MPLX (MPLX) 10-K 26 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 15, 2021	Jun. 30, 2020
Entity Information [Line Items]
Entity Registrant Name	MPLX LP
Entity File Number	001-35714
Entity Tax Identification Number	27-0005456
Entity Incorporation, State or Country Code	DE
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Well-known Seasoned Issuer	Yes
Entity Address, City or Town	Findlay
Entity Filer Category	Large Accelerated Filer
Entity Address, Postal Zip Code	45840-3229
Entity Address, Address Line One	200 E. Hardin Street
Entity Address, State or Province	OH
Entity Central Index Key	0001552000
Current Fiscal Year End Date	--12-31
Entity Voluntary Filers	No
Entity Shell Company	false
Entity Public Float			$ 6.8
Document Information [Line Items]
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
City Area Code	419
Local Phone Number	421-2414
Title of 12(b) Security	Common Units Representing Limited Partnership Interests
Trading Symbol	MPLX
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	true
Entity Common Stock, Shares Outstanding		1,037,169,607
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31

Description of the Business and

Description of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation (“MPC”) 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 was formed on March 27, 2012 as a Delaware limited partnership and completed its initial public offering on October 31, 2012. 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 10 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 4 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. On July 30, 2019, MPLX completed its acquisition by merger (the “Merger”) of Andeavor Logistics LP (“ANDX”). At the effective time of the Merger, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units. ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. See Note 4 for additional information regarding the Merger. Impairments – The outbreak of COVID-19 and its development into a pandemic in March 2020 resulted in significant economic disruption globally. Actions taken by various governmental authorities, individuals and companies around the world to prevent the spread of COVID-19 through social distancing have restricted travel, many business operations, public gatherings and the overall level of individual movement and in-person interaction across the globe. Although there have been some signs of economic improvement, these events significantly reduced global economic activity and resulted in a decline in the demand for the midstream services we provide beginning with the first quarter of 2020. Macroeconomic conditions and global geopolitical events have also resulted in significant price volatility related to those aforementioned products. 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 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 current environment, which impacted their current and expected future demand for our services, including the future utilization of our assets. Additionally, certain of our contracts have commodity price exposure, including NGL prices, which have experienced increased volatility as noted above. The table below provides information related to the impairments recognized during the first quarter of 2020 as well as the corresponding footnote where additional information can be found. No additional events or circumstances arose during the remainder of the year which would indicate the need for any additional impairment beyond those recognized during the first quarter. (In millions) Impairment Footnote Reference Goodwill $ 1,814 14 Equity method investments 1,264 5 Intangibles, net 177 14 Property, plant and equipment, net 174 13 Total impairments $ 3,429 Basis of Presentation – The accompanying consolidated financial statements of MPLX have been prepared in accordance with GAAP. The 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 a VIE 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 relation to the Merger described above and in Note 4, ANDX’s assets, liabilities and results of operations prior to the Merger are collectively included in what we refer to as the “Predecessor” from October 1, 2018, which was the date that MPC acquired Andeavor. MPLX’s acquisition of ANDX is considered a transfer between entities under common control due to MPC’s relationship with ANDX prior to the Merger. As an entity under common control with MPC, MPLX recorded the assets acquired and liabilities assumed on its consolidated balance sheets at MPC’s historical carrying value. For the acquiring entity, transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. Accordingly, the accompanying financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of ANDX beginning October 1, 2018.

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 6,896	$ 7,041		$ 5,449
Rental income		398	388		352
Rental income - related parties		952	1,196		846
(Loss)/income from equity method investments		(936)	290		247
Other income		5	12		7
Other income - related parties		254	114		99
Total revenues and other income		7,569	9,041		7,005
Costs and expenses:
Rental cost of sales		135	141		135
Rental cost of sales - related parties		160	165		31
Purchases - related parties		1,116	1,231		925
Depreciation and amortization		1,377	1,254		867
Impairment expense		2,165	1,197		0
General and administrative expenses		378	388		316
Restructuring Charges		37	0		0
Other taxes		125	113		83
Total costs and expenses		7,358	6,664		4,277
Income from operations		211	2,377		2,728
Related party interest and other financial costs		5	11		5
Interest expense		829	851		590
Other financial costs		62	53		119
(Loss)/income before income taxes		(685)	1,462		2,014
Provision for income taxes		2	0		8
Net (loss)/income		(687)	1,462		2,006
Less: Net income attributable to noncontrolling interests		33	28		16
Less: Net income attributable to Predecessor		0	401		172
Net (loss)/income attributable to MPLX LP	[1]	(720)	1,033		1,818
Limited partners’ interest in net (loss)/income attributable to MPLX LP		$ (842)	$ 935		$ 1,743
Weighted average limited partner units outstanding:
Common - basic (in units)		1,051	906	[2]	761
Common - diluted (in units)		1,051	907	[2]	761
Limited Partners Common Units
Costs and expenses:
Net (loss)/income attributable to MPLX LP	[1]	$ (842)	$ 910		$ 1,743
Net (loss)/income attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ (0.80)	$ 1		$ 2.29
Common - diluted (in USD per unit)		$ (0.80)	$ 1		$ 2.29
Weighted average limited partner units outstanding:
Common - basic (in units)		1,051	906	[2]	761
Common - diluted (in units)		1,051	907	[2]	761
Service [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 2,397	$ 2,498		$ 1,856
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3,580	3,455		2,404
Product [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		636	806		887
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[3]	128	142		87
Oil and Gas, Refining and Marketing [Member]
Costs and expenses:
Cost of Goods and Services Sold		1,326	1,489		1,096
Natural Gas, Midstream [Member]
Costs and expenses:
Cost of Goods and Services Sold		539	686		824
Service, Other [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		155	140		220
Series A Preferred Stock [Member] | Preferred Partner [Member]
Costs and expenses:
Net (loss)/income attributable to MPLX LP	[1]	81	81		75
Less: Series A preferred unit distributions		81	81		75
Series B Preferred Stock [Member] | Preferred Partner [Member]
Costs and expenses:
Net (loss)/income attributable to MPLX LP	[1]	41	42
Less: Series A preferred unit distributions		$ 41	$ 17		$ 0
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire year. See Notes 4 and 8 for additional information about the treatment of these units.
[3]	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 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 million, respectively.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Interest costs capitalized	$ 39	$ 51	$ 37

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (687)	$ 1,462	$ 2,006
Other comprehensive (loss)/income, net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	0	1	(2)
Comprehensive (loss)/income	(687)	1,463	2,004
Less comprehensive income attributable to:
Noncontrolling interests	33	28	16
Income attributable to Predecessor	0	401	172
Comprehensive (loss)/income attributable to MPLX LP	$ (720)	$ 1,034	$ 1,816

