,515
Liabilities, Current		$ 1,196				2,086
Equity Method Investment, Other than Temporary Impairment				1,264
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%
Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%
Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		17.00%
Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		38.00%
W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		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%
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%
Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		78.00%
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		9.19%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		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%
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		51.00%
G&P
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		$ (56)				(57)
Difference between carrying amount and underlying equity portion related to goodwill		31				31
G&P | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	696				698
G&P | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	552				557
G&P | MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	313				307
G&P | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	157				159
G&P | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	145				148
G&P | Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		137				145
G&P | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	248				237
L&S
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		331				331
Difference between carrying amount and underlying equity portion related to goodwill		167				167
L&S | MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[6]	462				465
L&S | Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investments		259				254
L&S | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments		257				252
L&S | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	193				194
L&S | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		187				188
L&S | Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	187				185
L&S | W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5],[7]	73				72
L&S | Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investments		69				72
L&S | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	105				103
Operating Segments | G&P
Schedule of Equity Method Investments [Line Items]
Revenues	[8]	1,045		(374)
Equity method investments		2,248				2,251
Income/(loss) from equity method investments(1)		34		(1,234)
Operating Segments | L&S
Schedule of Equity Method Investments [Line Items]
Revenues	[8]	1,294		1,366
Equity method investments		1,792				1,785
Income/(loss) from equity method investments(1)		36		50
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues		447		120
Costs and Expenses		238		236
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		209		(116)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		204		(151)
Assets, Current		633				848
Assets, Noncurrent		12,233				11,886
Liabilities, Current		532				510
Liabilities, Noncurrent		2,906				2,734
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues		284		337
Costs and Expenses		131		132
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		153		205
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		140		186
Income/(loss) from equity method investments(1)		31	[1]	38	[3]
Assets, Current		321				318
Assets, Noncurrent		4,967				4,997
Liabilities, Current		194				187
Liabilities, Noncurrent		853				830
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues		163		(217)
Costs and Expenses		107		104
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		56		(321)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		64		(337)
Income/(loss) from equity method investments(1)		39	[1]	$ (1,222)	[3]
Assets, Current		312				530
Assets, Noncurrent		7,266				6,889
Liabilities, Current		338				323
Liabilities, Noncurrent		$ 2,053				$ 1,904
[1]	Includes the impact of any basis differential amortization or accretion.
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[5]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.
[6]	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.
[7]	Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest Wink to Webster Pipeline LLC.
[8]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $129 million and $158 million for the three months ended March 31, 2021 and 2020, respectively. Third party revenues for the G&P segment were $989 million for the three months ended March 31, 2021 and third party losses were $425 million for the three months ended March 31, 2020.

Related Party Agreements and _3

Related Party Agreements and Transactions - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Restructuring Charges	$ 37

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	Mar. 31, 2021	Mar. 31, 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			$ 2,241	$ 1,667	$ 6,264
Line of Credit Facility, Interest Rate During Period			1.371%		2.278%
Repayments of Lines of Credit			$ 2,241	$ 2,261	$ 6,858
Long-term Line of Credit	[1]		$ 0		$ 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
		Mar. 31, 2021	Mar. 31, 2020
Related Party Transaction [Line Items]
Operating Lease, Lease Income		$ 99	$ 96
Related Party Transaction, Other Revenues from Transactions with Related Party		65	64
Revenue from Related Parties		9	2
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Operating Lease, Lease Income		242	234
Related Party Transaction, Other Revenues from Transactions with Related Party		49	48
Sales Revenue, Goods, Related Party, Net Zero		168	173
Other Affiliates [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Other Revenues from Transactions with Related Party		16	16
Product [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		42	33
Product [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[1]	42	33
Service [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		872	928
Service [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		871	927
Service [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		$ 1	$ 1
[1]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2021 and March 31, 2020, these sales totaled $168 million and $173 million, respectively.