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents		$ 15	$ 15
Receivables, net		452	593
Current Assets, Related Parties		677	656
Inventories		118	110
Other current assets		65	110
Disposal Group, Including Discontinued Operation, Assets, Current		188	0
Total current assets		1,515	1,484
Equity method investments		4,036	5,275
Property, plant and equipment, net		21,218	22,145
Intangibles, net		959	1,270
Goodwill		7,657	9,536
Operating Lease, Right-of-Use Asset		309	365
Other Assets, Related Parties, Noncurrent		672	303
Other noncurrent assets		48	52
Total assets		36,414	40,430
Current liabilities:
Accounts payable		152	242
Accrued liabilities		194	187
Current Liabilities, Related Parties		356	1,008
Accrued property, plant and equipment		84	283
Amounts due within one year		764	9
Accrued interest payable		222	210
Operating Lease, Liability, Current		63	66
Other current liabilities		150	127
Disposal Group, Including Discontinued Operation, Liabilities, Current		101	0
Total current liabilities		2,086	2,132
Long-term deferred revenue		314	217
Liabilities, Related Parties, Noncurrent		283	290
Long-term debt		19,375	19,704
Deferred Income Tax Liabilities, Net		12	12
Operating Lease, Liability, Noncurrent		244	302
Deferred credits and other liabilities		115	192
Total liabilities		22,429	22,849
Commitments and contingencies (see Note 23)
Series A preferred units		968	968
Equity
Total MPLX LP partners’ capital		12,772	16,364
Accumulated other comprehensive loss	[1]	(15)	(15)
Noncontrolling interests		245	249
Total equity		13,017	16,613
Total liabilities, preferred units and equity		36,414	40,430
Marathon Petroleum Corporation [Member]
Current assets:
Other current assets		1	0
Operating Lease, Right-of-Use Asset		231	232
Current liabilities:
Operating Lease, Liability, Current		1	1
Operating Lease, Liability, Noncurrent		$ 229	$ 230
Limited Partners Common Units | Public
Current liabilities:
Limited Partners' Capital Account, Units Issued		391,000,000	392,000,000
Equity
Total MPLX LP partners’ capital		$ 9,384	$ 10,800
Total equity		$ 9,384	$ 10,800
Units outstanding		391,000,000	392,000,000
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Current liabilities:
Limited Partners' Capital Account, Units Issued		647,000,000	666,000,000
Equity
Total MPLX LP partners’ capital		$ 2,792	$ 4,968
Total equity		$ 2,792	$ 4,968
Units outstanding		647,415,452	666,000,000
Series B Preferred Stock [Member]
Current liabilities:
Limited Partners' Capital Account, Units Issued		600,000	600,000
Equity
Total MPLX LP partners’ capital		$ 611	$ 611
Units outstanding		600,000	600,000
LOOP and Explorer
Equity
Accumulated other comprehensive loss		$ (15)	$ (15)
Retained Earnings [Member] | Marathon Petroleum Corporation [Member]
Equity
Total equity		$ 0	$ 0
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares	Dec. 31, 2020	Dec. 31, 2019
Limited Partners Common Units | Public
Units issued	391,000,000	392,000,000
Units outstanding	391,000,000	392,000,000
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Units issued	647,000,000	666,000,000
Units outstanding	647,415,452	666,000,000
Series B Preferred Stock [Member]
Units issued	600,000	600,000
Units outstanding	600,000	600,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Operating activities:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (687)	$ 1,462	$ 2,006
Adjustments to reconcile net income/(loss) to net cash provided by operating activities:
Amortization of deferred financing costs	61	42	55
Depreciation and amortization	1,377	1,254	867
Impairment expense	2,165	1,197	0
Deferred income taxes	(1)	(2)	8
Asset retirement expenditures	0	(1)	(7)
Loss/(Gain) on disposal of assets	4	(6)	3
Income (Loss) from Equity Method Investments	936	(290)	(247)
Distributions from unconsolidated affiliates	459	525	412
Changes in:
Current receivables	62	17	(104)
Inventories	(12)	(9)	(5)
Fair value of derivatives	3	2	(10)
Current accounts payable and accrued liabilities	36	(59)	88
Current assets/current liabilities - related parties	8	(163)	(61)
Increase (Decrease) in Other Operating Assets and Liabilities, Net	(5)	4	0
Deferred revenue	112	100	61
All other, net	3	9	5
Net cash provided by operating activities	4,521	4,082	3,071
Investing activities:
Additions to property, plant and equipment	(1,183)	(2,408)	(2,111)
Proceeds from Previous Acquisition	0	6
Acquisitions, net of cash acquired			(451)
Disposal of assets	56	30	8
Investments in unconsolidated affiliates	(266)	(713)	(341)
Distributions from unconsolidated affiliates - return of capital	123	18	16
All other, net	8	4	1
Net cash used in investing activities	(1,262)	(3,063)	(2,878)
Proceeds from long-term lines of credit	6,810	9,174	13,476
Financing activities:
Long-term debt - repayments	(6,414)	(7,924)	(6,946)
Related party debt - borrowings	6,264	9,313	3,962
Related party debt - repayments	(6,858)	(8,719)	(4,347)
Debt issuance costs	(25)	(20)	(76)
Payments for Repurchase of Common Stock	(33)	0	0
Distributions to MPC for acquisitions	0	0	(4,111)
Distributions to unitholders and general partner	(2,884)	(2,435)	(1,819)
Distributions to noncontrolling interests	(37)	(30)	(17)
Contributions from MPC	50	74	41
Contributions from noncontrolling interests	0	95	11
All other, net	(10)	(13)	19
Net Cash Provided by (Used in) Financing Activities	(3,259)	(1,089)	(117)
Net (decrease)/increase in cash, cash equivalents and restricted cash	0	(70)	76
Cash, cash equivalents and restricted cash at beginning of period	15	15	85
Cash, cash equivalents and restricted cash at end of period	15	15	85
Series A Preferred Stock [Member] | Preferred Partner [Member]
Financing activities:
Distributions to Series A preferred unitholders	(81)	(81)	(71)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Financing activities:
Distributions to Series A preferred unitholders	(41)	(21)	0
Payments of Distributions on Preferred Units from Predecessor	$ 0	$ (502)	$ (239)

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions	Total	Preferred Partner [Member]	PublicLimited Partners Common Units	Marathon Petroleum Corporation [Member]General Partner	Marathon Petroleum Corporation [Member]Limited Partners Common Units	AOCI Attributable to Parent	Noncontrolling Interest	Equity of PredecessorMarathon Petroleum Corporation [Member]
Beginning balance at Dec. 31, 2017	$ 9,973	$ 0	$ 8,379	$ (637)	$ 2,099	$ (14)	$ 146	$ 0
Contribution from MPC	(11,980)	0	0	0	0	0	0	(11,980)
Stockholders' Equity, Other	11	0	12	1	0	(2)	0	0
Net income (loss)	(1,931)	0	(667)	0	(1,076)	0	(16)	(172)
Allocation of MPC's net investment at acquisition	0	0	0	4,126	(5,172)	0	0	1,046
Distribution to MPC for acquisitions	(4,100)	0	0	(3,164)	(936)	0	0	0
Noncontrolling interests	(2,058)	0	(722)	0	(1,097)	0	0	(239)
(Placeholder)	(17)	0	0	0	0	0	(17)	0
Contributions from noncontrolling interests	11	0	0	0	0	0	11	0
Unit conversion	0	0	0	7,926	(7,926)	0	0	0
Ending balance at Dec. 31, 2018	17,731	0	8,336	0	(1,612)	(16)	156	10,867
Contribution from MPC	(346)	0	0	0	(315)	0	0	(31)
Stockholders' Equity, Other	13	0	12	0	0	1	0	0
Net income (loss)	(1,381)	(17)	(340)	0	(595)	0	(28)	(401)
Allocation of MPC's net investment at acquisition	0	(615)	(2,983)	0	(7,199)	0	0	10,797
Noncontrolling interests	(2,959)	(21)	(907)	0	(1,529)	0	0	(502)
(Placeholder)	(30)	0	0	0	0	0	(30)	0
Contributions from noncontrolling interests	95	0	0	0	0	0	95	0
Unit conversion	36	0	36	0	0	0	0	0
Ending balance at Dec. 31, 2019	16,613	611	10,800	0	4,968	(15)	249	0
Contribution from MPC	(261)	0	0	0	(261)	0	0	0
Stockholders' Equity, Other	5	0	6	0	(1)	0	0	0
Net income (loss)	768	(41)	307	0	535	0	(33)	0
Stock Repurchased and Retired During Period, Value	(33)	0	(33)	0	0	0	0	0
Allocation of MPC's net investment at acquisition	102	0	0	0	102	0	0	0
Noncontrolling interests	(2,922)	(41)	(1,082)	0	(1,799)	0	0	0
(Placeholder)	(37)	0	0	0	0	0	(37)	0
Contributions from noncontrolling interests	0
Ending balance at Dec. 31, 2020	$ 13,017	$ 611	$ 9,384	$ 0	$ 2,792	$ (15)	$ 245	$ 0

Summary of Principal Accounting

Summary of Principal Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Principal Accounting Policies	Summary of Principal Accounting Policies Use of Estimates – The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods. Actual results could differ materially from those estimates. Estimates are subject to uncertainties due to the levels of subjectivity and judgment necessary to account for highly uncertain matters or the susceptibility of such matters to change and affect items such as valuing identified intangible assets; determining the fair value of derivative instruments; evaluating impairments of long-lived assets, goodwill and equity investments; establishing estimated useful lives for long-lived assets; acquisition accounting; recognizing share-based compensation expense; estimating revenues, expense accruals and capital expenditures; valuing AROs; and determining liabilities, if any, for environmental and legal contingencies. Revenue Recognition – Revenue is measured based on consideration specified in a contract with a customer. MPLX recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. MPLX enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” MPLX provides services under the following types of arrangements: • Fee-based arrangements – Under fee-based arrangements, MPLX receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue MPLX earns from these arrangements is generally directly related to the volume of natural gas, NGLs, refined products or crude oil that is handled by or flows through MPLX’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, MPLX’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed. In certain instances when specifically stated in the contract terms, MPLX purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis, as MPLX takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, MPLX: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, MPLX delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes MPLX retains to third parties or related parties. Revenue is recognized on a net basis when MPLX acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, MPLX gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, MPLX must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require MPLX to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” on the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as MPLX is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by MPLX. • Purchase arrangements – Under purchase arrangements, MPLX purchases natural gas at either the wellhead or the tailgate of a plant. MPLX then gathers and delivers the natural gas to pipelines where MPLX may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs.” Often, MPLX earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements is reported as “Product sales” on the Consolidated Statements of Income and is recognized on a gross basis as MPLX purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, MPLX provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, MPLX records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of MPLX’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions, and the services offered and whether they are deemed distinct. MPLX allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. Revenue from MPLX’s service arrangements will generally be recognized over time as the performance obligation is satisfied as services are provided. MPLX has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period. In instances in which tiered pricing structures do not reflect our efforts to perform, MPLX will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue. Based on the terms of certain natural gas gathering, transportation and processing agreements, MPLX is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. Revenue and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as “Rental income” and “Rental cost of sales,” respectively, on the Consolidated Statements of Income. Revenue and Expense Accruals – MPLX routinely makes accruals based on estimates for both revenues and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling MPLX’s records with those of third parties. The delayed information from third parties includes, among other things, actual volumes purchased, transported or sold, adjustments to inventory and invoices for purchases, actual natural gas and NGL deliveries and other operating expenses. MPLX makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and MPLX’s internal records have been reconciled. Cash and Cash Equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with initial maturities of three months or less. Restricted Cash – Restricted cash consists of cash and investments that must be maintained as collateral for letters of credit issued to certain third-party producer customers. The balances will be outstanding until certain capital projects are completed and the third party releases the restriction. Restricted cash also consists of cash advances to be used for the operation and maintenance of an operated pipeline system. Restricted cash is included in “Other current assets” on the Consolidated Balance Sheets. Receivables – Receivables primarily consist of customer accounts receivable, which are recorded at the invoiced amount and generally do not bear interest. Allowances for doubtful accounts are generally recorded when it becomes probable that the receivable will not be collected and are recorded to bad debt expense. We review the allowance quarterly with past-due balances over 90 days and other higher-risk amounts being reviewed individually for collectability. Balances that remain outstanding after reasonable collection efforts have been unsuccessful are written off through a charge to the valuation allowance and a credit to accounts receivable. Leases – As part of the adoption of ASU No. 2016-02, Leases (“ASC 842”), we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to grandfather the historical accounting conclusions until a reassessment event is present. We also elected the practical expedient to not recognize short-term leases on the balance sheet, the practical expedient related to right of way permits and land easements which allows us to carry forward our accounting treatment for those existing agreements, and the practical expedient to combine lease and non-lease components for the majority of our underlying classes of assets except for our third-party contractor service and equipment agreements and boat and barge equipment agreements in which we are the lessee. We did not elect the practical expedient to combine lease and non-lease components for arrangements in which we are the lessor. In instances where the practical expedient was not elected, lease and non-lease consideration is allocated based on relative standalone selling price. Right of use (“ROU”) assets represent our right to use an underlying asset in which we obtain substantially all of the economic benefits and the right to direct the use of the asset during the lease term. Lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. We recognize ROU assets and lease liabilities on the balance sheet for leases with a lease term of greater than one year. Payments that are not fixed at the commencement of the lease are considered variable and are excluded from the ROU asset and lease liability calculations. In the measurement of our ROU assets and lease liabilities, the fixed lease payments in the agreement are discounted using a secured incremental borrowing rate for a term similar to the duration of the lease, as our leases do not provide implicit rates. Operating lease expense is recognized on a straight-line basis over the lease term. As a lessor under ASC 842, MPLX may be required to re-classify existing operating leases to sales-type leases upon modification and related reassessment of the leases. See Note 22 for further information regarding our ongoing evaluation of the impacts of lease reassessments as modifications occur. The net investment in a sales-type lease is recorded within “Current assets - related parties” and “Noncurrent assets - related parties” on the Consolidated Balance Sheets and is comprised of the present value of the sum of the future minimum lease payments representing the value of the lease receivable and the unguaranteed residual value of the leased assets. Management assesses the net investment in sales-type leases for recoverability quarterly. Inventories – Inventories consist primarily of natural gas, propane, other NGLs and materials and supplies to be used in operations. Natural gas, propane, and other NGLs are valued at the lower of cost or market value. Materials and supplies are stated at the lower of cost or market value. Cost for materials and supplies are determined primarily using the weighted-average cost method. Imbalances – Within our pipelines and storage assets, we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive imbalances are recorded as other current assets and negative imbalances are recorded as accounts payable. Positive and negative product imbalances are settled in cash, settled by physical delivery of gas from a different source, or tracked and settled in the future. Property, Plant and Equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis over the estimated useful lives of the assets. Expenditures that extend the useful lives of assets are capitalized. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment assessment is performed and the excess of the book value over the fair value is recorded as an impairment loss. When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported on the Consolidated Statements of Income. Gains on the disposal of property, plant and equipment are recognized when they occur, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale. Interest costs for the construction or development of long-lived assets are capitalized and amortized over the related asset’s estimated useful life. Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined using an income and/or market approach which is compared to the carrying value of the reporting unit. The fair value under the income approach is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future operating performance, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the excess, if any, of the book value over the fair value of the reporting unit up to the amount of goodwill recorded is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. Total goodwill at December 31, 2020 was $7.7 billion and no impairment was recorded as a result of our November 30, 2020 annual goodwill impairment analysis. As a result of MPLX’s interim goodwill impairment analysis at March 31, 2020 and annual goodwill impairment analysis at November 30, 2019, we recorded impairment charges of approximately $1.8 billion and $1.2 billion, respectively. See Note 14 for further details. Other Taxes – Other taxes primarily include real estate taxes. Environmental Costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. MPLX recognizes remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure and are discounted when the estimated amounts are reasonably fixed and determinable. If recoveries of remediation costs from third parties are probable, a receivable is recorded and is discounted when the estimated amount is reasonably fixed and determinable. Asset Retirement Obligations – An ARO is a legal obligation associated with the retirement of tangible long-lived assets that generally result from the acquisition, construction, development or normal operation of the asset. AROs are recorded at fair value in the period in which they are incurred, if a reasonable estimate of fair value can be made, and added to the carrying amount of the associated asset. This additional carrying amount is then depreciated over the life of the asset. The liability is determined using a credit adjusted risk free interest rate and increases due to the passage of time based on the time value of money until the obligation is settled. MPLX recognizes a liability of a conditional ARO as soon as the fair value of the liability can be reasonably estimated. A conditional ARO is defined as an unconditional legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within the control of the entity. AROs have not been recognized for certain assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates. Investment in Unconsolidated Affiliates – Equity investments in which MPLX exercises significant influence, but does not control and is not the primary beneficiary, are accounted for using the equity method and are reported in “Equity method investments” on the accompanying Consolidated Balance Sheets. This includes entities in which we hold majority ownership but the minority shareholders have substantive participating rights. Differences in the basis of the investments and the separate net asset values of the investees, if any, are amortized into net income over the remaining useful lives of the underlying assets and liabilities, except for the excess related to goodwill. MPLX believes the equity method is an appropriate means for it to recognize increases or decreases measured by GAAP in the economic resources underlying the investments. Regular evaluation of these investments is appropriate to evaluate any potential need for impairment. MPLX uses evidence of a loss in value to identify if an investment has an other than a temporary decline. During the first quarter of 2020, MPLX recorded an other than temporary impairment for three joint ventures totaling $1,264 million, of which $1,251 million was related to MarkWest Utica EMG and its investment in Ohio Gathering Company, L.L.C. The impairments were 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. Deferred Financing Costs – Deferred financing costs are an asset for credit facility costs and netted against debt for senior notes. These costs are amortized over the contractual term of the related obligations using the effective interest method or, in certain circumstances, accelerated if the obligation is refinanced. Derivative Instruments – MPLX may use commodity derivatives to economically hedge a portion of its exposure to commodity price risk. All derivative instruments (including derivatives embedded in other contracts) are recorded at fair value. Certain commodity derivatives are reflected on the consolidated balance sheets on a net basis by counterparty as they are governed by master netting arrangements. MPLX discloses the fair value of all derivative instruments under the captions “Other noncurrent assets,” “Other current liabilities” and “Deferred credits and other liabilities” on the Consolidated Balance Sheets. Changes in the fair value of derivative instruments are reported on the Consolidated Statements of Income in accounts related to the item whose value or cash flows are being managed. All derivative instruments are marked to market through “Product sales,” “Purchased product costs,” or “Cost of revenues” on the Consolidated Statements of Income. Revenue gains and losses relate to contracts utilized to manage the cash flow for the sale of a product, typically NGLs. Purchased product costs gains and losses relate to contracts utilized to manage the cost of natural gas purchases, typically related to keep-whole arrangements. Cost of revenues gains and losses relate to a contract utilized to manage electricity costs. Changes in risk management for unrealized activities are reported as an adjustment to net income in computing cash flow from operating activities on the accompanying Consolidated Statements of Cash Flows. During the years ended December 31, 2020, 2019 and 2018, MPLX did not elect hedge accounting for any derivatives. MPLX has elected the normal purchases and normal sales designation for certain contracts related to the physical purchase of electric power and the sale of some commodities. Fair Value of Financial Instruments – Management believes the carrying amount of financial instruments, including cash and cash equivalents, receivables, receivables from related parties, other current assets, accounts payable, accounts payable to related parties and accrued liabilities approximate fair value because of the short-term maturity of these instruments. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximate fair value due to the variable interest rate that approximates current market rates (see Note 15). Derivative instruments are recorded at fair value, based on available market information (see Note 16). Fair Value Measurement – Financial assets and liabilities recorded at fair value in the Consolidated Balance Sheets are categorized based upon the fair value hierarchy established by GAAP, which classifies the inputs used to measure fair value into Level 1, Level 2 or Level 3. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The methods and assumptions utilized may produce a fair value that may not be realized in future periods upon settlement. Furthermore, while MPLX believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. For further discussion, see Note 15. Equity-Based Compensation Arrangements – MPLX issues phantom units under its share-based compensation plan as described further in Note 21. A phantom unit entitles the grantee a right to receive a common unit upon the issuance of the phantom unit. The fair value of phantom unit awards granted to employees and non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. The fair value of the units awarded is amortized into earnings using a straight-line amortization schedule over the period of service corresponding with the vesting period. For phantom units that vest immediately and are not forfeitable, equity-based compensation expense is recognized at the time of grant. Performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards. Equity-classified performance units with a market condition use a Monte Carlo valuation model to calculate a grant date fair value of market conditions. Equity-classified performance units with a performance condition are valued based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. To satisfy common unit awards, MPLX may issue new common units, acquire common units in the open market or use common units already owned by the general partner. Income Taxes – MPLX is not a taxable entity for United States federal income tax purposes or for the majority of the states that impose an income tax. Taxes on MPLX’s net income generally are borne by its partners through the allocation of taxable income. MPLX’s taxable income or loss, which may vary substantially from the net income or loss reported on the Consolidated Statements of Income, is includable in the federal income tax returns of each partner. MPLX and certain legal entities are, however, taxable entities under certain state jurisdictions. MPLX accounts for income taxes under the asset and liability method. Deferred income taxes are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, capital loss carryforwards and net operating loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect of any tax rate change on deferred taxes is recognized as tax expense/(benefit) from continuing operations in the period that includes the enactment date of the tax rate change. Realizability of deferred tax assets is assessed and, if not more likely than not, a valuation allowance is recorded to reflect the deferred tax assets at net realizable value as determined by management. All deferred tax balances are classified as long-term in the accompanying Consolidated Balance Sheets. All changes in the tax bases of assets and liabilities are allocated among operations and items charged or credited directly to equity. Distributions – 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, as discussed in Notes 8 and 9, and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued as a liability until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described below. Net Income Per Limited Partner Unit – MPLX uses the two-class method when calculating the net income per unit applicable to limited partners, because there is more than one class of participating security. The classes of participating securities include common units, Series A and Series B preferred units and certain equity-based compensation awards. Net income attributable to MPLX LP is allocated to the unitholders differently for preparation of the Consolidated Statements of Equity and the calculation of net income per limited partner unit. 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 and subsequently allocated to remaining unitholders in accordance with their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 7. In preparing net income per limited partner units, during periods in which a net loss attributable to MPLX is reported or periods in which the total distributions exceed the reported net income attributable to MPLX’s unitholders, the amount allocable to certain equity-based compensation awards is based on actual distributions to the equity-based compensation awards. Diluted earnings per unit is calculated by dividing net income attributable to MPLX’s common unitholders, after deducting amounts allocable to other participating securities, by the weighted average number of common units and potential common units outstanding during the period. Potential common units are excluded from the calculation of diluted earnings per unit during periods in which net income attributable to MPLX’s unitholders, after deducting amounts that are allocable to the outstanding equity-based compensation awards and preferred units, is a loss, as the impact would be anti-dilutive. Business Combinations – MPLX recognizes and measures the assets acquired and liabilities assumed in a business combination based on their estimated fair values at the acquisition date, with any remaining difference recorded as goodwill or gain from a bargain purchase. Depending on the nature of the transaction, management may engage an independent valuation specialist to assist with the determination of fair value of the assets acquired, liabilities assumed, noncontrolling interests, if any, and goodwill, based on recognized business valuation methodologies. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent

Accounting Standards

Accounting Standards	12 Months Ended
	Dec. 31, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Accounting Standards	Accounting Standards Recently Adopted ASU 2016-13, Credit Losses - Measurement of Credit Losses on Financial Instruments Effective January 1, 2020, we adopted ASU 2016-13 using the modified retrospective transition method. This ASU requires entities to consider a broader range of information to estimate expected credit losses, which may result in earlier recognition of losses. The ASU requires the company to utilize an expected loss methodology in place of the incurred loss methodology for financial instruments, including trade receivables, and off-balance sheet credit exposures. Adoption of the standard did not have a material impact on our financial statements. We are exposed to credit losses, primarily as a result of the midstream services that we provide. We assess each customer’s ability to pay through our credit review process, which considers various factors such as external credit ratings; a review of financial statements to determine liquidity, leverage, trends and business specific risks; market information; pay history and our business strategy. We monitor our ongoing credit exposure through timely review of customer payment activity. At December 31, 2020, we reported $452 million of third-party accounts receivable, net of allowances of $1 million We also adopted the following ASUs during 2020, which did not have a material impact to our financial statements or financial statement disclosures: ASU Topic Effective Date 2018-13 Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement January 1, 2020 2020-04 Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting April 1, 2020

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Acquisitions	Acquisitions and Dispositions Javelina Held-for-Sale On December 23, 2020, MarkWest Energy Operating Company, L.L.C., (“MarkWest Energy”) a wholly owned subsidiary of MPLX, entered into an Equity Purchase Agreement with a third party under which MarkWest Energy has agreed to sell 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”). This agreement includes adjustments for working capital as well as an earnout provision based on the performance of the assets. This sale was closed on February 12, 2021. Javelina’s assets and liabilities to be sold are shown on the Consolidated Balance Sheet as “Assets held for sale” and “Liabilities held for sale”, respectively, for the year ended December 31, 2020. Javelina is currently reported within the G&P segment. Wholesale Exchange On July 31, 2020, MPLX entered into a Redemption Agreement (the “Redemption Agreement”) with WRSW, a wholly owned subsidiary of MPC, pursuant to which MPLX agreed to transfer to WRSW all of the outstanding membership interests in WRW in exchange for the redemption of MPLX common units held by WRSW. The transaction effects the transfer to MPC of the Western wholesale distribution business that MPLX acquired as a result of its acquisition of ANDX as described below. Per the terms of the Redemption Agreement, MPLX redeemed 18,582,088 common units (the “Redeemed Units”) held by WRSW on July 31, 2020. The number of Redeemed Units was calculated by dividing WRW’s aggregate valuation of $340 million by the simple average of the volume weighted average NYSE prices of an MPLX common unit for the ten trading days ending at market close on July 27, 2020. MPLX canceled the Redeemed Units immediately following the Wholesale Exchange. The carrying value of the net assets of WRW transferred to MPC was approximately $90 million as of July 31, 2020, resulting in $250 million being recorded to “Common Unit-holder MPC” within the Consolidated Statements of Equity, netted against the fair value of the redeemed units. Included within the $90 million carrying value of the WRW net assets was approximately $65 million of goodwill. Acquisition of Andeavor Logistics LP On May 7, 2019, ANDX, Tesoro Logistics GP, LLC, then the general partner of ANDX, MPLX, MPLX GP LLC, the general partner of MPLX (“MPLX GP”), and MPLX MAX LLC, a wholly owned subsidiary of MPLX (“Merger Sub”), entered into an Agreement and Plan of Merger (the “Merger Agreement”) that provided for, among other things, the merger of Merger Sub with and into ANDX. On July 30, 2019, the Merger was completed, and ANDX survived the Merger as a wholly owned subsidiary of MPLX. At the effective time of the Merger, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units. ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. See Note 8 for information on common units issued in connection with the Merger as well as Series B preferred units. As a result of the Merger, the ANDX Special Limited Partner Interest outstanding immediately prior to the effective time of the Merger was converted into a right for WRSW, as the holder of all such interest, to receive a substantially equivalent special limited partner interest in MPLX (the “MPLX Special Limited Partner Interest”). By virtue of the conversion, the ANDX Special Limited Partner Interest was cancelled and ceased to exist as of the effective time of the Merger. For information on ANDX’s preferred units, please see Note 8. The assets of ANDX consist of a network of owned and operated crude oil, refined product and natural gas pipelines; crude oil and water gathering systems; refining logistics assets; terminals with crude oil and refined products storage capacity; rail facilities; marine terminals including storage; bulk petroleum distribution facilities; a trucking fleet; and natural gas processing and fractionation complexes. The assets are located in the western and inland regions of the United States and complement MPLX’s existing business and assets. MPC accounted for its October 1, 2018 acquisition of Andeavor (through which it acquired control of ANDX) using the acquisition method of accounting, which required Andeavor assets and liabilities to be recorded by MPC at the acquisition date fair value. The Merger was closed on July 30, 2019, and the results of ANDX have been incorporated into the results of MPLX as of October 1, 2018, which is the date that common control was established. As a result of MPC’s relationship with both MPLX and ANDX, the Merger has been treated as a common control transaction, which requires the recasting of MPLX’s historical results and the recognition of assets acquired and liabilities assumed using MPC’s historical carrying value. The fair value of assets acquired and liabilities assumed shown below represents MPC’s historical carrying values as of October 1, 2018. (In millions) As Originally Reported Adjustments (1) As Adjusted Cash and cash equivalents $ 83 $ (53) $ 30 Receivables, net 241 259 500 Inventories 21 — 21 Other current assets (2) 59 (7) 52 Equity method investments 731 (89) 642 Property, plant and equipment, net 6,709 (427) 6,282 Intangibles, net (3) 960 74 1,034 Other noncurrent assets (4) 31 (8) 23 Total assets acquired 8,835 (251) 8,584 Accounts payable 198 265 463 Other current liabilities (5) 188 (41) 147 Long-term debt 4,916 — 4,916 Deferred credits and other long-term liabilities (6) 75 1 76 Total liabilities assumed 5,377 225 5,602 Net assets acquired excluding goodwill 3,458 (476) 2,982 Goodwill 7,428 724 8,152 Total purchase price $ 10,886 $ 248 $ 11,134 (1) Inclusive of activity recorded subsequent to the acquisition of ANDX on July 30, 2019, a portion of which was recorded as a non-cash contribution from MPC. (2) Includes both related party and third party other current assets. (3) Includes approximately $4 million of favorable lease assets. In connection with the implementation of ASC 842, this balance was reclassed to “Right of use assets” on the Consolidated Balance Sheets during 2019. (4) Includes both related party and third party other noncurrent assets as well as right of use assets associated with leases. (5) Includes accrued liabilities, operating lease liabilities, long-term debt due within one year, as well as related party and third party other current liabilities. (6) Includes deferred revenue and deferred income taxes, as well as related party and third party other noncurrent liabilities. Details of our valuation methodology and significant inputs for fair value measurements are included by asset class below. The fair value measurements for equity method investments; property, plant and equipment; intangible assets and long-term debt are based on significant inputs that are not observable in the market and, therefore, represent Level 3 measurements. Goodwill The purchase consideration allocation resulted in the recognition of $8.2 billion in goodwill, which has been allocated between the L&S segment and the G&P segment at $7.2 billion and $1.0 billion, respectively. See Note 14 for further information related to goodwill. Inventory The fair value of inventory was recorded at cost as of October 1, 2018, as these items are related to spare parts as well as materials and supplies and approximate fair value. Equity Method Investments The fair value of the equity method investments was $642 million, which was determined based on applying income and market approaches. The income approach relied on the discounted cash flow method and the market approach relied on a market multiple approach considering historical and projected financial results. Discount rates for the discounted cash flow models were based on capital structures for similar market participants and included various risk premiums that account for risks associated with the specific investments. Property, Plant and Equipment The fair value of property, plant and equipment was $6.3 billion, which was based primarily on the cost approach. Key assumptions in the cost approach include determining the replacement cost by evaluating recent purchases of similar assets or published data, and adjusting replacement cost for economic and functional obsolescence, location, normal useful lives, and capacity (if applicable). Acquired Intangible Assets The fair value of the acquired identifiable intangible assets was $1.0 billion, which represents the value of various customer contracts and relationships and other intangible assets. The fair value of customer contracts and relationships was $950 million, which was valued by applying the multi-period excess earnings method, which is an income approach. Key assumptions in the income approach include the underlying contract cash flow estimates, remaining contract term, probability of renewal, growth rates and discount rates. The intangible assets are all finite lived and will be amortized over two Debt The fair value of the ANDX notes was measured using a market approach, based upon the average of quotes for the acquired debt from major financial institutions and a third-party valuation service. Additionally, approximately $1.1 billion of borrowings under revolving credit agreements approximated fair value. The ANDX revolving credit facilities with total capacity of $2.1 billion were terminated upon closing of the Merger and were repaid with borrowings under the MPLX revolving credit facility. Acquisition Costs We recognized $14 million in acquisition costs during 2019, which are reflected in general and administrative expenses. ANDX Revenue and Net Income For the year ended December 31, 2019, we recognized $2,400 million of revenues and other income related to ANDX and $266 million of net loss related to ANDX, which was impacted by the goodwill impairment discussed in Note 14. For the year ended December 31, 2018, we recognized $580 million of revenues and other income related to ANDX and $172 million of net income related to ANDX. Pro Forma Financial Information The following unaudited pro forma information combines the historical operations of MPLX and ANDX, giving effect to the Merger as if it had been consummated on January 1, 2018, the beginning of the earliest period presented. (In millions) 2019 2018 Total revenues and other income $ 9,041 $ 8,666 Net income attributable to MPLX LP $ 1,434 $ 2,446 The pro forma information includes adjustments to align accounting policies, which include adjustments for capitalization of assets and treatment of planned major maintenance costs. The pro forma information also includes adjustments related to: eliminating transactions between MPLX and ANDX, which previously would have been recorded as transactions between related parties; basis differences on equity method investments as a result of recognition of MPC’s investments in ANDX’s equity method investments; depreciation and amortization expense to reflect the increased fair value of property, plant and equipment and increased amortization expense related to identifiable intangible assets, as well as adjustments to interest expense for the amortization of fair value adjustments over the remaining term of ANDX’s outstanding debt, reversal of ANDX’s historical amortization of debt issuance costs and debt discounts and to adjust for the difference in the weighted average interest rate between MPLX’s revolving credit facility and ANDX’s revolving credit facilities. Mt. Airy Terminal On September 26, 2018, MPLX acquired an eastern U.S. Gulf Coast export terminal (the “Mt. Airy Terminal”) from Pin Oak Holdings, LLC for total consideration of $451 million. At the time of the acquisition, the terminal included tanks with 4 million barrels of third-party leased storage capacity and a dock with 120 mbpd of capacity. The Mt. Airy Terminal is located on the Mississippi River between New Orleans and Baton Rouge, is in close proximity to several Gulf Coast refineries including MPC’s Garyville Refinery and is near numerous rail lines and pipelines. The Mt. Airy Terminal is accounted for within the L&S segment. In the first quarter of 2019, an adjustment to the initial purchase price was made for approximately $5 million related to the final settlement of the acquisition, which was paid in the first six months of 2019 as shown on the statement of cash flow. This reduced the total purchase price to $446 million and resulted in $336 million of property, plant and equipment, $121 million of goodwill and the remainder being attributable to net liabilities assumed. The amount of revenue and income from operations associated with the acquisition of the Mt. Airy Terminal included on the Consolidated Statement of Income since the September 26, 2018 acquisition date was not material to the financial statements. Assuming the acquisition had occurred on January 1, 2018, the consolidated pro forma results would not have been materially different from the reported results. Refining Logistics and Fuels Distribution Acquisition On February 1, 2018, MPC and MPLX LP closed on an agreement for the dropdown of refining logistics assets and fuels distribution services to MPLX LP. MPC contributed these assets and services in exchange for $4.1 billion in cash and a fixed number of MPLX LP common units and general partner units of 111,611,111 and 2,277,778, respectively. The fair value of the common and general partner units issued as of the acquisition date was $4.3 billion based on the closing common unit price as of February 1, 2018, as recorded on the Consolidated Statements of Equity, for a total purchase price of $8.4 billion. The equity issued consisted of: (i) 85,610,278 common units to MPLX GP, (ii) 18,176,666 common units to MPLX Logistics Holdings LLC and (iii) 7,824,167 common units to MPLX Holdings Inc. MPLX also issued 2,277,778 general partner units to MPLX GP in order to maintain its two general partner interest (“GP Interest”) in MPLX. MPC agreed to waive approximately one-third of the first quarter 2018 distributions on the common units issued in connection with this transaction. As a result of this waiver, MPC did not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. Immediately following this transaction, the GP Interest was converted into a non-economic general partner interest as discussed in Note 8. MPLX recorded this transaction on a historical basis as required for transactions between entities under common control. No effect was given to the prior periods as these entities were not considered businesses prior to the February 1, 2018 dropdown. In connection with the dropdown, approximately $830 million of net property, plant and equipment was recorded in addition to $85 million and $130 million of goodwill allocated to MPLX Refining Logistics LLC (“Refining Logistics”) and MPLX Fuels Distribution LLC (“Fuels Distribution”), respectively. Both the Refining Logistics assets and the Fuels Distribution services are accounted for within the L&S segment. As of the transaction date, the Refining Logistics assets included 619 tanks with approximately 56 million barrels of storage capacity (crude, finished products and intermediates), 32 rail and truck racks, 18 docks, and gasoline blenders. These assets generate revenue through storage services agreements with MPC. Refining Logistics provides certain services to MPC related to the receipt, storage, throughput, custody and delivery of petroleum products in and through certain storage and logistical facilities and assets associated with MPC’s refineries. Fuels Distribution, which is a wholly owned subsidiary of MPLXT, generates revenue through a Fuels Distribution Services Agreement with MPC. Fuels Distribution is structured to provide a broad range of scheduling and marketing services as MPC’s agent. The amounts of revenue and income from operations associated with these investments included on the Consolidated Statements of Income, since the February 1, 2018 acquisition date, were as follows: (In millions) Twelve Months Ended Revenues and other income $ 1,359 Income from operations $ 874