Summary of Charges for Employee

Summary of Charges for Employee Services and Omnibus Agreements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 298	$ 276
General and Administrative Expense	86	97
Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	32	35
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	294	271
General and Administrative Expense	57	64
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	39	46
Other Affiliates [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	4	5
Construction-in-progress | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 12	$ 36

Other Assets and Liabilities fr

Other Assets and Liabilities from Related Parties (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Other current assets	$ 49	$ 65
Current Assets, Related Parties	650	677
Due from Related Parties, Noncurrent	676	672
Operating Lease, Right-of-Use Asset	296	309
Due to Related Parties, Current	338	356
Operating Lease, Liability, Current	63	63
Operating Lease, Liability, Noncurrent	230	244
Liabilities, Related Parties, Noncurrent	279	283
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	586	615
Prepaid Insurance	17	4
Other current assets	1	1
Sales-type Lease, Interest Income	32	30
Due from Related Parties, Noncurrent	32	32
Operating Lease, Right-of-Use Asset	230	231
Net Investment in Lease, Noncurrent	388	386
Sales-type Lease, Unguaranteed Residual Asset	26	23
Due to Related Parties, Current	213	215
Operating Lease, Liability, Current	1	1
Operating Lease, Liability, Noncurrent	229	229
Other Affiliates [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	14	27
Due to Related Parties, Current	26	43
Minimum Committed Volume Contracts [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	65	66
Reimbursable Projects [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	32	30
Deferred Revenue, Noncurrent, Related Parties	43	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
		Mar. 31, 2021	Mar. 31, 2020
Net Income Per Share [Line Items]
Potentially dilutive units		1	1
Net Income (Loss) Attributable to Parent	[1]	$ 739	$ (2,724)
Distribution Made to Limited Partner, Cash Distributions Declared		738	759
Undistributed net income (loss) attributable to MPLX LP		1	(3,483)
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	708	(2,755)
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	707	728
Undistributed net income (loss) attributable to MPLX LP		1	(3,483)
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	20	20
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	20	20
Undistributed net income (loss) attributable to MPLX LP		0	0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent	[1]	11	11
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	11	11
Undistributed net income (loss) attributable to MPLX LP		$ 0	$ 0
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	See Note 7 for distribution information.

Net Income Per Limited Partne_5

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Net income attributable to MPLX LP per limited partner unit:
Distribution Made to Limited Partner, Cash Distributions Declared		$ 738	$ 759
Undistributed net income (loss) attributable to MPLX LP		1	(3,483)
Net Income (Loss) Attributable to Parent	[1]	$ 739	$ (2,724)
Limited Partners Common Units [Member]
Weighted average units outstanding:
Common - basic (in shares)		1,037	1,058
Common - diluted (in shares)		1,037	1,058
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.68	$ (2.60)
Diluted (in USD per unit)		$ 0.68	$ (2.60)
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	$ 707	$ 728
Undistributed net income (loss) attributable to MPLX LP		1	(3,483)
Net Income (Loss) Attributable to Parent	[1]	708	(2,755)
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net income attributable to MPLX LP per limited partner unit:
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	20	20
Undistributed net income (loss) attributable to MPLX LP		0	0
Net Income (Loss) Attributable to Parent	[1]	20	20
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net income attributable to MPLX LP per limited partner unit:
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	11	11
Undistributed net income (loss) attributable to MPLX LP		0	0
Net Income (Loss) Attributable to Parent	[1]	$ 11	$ 11
[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]	See Note 7 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ / shares in Units, $ in Millions		Apr. 27, 2021	Apr. 05, 2021	Feb. 16, 2021	Jul. 31, 2020	Apr. 28, 2020	Jul. 29, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity								$ (729)	$ 2,736
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders								(735)	(738)
Distribution Made to Limited Partner, Cash Distributions Declared								738	759
Stock Repurchase Program, Authorized Amount								$ 1,000
Stock Repurchased and Retired During Period, Shares								(6,272,981)
Treasury Stock Acquired, Average Cost Per Share								$ 24.78
Stock Repurchased and Retired During Period, Value								$ 155
Stock Repurchase Program, Remaining Authorized Repurchase Amount								812
Total MPLX LP partners’ capital								$ 12,606		$ 12,772
Cash distributions declared per limited partner common unit						$ 0.6875
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2020			1,032,648,244					1,038,777,978
Unit-based compensation awards								143,247
Balance at March 31, 2021								1,032,648,244
Distribution Made to Limited Partner, Cash Distributions Declared	[1]							$ 707	728
Series B Preferred Stock [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity								(11)	(11)
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders								(21)	(21)
Stock Repurchased and Retired During Period, Value								0
Total MPLX LP partners’ capital								601	601	$ 611	$ 611
Subsequent Event
Stockholders Equity [Line Items]
Stock Repurchased and Retired During Period, Shares			(291,400)
Stock Repurchased and Retired During Period, Value			$ 7
Cash distributions declared per limited partner common unit		$ 0.6875
Subsequent Event | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared		$ 707
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	[1]							$ 11	$ 11
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
Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Contributed					(18,582,088)
[1]	See Note 7 for distribution information.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions		May 14, 2021	May 07, 2021	Apr. 27, 2021	Feb. 16, 2021	Apr. 28, 2020	Jul. 29, 2019	Mar. 31, 2021	Mar. 31, 2020
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared								$ 738	$ 759
Cash distributions declared per limited partner common unit						$ 0.6875
Partners' Capital Account, Distributions								735	738
Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date					Feb. 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				Apr. 27, 2021
Cash distributions declared per limited partner common unit				$ 0.6875
Date of record			May 7, 2021
Subsequent Event | Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution date		May 14, 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	[1]							707	728
Limited Partners Common Units [Member] | Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared				$ 707
Series A Preferred Stock [Member] | Series B Preferred Stock [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]							11	11
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	[1]							$ 20	$ 20
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]	See Note 7 for distribution information.