Investments and Noncontrolling

Investments and Noncontrolling Interests	12 Months Ended
	Dec. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests	Investments and Noncontrolling Interests The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at December 31, December 31, (In millions, except ownership percentages) 2020 2020 2019 L&S MarEn Bakken Company LLC (1) 25% $ 465 $ 481 Illinois Extension Pipeline Company, L.L.C. 35% 254 265 LOOP LLC 41% 252 238 Andeavor Logistics Rio Pipeline LLC (2)(3) 67% 194 202 Minnesota Pipe Line Company, LLC (2) 17% 188 190 Whistler Pipeline LLC (3) 38% 185 134 W2W Holdings LLC (3)(4) 50% 72 — Wink to Webster Pipeline LLC (3)(4) 15% — 126 Explorer Pipeline Company 25% 72 83 Other (2)(3) 103 55 Total L&S 1,785 1,774 G&P MarkWest Utica EMG, L.L.C. (3) 57% 698 1,984 Sherwood Midstream LLC (3) 50% 557 537 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (3) 67% 307 302 Rendezvous Gas Services, L.L.C. (2)(3) 78% 159 170 Sherwood Midstream Holdings LLC (3) 51% 148 157 Centrahoma Processing LLC 40% 145 153 Other (2)(3) 237 198 Total G&P 2,251 3,501 Total $ 4,036 $ 5,275 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture 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) These investments as well as certain investments included within “Other” for both L&S and G&P are investments acquired as part of the Merger. The December 31, 2019 balance reflects all purchase accounting adjustments identified by MPC as part of its acquisition of Andeavor. (3) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (4) During the year ended December 31, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC. For those entities that have been deemed to be VIEs, neither MPLX nor any of its subsidiaries have been deemed to be the primary beneficiary due to voting rights on significant matters. While we have the ability to exercise influence through participation in the management committees which make all significant decisions, we have equal influence over each committee as a joint interest partner and all significant decisions require the consent of the other investors without regard to economic interest and as such we have determined that these entities should not be consolidated and apply the equity method of accounting with respect to our investments in each entity. Sherwood Midstream 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 December 31, 2020, MPLX had a 24.5 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 years ended December 31, 2020, 2019 and 2018. 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 fair value of the investments was determined based upon applying the discounted cash flow method, which is an income approach. The discounted cash flow fair value estimate is based on known or knowable information at the interim measurement date. The significant assumptions that were used to develop the estimate of the fair value under the discounted cash flow method include management’s best estimates of the expected future cash flows, including prices and volumes, the weighted average cost of capital and the long-term growth rate. Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As such, the fair value of these equity method investments represents a Level 3 measurement. As a result, there can be no assurance that the estimates and assumptions made for purposes of the impairment test will prove to be an accurate prediction of the future. The impairment was recorded through “Income from equity method investments.” The impairments were largely due to a reduction in forecasted volumes gathered and processed by the systems operated by the joint ventures. There were no additional impairments recorded during the remainder of 2020. During the fourth quarter of 2019, two joint ventures in which we have an interest recorded impairments, which impacted the amount of income from equity method investments during the period by approximately $28 million and took the carrying value of one of the investments to zero. For the other joint venture, we had a basis difference recorded which was being amortized over the life of the underlying assets. As a result of the impairment recorded by the joint venture, we assessed our investment, including the related basis difference, for impairment and recorded an additional $14 million of impairment during the quarter related to our basis difference. The fair value of the investment was determined based upon applying the discounted cash flow method, which is an income approach. The discounted cash flow fair value estimate is based on known or knowable information at the interim measurement date. The significant assumptions that were used to develop the estimate of the fair value under the discounted cash flow method include management’s best estimates of the expected future results using a probability-weighted average set of cash flow forecasts and the discount rate. The impairment of the basis difference was also recorded through “Income from equity method investments” for a total impact during the quarter of approximately $42 million. The impairments were largely due to a reduction in forecasted volumes of the joint ventures. Summarized financial information for MPLX’s equity method investments for the years ended December 31, 2020, 2019 and 2018 is as follows: December 31, 2020 (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 298 $ 1,208 $ 1,506 Costs and expenses 414 531 945 (Loss)/income from operations (116) 677 561 Net (loss)/income (175) 615 440 (Loss)/income from equity method investments (2) $ (1,100) $ 164 $ (936) December 31, 2019 (1) (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 650 $ 1,417 $ 2,067 Costs and expenses 375 568 943 Income from operations 275 849 1,124 Net income 215 752 967 Income from equity method investments (2) $ 103 $ 187 $ 290 December 31, 2018 (1) (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 484 $ 1,421 $ 1,905 Costs and expenses 286 738 1,024 Income from operations 198 683 881 Net income 197 606 803 Income from equity method investments (2) $ 67 $ 180 $ 247 (1) The financial information for equity method investments for 2019 includes financial information of equity method investments acquired as part of the Merger. The financial information for equity method investments for 2018 includes financial information of equity method investments acquired as part of the Merger for the last three months of 2018. See Note 1 for additional information. (2) “Income from equity method investments” includes the impact of any basis differential amortization or accretion. The 2020 and 2019 amounts include impairment of $1,264 million and $42 million, respectively. Summarized balance sheet information for MPLX’s equity method investments as of December 31, 2020 and 2019 is as follows: December 31, 2020 (In millions) Other 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 December 31, 2019 (In millions) Other VIEs Non-VIEs Total Current assets $ 534 $ 330 $ 864 Noncurrent assets 5,862 5,134 10,996 Current liabilities 192 245 437 Noncurrent liabilities $ 305 $ 822 $ 1,127 As of 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 $57 million. At December 31, 2019, the carrying value of MPLX’s equity method investments in the G&P segment exceeded the underlying net assets of its investees by approximately $1.0 billion. As of December 31, 2020 and 2019, the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $331 million and $329 million, respectively. At December 31, 2020 and 2019, the G&P basis difference was being amortized into net income over the remaining estimated useful lives of the underlying net assets, except for $31 million and $498 million of excess related to goodwill, respectively. At December 31, 2020 and 2019, the L&S basis difference was being amortized into net income over the remaining estimated useful lives of the underlying net assets, except for $167 million of excess related to goodwill, respectively.

Related Party Agreements and Tr

Related Party Agreements and Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions Disclosure [Text Block]	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. In addition, 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 as discussed below. The commercial agreements with MPC include: • MPLX has a fuels distribution agreement with MPC under which MPC pays MPLX a tiered monthly volume-based fee for marketing and selling MPC’s products. This agreement is subject to a minimum quarterly volume and has an initial term of 10 years, subject to a 5-year renewal period under terms to be renegotiated at that time. • MPLX has various pipeline transportation agreements under which MPC pays MPLX fees for transporting crude and refined products on MPLX’s pipeline systems. These agreements are subject to minimum throughput volumes under which MPC will pay MPLX deficiency payments for any period in which they do not ship the minimum committed volume. These deficiency payments can be applied as credits to future periods in which MPC ships volumes in excess of the minimum volume, subject to a limited period of time. These agreements are subject to various terms and renewal periods. • MPLX has a marine transportation agreement with an initial term of 6 years under which MPC pays MPLX fees for providing marine transportation of crude oil, feedstock and refined petroleum products, and related services. This agreement is subject to two automatic renewal periods of 5 years each. • MPLX has various trucking transportation services agreements with terms ranging from month-to-month to 10 years, under which MPC pays MPLX fees for gathering barrels and providing trucking, dispatch, delivery and data services. Most of these agreements are subject to minimum volume commitments and have various terms regarding carry-forward of deficiency payments as credits towards excess volumes shipped in future periods. These agreements are subject to various terms and renewal periods. • MPLX has numerous storage services agreements governing storage services at various types of facilities including terminals, pipeline tank farms, caverns and refineries, under which MPC pays MPLX per-barrel fees for providing storage services. Some of these agreements provide MPC with exclusive access to storage at certain locations, such as storage located at MPC’s refineries or storage in certain caverns. Under these agreements, MPC pays MPLX a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. Many of the refinery storage agreements also contain provisions for logistical services to be provided by MPLX, for which MPC pays monthly fees. These agreements are subject to various terms and renewal periods. • MPLX has a 10-year terminal services agreement governing certain terminals under which MPC pays MPLX fees for terminal storage for refined petroleum products. Under this agreement MPC pays MPLX agreed upon fees relating to MPC product deliveries as well as any viscosity surcharges, loading, handling, transfers or other related charges. This agreement is subject to minimum volume throughput commitments under which MPC pays a deficiency payment for any period in which they do not meet the minimum committed volume. The terminal services agreement with MPC includes automatic renewal terms ranging from one one • MPLX has a year to year 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. The pricing structure under this agreement provides for a base volume subject to a base rate and incremental volumes subject to variable rates which are calculated with reference to certain of our costs incurred as processor of the volumes. The pricing for both the base and incremental volumes are subject to revision each year. In many cases, agreements are location-based hybrid agreements, containing provisions relating to multiple of the types of agreements and services described above. Operating Agreements MPLX operates various pipelines owned by MPC under operating services agreements. Under these operating services agreements, MPLX receives an operating fee for operating the assets and is reimbursed for all direct and indirect costs associated with operating the assets. Most of these agreements are indexed for inflation. These agreements range from one Co-location Services Agreements MPLX is party to co-location services agreements with MPC’s refineries, under which MPC provides management, operational and other services to the subsidiaries of Refining Logistics. Refining Logistics pays MPC monthly fixed fees and direct reimbursements for such services calculated as set forth in the agreements. These agreements have initial terms of 50 years. Ground Lease Agreements MPLX is party to ground lease agreements with certain of MPC’s refineries under which MPLX is the lessor of certain sections of property which contain facilities owned by Refining Logistics and are within the premises of MPC’s refineries. Refining Logistics pays MPC monthly fixed fees under these ground leases. These agreements have initial terms of 50 years. Marine Services Agreements with MPC MPLX has a management services agreement and a loss control agreement with MPC under which it provides management and loss control services to assist MPC in the oversight and management of the marine business. MPLX receives fixed annual fees for providing the required services, which are subject to predetermined annual escalation rates. These agreements are subject to an initial terms of five years and automatically renew for one additional five Omnibus Agreements MPLX has omnibus agreements with MPC that address MPLX’s payment of fixed annual fees to MPC for the provision of executive management services by certain executive officers of the general partner and MPLX’s reimbursement of MPC for the provision of certain general and administrative services to it. They also provide for MPC’s indemnification to MPLX for certain matters, including environmental, title and tax matters; as well as our indemnification of MPC for certain matters under these agreements. Employee Services Agreements MPLX has various employee services agreements and secondment agreements with MPC under which MPLX reimburses MPC for employee benefit expenses, along with the provision of operational and management services in support of both our L&S and G&P segments’ operations. Loan Agreement MPLX is party to a loan agreement with 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. On April 27, 2018, MPLX and MPC Investment entered into an amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion. In connection with the Merger, on July 31, 2019, MPLX and MPC Investment entered into a second amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement to $1.5 billion in aggregate principal amount of all loans outstanding at any one time. The 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 the maturity date. Borrowings under the MPC Loan Agreement prior to July 31, 2019 bore interest at LIBOR plus 1.50 percent, while borrowings as of and after July 31, 2019 will bear interest at the one-month 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 17. Activity on the MPC Loan Agreement was as follows: (In millions, except %) December 31, 2020 December 31, 2019 Borrowings $ 6,264 $ 8,540 Average interest rate of borrowings 2.278 % 3.441 % Repayments $ 6,858 $ 7,946 Outstanding balance at end of period $ — $ 594 Prior to the Merger, ANDX was also party to a loan agreement with MPC (“ANDX-MPC Loan Agreement”). This facility was entered into on December 21, 2018, with a borrowing capacity of $500 million. In connection with the Merger, on July 31, 2019, MPLX repaid the entire outstanding balance and terminated the ANDX-MPC Loan Agreement. There was no activity on the ANDX-MPC Loan Agreement in 2018. Activity on the agreement during 2019 prior to the Merger was as follows: (In millions, except %) December 31, 2019 Borrowings $ 773 Average interest rate of borrowings 4.249 % Repayments $ 773 Outstanding balance at end of period $ — Related Party Revenue Related party sales to MPC consist of crude oil and refined products pipeline and trucking transportation services based on tariff/contracted rates; storage, terminal and fuels distribution services based on contracted rates; and marine transportation services. Related party sales to MPC also consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: (In millions) 2020 2019 2018 Service revenue MPC $ 3,578 $ 3,455 $ 2,404 Other 2 — — Total Service revenue - related parties 3,580 3,455 2,404 Rental income MPC 952 1,196 846 Product sales (1) MPC 128 140 87 Other — 2 — Total Product sales - related parties 128 142 87 Other income MPC 192 47 41 Other 62 67 58 Total Other income - related parties $ 254 $ 114 $ 99 (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 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 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 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 rent and lease agreements with MPC. MPC has also been advancing certain strategic priorities to lay a foundation for long-term success, including plans to optimize its assets and structurally lower costs in 2021 and beyond, which included an involuntary workforce reduction plan. The workforce reduction plan, together with employee reductions resulting from MPC's indefinite idling of its Martinez, California and Gallup, New Mexico refineries, affected approximately 2,050 employees. All of the employees that conduct MPLX’s business are directly employed by affiliates of MPC, and certain of those employees were affected by MPC’s workforce reductions. During 2020, MPLX reimbursed MPC for $37 million related to severance and employee benefits related expenses that MPC recorded in connection with its workforce reductions. These costs are shown on the Consolidated Statements of Income as “Restructuring expenses.” 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: (In millions) 2020 2019 2018 Rental cost of sales - related parties $ 160 $ 165 $ 31 Purchases - related parties MPC 1,099 1,210 919 Other 17 21 6 Total Purchases - related parties 1,116 1,231 925 General and administrative expenses MPC 254 243 199 Restructuring expenses MPC $ 37 $ — $ — Some charges incurred under the omnibus and employee service 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 2020, 2019 and 2018, these charges totaled $97 million, $169 million and $152 million, respectively. Related Party Assets and Liabilities December 31, (In millions) 2020 2019 Current assets - related parties Receivables - MPC $ 615 $ 621 Receivables - Other 27 22 Prepaid - MPC 4 9 Other - MPC 1 — Lease Receivables - MPC 30 4 Total 677 656 Noncurrent assets - related parties Long-term receivables - MPC 32 21 Right of use assets - MPC 231 232 Long-term lease receivables - MPC 386 43 Unguaranteed residual asset - MPC 23 7 Total 672 303 Current liabilities - related parties Payables - MPC 215 911 Payables - Other 43 37 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 66 42 Deferred revenue - Project reimbursements - MPC 30 16 Deferred revenue - Project reimbursements - Other 1 1 Total 356 1,008 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 230 Long-term deferred revenue - Project reimbursements - MPC 47 53 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 283 $ 290