Series A Preferred Units (Narra

Series A Preferred Units (Narrative) (Details) - $ / shares shares in Millions	Apr. 27, 2021	Apr. 28, 2020	May 13, 2016	Mar. 31, 2021
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Distributions Declared, Per Unit		$ 0.6875
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units			30.8
Dividend rate, percentage			6.50%
Price per share			$ 32.50
Dividend rate, per-dollar-amount			$ 0.528125
Description			The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate.
Partners' Capital Account, Units				29.6
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Declaration Date	Apr. 27, 2021
Distribution Made to Limited Partner, Distributions Declared, Per Unit	$ 0.6875

Series A Preferred Units (Rollf

Series A Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Redeemable Noncontrolling Interest [Line Items]
Distributions to unitholders and general partner	$ (713)	$ (717)
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2020	968
Net income allocated	20
Distributions to unitholders and general partner	(20)
Balance at March 31, 2021	$ 968

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 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
		Mar. 31, 2021		Mar. 31, 2020
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	$ 329		$ 325
Total revenues from contracts with customers		1,862		1,781
Income/(loss) from equity method investments(1)		70	[2],[3]	(1,184)	[4],[5]
Other income		1		1
Total segment revenues and other income		(2,339)		(992)
Investments in unconsolidated affiliates		(35)		(91)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[6]	(3)		15
L&S
Segment Reporting Information [Line Items]
Depreciation and amortization		147		138
Total revenues from contracts with customers		957		1,023
Investments in unconsolidated affiliates		(9)		(54)
G&P
Segment Reporting Information [Line Items]
Depreciation and amortization		182		187
Total revenues from contracts with customers		905		758
Investments in unconsolidated affiliates		(26)		(37)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA		1,352		1,294
Operating Segments | L&S
Segment Reporting Information [Line Items]
Rental income		249		242
Income/(loss) from equity method investments(1)		36		50
Other income		52		51
Total segment revenues and other income	[7]	(1,294)		(1,366)
Adjusted EBITDA	[8]	896		872
Capital Expenditure		59		184
Operating Segments | G&P
Segment Reporting Information [Line Items]
Rental income		92		88
Income/(loss) from equity method investments(1)		34		(1,234)
Other income		14		14
Total segment revenues and other income	[7]	(1,045)		374
Adjusted EBITDA	[8]	456		422
Capital Expenditure		30		134
Service [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		589		612
Service [Member] | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		84		84
Service [Member] | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		505		528
Service [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		953		1,004
Service [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		508		536
Product [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		282		169
Product [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		4		19
Product [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		397		222
Third Party [Member] | L&S
Segment Reporting Information [Line Items]
Total segment revenues and other income		(129)		(158)
Third Party [Member] | G&P
Segment Reporting Information [Line Items]
Total segment revenues and other income		$ (989)		$ 425
[1]	Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020.
[2]	Includes the impact of any basis differential amortization or accretion.
[3]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[5]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[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.
[7]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $129 million and $158 million for the three months ended March 31, 2021 and 2020, respectively. Third party revenues for the G&P segment were $989 million for the three months ended March 31, 2021 and third party losses were $425 million for the three months ended March 31, 2020.
[8]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 24	$ 15
Assets	36,030	36,414
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	20,787	20,938
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 15,219	$ 15,461