Net Income (Loss) Per Limited P

Net Income (Loss) Per Limited Partner Unit	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings Per Share [Text Block]	Net Income/(Loss) Per Limited Partner Unit Net income/(loss) per unit applicable to common limited partner 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. Additional MPLX common units and MPLX Series B preferred units were issued on July 30, 2019 as a result of the merger with ANDX as discussed in Note 4. Distributions declared on these newly issued common and Series B preferred units are a reduction to income available to MPLX common unit holders due to their participation in distributions of income. Classes of participating securities for 2020, 2019 and 2018 include: 2020 2019 2018 Common Units P P P Equity-based compensation awards P P P Series A preferred units P P P Series B preferred units P P The Merger was a transfer between entities under common control as discussed in Note 4. As entities under common control with MPC, prior periods were retrospectively adjusted to furnish comparative information. Accordingly, the prior period earnings have been allocated to the general partner and do not affect the net income/(loss) per unit calculation. The earnings for the entities acquired under common control will be included in the net income/(loss) per unit calculation prospectively as described above. In 2020, 2019 and 2018, MPLX had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the years ended December 31, 2020, 2019 and 2018 were less than 1 million. (In millions) 2020 2019 2018 Net (loss)/income attributable to MPLX LP $ (720) $ 1,033 $ 1,818 Less: Distributions declared on Series A preferred units (1) 81 81 75 Distributions declared on Series B preferred units (1) 41 42 — Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1)(2) 2,872 2,635 1,985 Undistributed net loss attributable to MPLX LP $ (3,714) $ (1,725) $ (242) (1) See Note 8 for distribution information. (2) Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates. 2020 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net (loss)/income attributable to MPLX LP per unit: Net (loss)/income attributable to MPLX LP: Distributions declared $ 2,872 $ 81 $ 41 $ 2,994 Undistributed net loss attributable to MPLX LP (3,714) — — (3,714) Net (loss)/income attributable to MPLX LP (1) $ (842) $ 81 $ 41 $ (720) Weighted average units outstanding: Basic 1,051 1,051 Diluted 1,051 1,051 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.80) Diluted $ (0.80) 2019 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared $ 2,635 $ 81 $ 42 $ 2,758 Undistributed net loss attributable to MPLX LP (1,725) — — (1,725) Net income attributable to MPLX LP (1) $ 910 $ 81 $ 42 $ 1,033 Weighted average units outstanding: Basic (2) 906 906 Diluted (2) 907 907 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.00 Diluted $ 1.00 2018 (In millions, except per unit data) Limited Series A Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distribution declared $ 1,985 $ 75 $ 2,060 Undistributed net loss attributable to MPLX LP (242) — (242) Net income attributable to MPLX LP (1) $ 1,743 $ 75 $ 1,818 Weighted average units outstanding: Basic 761 761 Diluted 761 761 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.29 Diluted $ 2.29 (1) Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire year. See Notes 4 and 8 for additional information about the treatment of these units.

Equity

Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Equity	Equity Units Outstanding MPLX had 1,038,777,978 common units outstanding as of December 31, 2020. Of that number, 647,415,452 were owned by MPC, which also owns the non-economic GP Interest as described below. MPLX had 600,000 Series B preferred units outstanding as of December 31, 2020. The sections below describe activities and events which impacted our unit balances throughout the year. 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 year ended December 31, 2020, 1,473,843 public common units were repurchased at an average cost per unit of $22.29 per unit. Total cash paid for units repurchased during the year was $33 million with $967 million remaining available under the program for future repurchases as of December 31, 2020. As of December 31, 2020, we had agreements to acquire 99,406 additional common units for $2 million, which settled in early January 2021. Wholesale Exchange and Merger In connection with the Wholesale Exchange as discussed in Note 4, 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. In connection with the Merger and as discussed in Note 4, each common unit held by ANDX’s public unitholders was converted into the right to receive 1.135 MPLX common units while ANDX common units held by certain affiliates of MPC were converted into the right to receive 1.0328 MPLX common units. This resulted in the issuance of MPLX common units of approximately 102 million units to public unitholders and approximately 161 million units to MPC in connection with MPLX's acquisition of ANDX on July 30, 2019. Series A Redeemable Preferred Unit Conversions - During 2019, certain holders of Series A preferred units exercised their rights to convert their Series A preferred units into approximately 1.2 million common units as discussed in Note 9. ATM Program – On March 13, 2018, MPLX entered into a Third Amended and Restated Distribution Agreement, which provided for the at-the-market issuances of common units having an aggregate offering price of up to approximately $1.7 billion, in amounts, at prices and on terms determined by market conditions and other factors at the time of the offerings (such continuous offering program, or at-the-market program is referred to as the “ATM Program”). At December 31, 2020, the ATM Program remains in effect, although MPLX has not issued any units under this program during 2020, 2019 or 2018. The table below summarizes the changes in the number of units outstanding for the years ended December 31, 2018, 2019, and 2020: (In units) Common General Partner (1) Total Balance at December 31, 2017 407,130,020 8,308,773 415,438,793 Unit-based compensation awards 348,387 140 348,527 Contribution of Refining Logistics and Fuels Distribution (See Note 4) 111,611,111 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 (10,586,691) 264,413,309 Balance at December 31, 2018 794,089,518 — 794,089,518 Unit-based compensation awards 288,031 — 288,031 Issuance of units in connection with the Merger (See Note 4) 262,829,592 — 262,829,592 Conversion of Series A preferred units 1,148,330 — 1,148,330 Balance at December 31, 2019 1,058,355,471 — 1,058,355,471 Unit-based compensation awards 478,438 — 478,438 Wholesale Exchange (18,582,088) — (18,582,088) Units redeemed in unit repurchase program (1,473,843) — (1,473,843) Balance at December 31, 2020 1,038,777,978 — 1,038,777,978 (1) Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisition of Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two GP Interest. Series B Preferred Units - Prior to the Merger, ANDX issued 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of ANDX at a price to the public of $1,000 per unit. Upon completion of the Merger, the ANDX preferred units converted to preferred units of MPLX representing substantially equivalent limited partnership interests in MPLX. The Series B preferred units are pari passu with the Series A preferred units with respect to distribution rights and rights upon liquidation. Distributions on the Series B preferred units are payable semi-annually in arrears on the 15th day, or the first business day thereafter, of February and August of each year up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent. The changes in the Series B preferred unit balance from the Merger through December 31, 2020 are summarized below and are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Equity of Predecessor” for the period prior to the Merger and within “Series B preferred units” for the period following the Merger. The Series B preferred units are recorded at fair value as of July 30, 2019. (In millions) 2020 2019 Balance at beginning of period $ 611 $ — Balance acquired at Merger 615 Net income allocated 41 17 Distributions received by Series B preferred unitholders (41) (21) Balance at end of period $ 611 $ 611 Issuance of Additional Securities – The Partnership Agreement authorizes MPLX to issue an unlimited number of additional securities for the consideration and on the terms and conditions determined by the general partner without the approval of the unitholders. Net Income Allocation – In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders first and subsequently allocated to the limited partner unitholders in accordance with their respective ownership percentages. Cash Distributions – The Partnership Agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common unitholders and preferred unitholders will receive. On January 28, 2021, MPLX declared a quarterly cash distribution, based on the results of the fourth quarter of 2020, totaling $714 million, or $0.6875 per common unit. This rate was also received by Series A preferred unitholders. These distributions were paid on February 12, 2021 to unitholders of record on February 8, 2021. Distributions for the fourth quarter of 2019 were $0.6875 per common unit while distributions for the twelve months ended December 31, 2020 and 2019 were $2.7500 and $2.6900 per common unit, respectively. Additionally, in accordance with the distribution rights discussed above, MPLX made a cash distribution to holders of the Series B preferred unitholders on February 16, 2021 for approximately $21 million. The allocation of total quarterly cash distributions to general, limited, and preferred unitholders is as follows for the years ended December 31, 2020, 2019 and 2018. 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. (In millions) 2020 2019 2018 Limited partners' distributions: Common unitholders, includes common units of general partner $ 2,872 $ 2,635 $ 1,985 Series A preferred unit distributions 81 81 75 Series B preferred unit distribution 41 42 — Total cash distributions declared $ 2,994 $ 2,758 $ 2,060