Segment Information - Reconcili

Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
		Mar. 31, 2021		Mar. 31, 2020		Dec. 31, 2020
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization	[1]	$ 329		$ 325
Income Tax Expense (Benefit)		(1)		0
Amortization of Debt Issuance Costs		(17)		(14)
Share-based Payment Arrangement, Expense		(3)		(5)
Goodwill and Intangible Asset Impairment		0		(2,165)
Interest and Other Financial Costs		(220)		(216)
Extinguishment of Debt, Gain (Loss), Net of Tax		12		0
Income/(loss) from equity method investments(1)		70	[2],[3]	(1,184)	[4],[5]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(119)		(119)
Other Cost and Expense, Operating		2		1
Net income		748		(2,716)
Marathon Petroleum Corporation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges						$ (37)
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		1,352		1,294
Amortization of Debt Issuance Costs		(17)		(14)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(121)		(124)
Adjusted EBITDA attributable to noncontrolling interests		10		9
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		147		138
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[6]	896		872
Income/(loss) from equity method investments(1)		36		50
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		182		187
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[6]	456		422
Income/(loss) from equity method investments(1)		34		(1,234)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[7]	$ (3)		$ 15
[1]	Depreciation and amortization attributable to L&S was $147 million for the three months ended March 31, 2021 and $138 million for the three months ended March 31, 2020. Depreciation and amortization attributable to G&P was $182 million for the three months ended March 31, 2021 and $187 million for the three months ended March 31, 2020.
[2]	Includes the impact of any basis differential amortization or accretion.
[3]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[5]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[6]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
[7]	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	Mar. 31, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
NGLs	$ 5	$ 5
Line fill	20	13
Spare parts, materials and supplies	103	100
Total inventories	$ 128	$ 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	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 174
Property, plant and equipment, gross		$ 26,937	$ 26,875
Less accumulated depreciation		5,941	5,657
Property, plant and equipment, net		20,996	21,218
L&S
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,855	12,813
L&S | Terminals
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		6,045	6,026
L&S | Marine
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		965	965
L&S | Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,649	1,643
L&S | Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		2,335	2,333
L&S | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,584	1,584
L&S | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		277	262
G&P
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		14,082	14,062
G&P | Pipelines
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		7,559	7,547
G&P | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		5,725	5,721
G&P | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		510	507
G&P | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 288	$ 287

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	Mar. 31, 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)
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

Goodwill and Intangibles Intang

Goodwill and Intangibles Intangibles (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021		Mar. 31, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived			$ 177
Finite-Lived Intangible Assets, Gross	$ 1,571			$ 1,571
Finite-Lived Intangible Asset, Expected Amortization, Remainder of Fiscal Year	95
Finite-Lived Intangible Asset, Expected Amortization, Year One	127
Finite-Lived Intangible Asset, Expected Amortization, Year Two	127
Finite-Lived Intangible Asset, Expected Amortization, Year Four	113
Finite-Lived Intangible Asset, Expected Amortization, after Year Five	338
Finite-Lived Intangible Assets, Net	927
Finite-Lived Intangible Assets, Accumulated Amortization	(644)	[1]		(612)	[2]
Intangibles, net	927			959
Finite-Lived Intangible Asset, Expected Amortization, Year Three	127
L&S
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	9
Finite-Lived Intangible Assets, Gross	283			283
Finite-Lived Intangible Assets, Accumulated Amortization	(90)	[1]		(81)
Intangibles, net	$ 193			202
L&S | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	6 years
L&S | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	8 years
G&P
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 23
Finite-Lived Intangible Assets, Gross	1,288			1,288
Finite-Lived Intangible Assets, Accumulated Amortization	(554)	[1]		(531)	[2]
Intangibles, net	$ 734			$ 757
G&P | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	6 years
G&P | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	25 years
[1]	Amortization expense attributable to the L&S and G&P segments for the three months ended March 31, 2021 was $9 million and $23 million, respectively.
[2]	Impairment charge of $177 million is included within the G&P accumulated amortization.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions		Mar. 31, 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]	(66)	(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
Derivative liability		66	$ 63
Derivative Asset, Fair Value, Gross Asset		$ 0
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details)		3 Months Ended
		Mar. 31, 2021USD ($)$ / gal	Dec. 31, 2020USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, Average Forward Price | $ / gal		0.62
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]	$ (66,000,000)	$ (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%
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		$ 0.48
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		$ 1.36
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 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
Total gains (losses) (realized and unrealized) included in earnings	[1]	0	0
Settlements		0	0
Fair value at end of period		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
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
Derivative Liability, Fair Value, Gross Liability	[2]	(66)		$ (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		(63)	(60)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(6)	14
Settlements		3	1
Fair value at end of period		(66)	(45)
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		$ (5)	$ 13
[1]	Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
[2]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including amounts due within one year)	$ 20,156	$ 20,244
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including amounts due within one year)	$ 21,988	$ 22,951

Derivative Financial Instrume_3

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)	Dec. 31, 2020USD ($)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Derivative [Line Items]
Derivative Liability	$ (66)	$ (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		Mar. 31, 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]	66	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]	10	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]	$ 56	$ 56
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Derivative [Line Items]
Total gain (loss)		$ (6)	$ 14
Purchased product costs
Derivative [Line Items]
Realized gain (loss)		(3)	(1)
Total gain (loss)		(6)	14
Not Designated as Hedging Instrument [Member] | Purchased product costs
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[1]	$ (3)	$ 15
[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		Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total		$ 20,445	$ 20,536
Unamortized debt issuance costs		(112)	(116)
Unamortized discount/premium		(279)	(281)
Amounts due within one year		(2)	(764)
Total long-term debt due after one year		20,052	19,375
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		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(1)	[1]	10	11
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		$ 835	$ 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%
[1]	See Note 20 for lease information.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions		Sep. 09, 2019	Mar. 31, 2021	Jan. 15, 2021	Dec. 31, 2020	Jul. 30, 2019
Debt Instrument [Line Items]
Debt and Lease Obligation			$ 20,445		$ 20,536
Unamortized Debt Issuance Expense			112		116
Debt Instrument, Unamortized Discount			279		281
Debt, Current			2		764
Long-term debt			20,052		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			835		175
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit			$ 1,910
Debt Instrument, Interest Rate, Effective Percentage			1.347%
Repayments of Long-term Lines of Credit			$ 1,250
Letters of Credit Outstanding			1
Line of Credit Facility, Remaining Borrowing Capacity			$ 2,670
Line of Credit Facility, Remaining Borrowing Capacity, Percentage			76.10%
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]
Debt Instrument [Line Items]
Percent of Par		100.00%
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			1,000		1,000
Variable Rate Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR plus 1.1 percent per annum
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Finance Lease, Liability	[1]		$ 10		$ 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%
[1]	See Note 20 for lease information.

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021		Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		$ 1,862		$ 1,781
Income/(loss) from equity method investments(1)		70	[1],[2]	(1,184)	[3],[4]
Other Operating Income		1		1
Related Party Transaction, Other Revenues from Transactions with Related Party		65		64
Operating Income (Loss)	[5]	477		(789)
Total segment revenues and other income		2,339		992
L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		957		1,023
G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		905		758
Service [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		589		612
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		872		928
Service [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		84		84
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		869		920
Service [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		505		528
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3		8
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		77		39
Service, Other [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		0		0
Service, Other [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		77		39
Product [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		282		169
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		42		33
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		282		169
Product [Member] | L&S
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3		4
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		1		15
Product [Member] | G&P
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		39		29
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		$ 281		$ 154
[1]	Includes the impact of any basis differential amortization or accretion.
[2]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	The 2020 period includes $1,264 million of impairment expense. See Note 4.
[5]	Non-ASC 606 Revenue includes rental income, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.

Revenue Contract Balances (Deta

Revenue Contract Balances (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	[1]
Long-term deferred revenue, beginning balance		$ 314
Long-term deferred revenue, ending balance		332
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)		(27)	(27)
Contract assets, revenue recognized	[2]	0	0
Contract assets, ending balance		13	12
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)		7	5
Deferred revenue, revenue recognized	[2]	(1)	(3)
Deferred revenue, ending balance		43	25
Deferred revenue - related parties, beginning balance	[1]	91	53
Deferred revenue - related party, additions/(deletions)		21	12
Deferred revenue - related parties, revenue recognized	[2]	(20)	(16)
Deferred revenue - related parties, ending balance		92	49
Long-term deferred revenue, beginning balance	[1]	119	90
Long-term deferred revenue, additions/(deletions)		4	6
Long-term deferred revenue, revenue recognized	[2]	0	0
Long-term deferred revenue, ending balance		123	96
Long-term deferred revenue - related parties, beginning balance	[1]	48	55
Long-term deferred revenue - related party, additions/(deletions)		(4)	1
Long-term deferred revenue - related parties, revenue recognized	[2]	0	0
Long-term deferred revenue - related parties, ending balance		44	$ 56
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	Mar. 31, 2021USD ($)
Revenue from Contract with Customer [Abstract]
Contract with customer, liability	$ 301
Revenue, Remaining Performance Obligation, Amount	11,073	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-04-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,435
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	9 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	$ 1,838
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	$ 1,674
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	$ 1,546
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	$ 4,580
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	23 years
[1]	All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded.
[2]	Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table.
[3]	Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jul. 31, 2020
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 231	$ 210
Non-cash investing and financing activities
Net transfers of property, plant and equipment (to)/from materials and supplies inventories	$ 0	$ (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	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Decrease in capital accruals	$ (37)	$ (61)

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021		Mar. 31, 2020		Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax	[1]	$ (2)		$ (1)
LOOP and Explorer
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss		(17)	[2]	(16)	[2]	$ (15)	$ (15)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax		(2)		(1)
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | LOOP and Explorer
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss		(15)	[2]	(14)	[2]	(13)	(14)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax		(2)		0
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | LOOP and Explorer
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss		(2)	[2]	(2)	[2]	$ (2)	$ (1)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax		$ 0		$ (1)
[1]	Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.
[2]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Pla_2

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Number of Units
Outstanding at December 31, 2020 | shares	644,023
Granted | shares	321,927
Settled | shares	143,088
Forfeited | shares	1,270
Outstanding at March 31, 2021 | shares	821,592
Weighted Average Fair Value
Outstanding at December 31, 2020 | $ / shares	$ 28.65
Granted | $ / shares	24.88
Settled | $ / shares	29.73
Forfeited | $ / shares	33.09
Outstanding at March 31, 2021 | $ / shares	$ 26.98

Equity-Based Compensation Pla_3

Equity-Based Compensation Plan - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021$ / shares
Maximum [Member] | Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 2

Equity-Based Compensation Pla_4

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail)	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2020	3,092,286
Granted	375,730	[1]
Settled	1,347,177
Forfeited	(1,761)
Outstanding at March 31, 2021	2,119,078
Maximum [Member] | Performance Unit Performance Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Performance units grant date fair value (in USD per unit) | $ / shares	$ 2
[1]	No new performance unit awards were granted during the three months ended March 31, 2021. The performance units shown above as granted represents subsequent tranches of the 2020 and 2019 performance unit awards that were subject to a performance condition based on MPLX LP’s 2021 distributable cash flow, which, due to the nature of the award terms, did not meet the criteria for a grant date fair value until the first quarter of 2021, at which time we began recognizing units and expense related to these tranches. Grant date fair value of the performance condition is based on potential payouts of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes.

Leases Leases Narrative (Detail

Leases Leases Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Operating Lease, Lease Income	$ 99	$ 96
Marathon Petroleum Corporation [Member]
Operating Lease, Lease Income	242	234
Sales-type Lease, Unguaranteed Residual Asset	26		$ 23
Refining Logistics [Member]
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount		171
Refining Logistics [Member] | Marathon Petroleum Corporation [Member]
Sales-type Lease, Lease Receivable		370
Sales-type Lease, Unguaranteed Residual Asset		10
Sales-type Lease, Selling Profit (Loss)		$ 209
Third Party [Member]
Sales-type Lease, Selling Profit (Loss)	$ 0

Leases Lessor Lease Revenues (D

Leases Lessor Lease Revenues (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	$ 202	$ 186
Sales-type Lease, Interest Income		37	38
Sales-type Lease, Variable Lease Income		0	0
Third Party [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	68	63
Sales-type Lease, Selling Profit (Loss)		0
Sales-type Lease, Interest Income		0
Sales-type Lease, Variable Lease Income		0
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Sales-type Lease, Selling Profit (Loss)		$ 0	$ 0
[1]	These amounts are presented net of executory costs

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2021
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation	$ 17	$ 18
Due to Related Parties, Current	356	338
Loss Contingency, Damages Sought, Value	187
Contractual commitments to acquire property, plant and equipment		101
Loss Contingency, Damages Paid, Value	4
Marathon Petroleum Corporation [Member]
Commitments And Contingencies [Line Items]
Due to Related Parties, Current	215	213
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