Redeemable Preferred Units

Redeemable Preferred Units	12 Months Ended
	Dec. 31, 2020
Redeemable Preferred Units Disclosure [Abstract]
Redeemable Preferred Units	Series A Preferred Units Private Placement of Preferred Units – On May 13, 2016, MPLX completed the private placement of approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The aggregate net proceeds of approximately $984 million from the sale of the Series A preferred units were used for capital expenditures, repayment of debt and general business purposes. Preferred Unit Distribution Rights - 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 received cumulative quarterly distributions equal to $0.528125 per unit for each quarter prior to the second quarter of 2018. Beginning with the second quarter of 2018, 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 January 28, 2021, MPLX declared a quarterly cash distribution of $0.6875 per common unit for the fourth quarter of 2020. Holders of the Series A preferred units will receive the common unit rate in lieu of the lower $0.528125 base amount. The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then applicable change of control conversion rate. On September 20, 2019, certain holders exercised their right to convert a total of 1.2 million Series A preferred units into common units. As a result of the transaction, approximately 29.6 million Series A preferred units remain outstanding as of December 31, 2020 and 2019. The changes in the redeemable preferred balance for 2020 and 2019 are summarized below: (In millions) 2020 2019 Balance at beginning of period $ 968 $ 1,004 Net income allocated 81 81 Distributions received by preferred unitholders (81) (81) Conversion of preferred units to common units — (36) Balance at end of period $ 968 $ 968

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Segment Information	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; 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 interests; 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 for the years ended December 31, 2020, 2019 and 2018 as well as total assets for our reportable segments as of December 31, 2020 and 2019: (In millions) 2020 2019 2018 L&S Service revenue $ 3,889 $ 3,765 $ 2,575 Rental income 985 1,235 856 Product related revenue 51 91 23 Income from equity method investments 154 200 171 Other income 206 61 47 Total segment revenues and other income (1) 5,285 5,352 3,672 Segment Adjusted EBITDA (2) 3,488 2,748 2,057 Restructuring expenses 29 — — Capital expenditures (3) 498 1,060 708 Investments in unconsolidated affiliates 141 289 3 G&P Service revenue 2,088 2,188 1,685 Rental income 365 349 342 Product related revenue 868 997 1,171 (Loss)/income from equity method investments (1,090) 90 76 Other income 53 65 59 Total segment revenues and other income (1) 2,284 3,689 3,333 Segment Adjusted EBITDA (2) 1,723 1,586 1,418 Restructuring expenses 8 — — Capital expenditures (3) 441 1,203 1,545 Investments in unconsolidated affiliates $ 125 $ 424 $ 338 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $567 million, $660 million and $371 million for 2020, 2019 and 2018, respectively. Third party revenues for the G&P segment were $2,088 million, $3,474 million and $3,198 million for 2020, 2019 and 2018, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (3) Capital expenditures do not include adjustments for asset retirement expenditures. December 31, (In millions) 2020 2019 Segment Assets Cash and cash equivalents $ 15 $ 15 L&S 20,938 20,810 G&P 15,461 19,605 Total assets $ 36,414 $ 40,430 The table below provides a reconciliation between “Net income” and Segment Adjusted EBITDA. (In millions) 2020 2019 2018 Reconciliation to Net (loss)/income: L&S Segment Adjusted EBITDA $ 3,488 $ 2,748 $ 2,057 G&P Segment Adjusted EBITDA 1,723 1,586 1,418 Total reportable segments 5,211 4,334 3,475 Depreciation and amortization (1) (1,377) (1,254) (867) Provision for income taxes (2) — (8) Amortization of deferred financing costs (61) (42) (55) Gain/(loss) on extinguishment of debt 19 — (46) Non-cash equity-based compensation (14) (22) (23) Impairment expense (2,165) (1,197) — Net interest and other financial costs (854) (873) (613) (Loss)/income from equity method investments (936) 290 247 Distributions/adjustments related to equity method investments (499) (562) (458) Unrealized derivative (losses)/gains (2) (3) 1 5 Acquisition costs — (14) (4) Restructuring expenses (37) — — Other (6) (1) — Adjusted EBITDA attributable to noncontrolling interests 37 32 18 Adjusted EBITDA attributable to Predecessor (3) — 770 335 Net (loss)/income $ (687) $ 1,462 $ 2,006 (1) Depreciation and amortization attributable to L&S was $633 million, $503 million and $308 million for the years ended 2020, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $744 million, $751 million and $559 million for 2020, 2019 and 2018, respectively. (2) MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (3) The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.

Major Customers and Concentrati

Major Customers and Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2020
Risks and Uncertainties [Abstract]
Major Customers and Concentration of Credit Risk	Major Customers and Concentration of Credit Risk The table below shows, by segment, the percentage of operating revenues as well as total revenues and other income with MPC which is our most significant customer and our largest concentration of credit risk. 2020 (1) 2019 (1) 2018 (1) Operating revenues (2) L&S 92 % 91 % 94 % G&P 4 % 4 % 3 % Total 56 % 56 % 50 % Total revenues and other income (3) L&S 89 % 88 % 90 % G&P 4 % 4 % 2 % Total 55 % 53 % 48 % (1) The percent calculations exclude revenues attributable to volumes shipped by MPC under joint tariffs with third parties, which are treated as third-party revenue for accounting purposes. (2) Operating revenues consist of service revenue, service revenue - product related, rental income and product sales. (3) The percent calculations exclude losses attributable to the impairment of equity method investments.

Inventories

Inventories	12 Months Ended
	Dec. 31, 2020
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: December 31, (In millions) 2020 2019 NGLs $ 5 $ 5 Line fill 13 10 Spare parts, materials and supplies 100 95 Total inventories $ 118 $ 110

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: Estimated December 31, (In millions) 2020 2019 L&S Pipelines 2-51 years $ 6,026 $ 5,572 Refining logistics 13-40 years 2,333 2,870 Terminals 4-40 years 1,643 1,109 Marine 15-20 years 965 906 Land, building and other 1-61 years 1,584 1,817 Construction-in progress 262 660 Total L&S property, plant and equipment 12,813 12,934 G&P Gathering and transportation 5-40 years 7,547 7,159 Processing and fractionation 10-40 years 5,721 5,545 Land, building and other 3-40 years 507 484 Construction-in-progress 287 745 Total G&P property, plant and equipment 14,062 13,933 Total property, plant and equipment 26,875 26,867 Less accumulated depreciation (1) 5,657 4,722 Property, plant and equipment, net $ 21,218 $ 22,145 (1) The December 31, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below. Long-lived assets used in operations are assessed for impairment whenever changes in facts and circumstances indicate that the carrying value of the assets may not be recoverable based on the expected undiscounted future cash flow of an asset group. For purposes of impairment evaluation, long-lived assets must be grouped at the lowest level for which independent cash flows can be identified, which is at least at the segment level and in some cases for similar assets in the same geographic region where cash flows can be separately identified. If the sum of the undiscounted cash flows is less than the carrying value of an asset group, fair value is calculated, and the carrying value is written down if greater than the calculated fair value. During the first quarter of 2020, we identified an impairment trigger relating to asset groups within our Western G&P reporting unit as a result of significant impacts to forecasted cash flows for these asset

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values – Recurring Fair value measurements and disclosures relate primarily to MPLX’s derivative positions as discussed in Note 16. The following table presents the financial instruments carried at fair value on a recurring basis as of December 31, 2020 and 2019 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. December 31, 2020 2019 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (63) $ — $ (60) Total carrying value on Consolidated Balance Sheets $ — $ (63) $ — $ (60) Level 3 instruments include an embedded derivative in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.47 to $1.09 per gallon with a weighted average of $0.59 per gallon and (2) the probability of renewal of 100 percent for the first and second 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. 2020 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (60) $ — $ (61) Total losses (realized and unrealized) included in earnings (1) — (9) — (5) Settlements — 6 — 6 Fair value at end of period — (63) — (60) The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (4) $ — $ (5) (1) Gains and losses on commodity derivatives 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 16). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. December 31, 2020 2019 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 22,951 $ 20,244 $ 21,054 $ 19,800

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments As of December 31, 2020, MPLX had no outstanding commodity contracts. Embedded Derivative - MPLX has a natural gas purchase commitment embedded in a keep-whole processing agreement with a producer customer in the Southern Appalachia region expiring in December 2022. The customer has the unilateral option to extend the agreement for two consecutive five Income. As of December 31, 2020 and 2019, the estimated fair value of this contract was a liability of $63 million and $60 million, respectively. Certain derivative positions are subject to master netting agreements; therefore, MPLX has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of December 31, 2020 and 2019, there were no derivative assets or liabilities that were offset on the Consolidated Balance Sheets. The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: December 31, (In millions) 2020 2019 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets /Other current liabilities $ — $ (7) $ — $ (5) Other noncurrent assets /Deferred credits and other liabilities — (56) — (55) Total $ — $ (63) $ — $ (60) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, including the effect of master netting arrangements or collateral, see Note 15. See Note 2 for a discussion of derivatives MPLX may use and the reasons for them. 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: (In millions) 2020 2019 2018 Product sales Realized gains $ — $ — $ 4 Unrealized gains — — 2 Total derivative gains related to product sales — — 6 Purchased product costs Realized losses (6) (6) (12) Unrealized (losses)/gains (3) 1 3 Total derivative loss related to purchased product costs (9) (5) (9) Total derivative losses $ (9) $ (5) $ (3)

Debt

Debt	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings at December 31, 2020 and 2019 consisted of the following: December 31, (In millions) 2020 2019 MPLX LP: Bank revolving credit facility due July 30, 2024 $ 175 $ — Term loan facility due September 26, 2021 — 1,000 Floating rate senior notes due September 9, 2021 — 1,000 Floating rate senior notes due September 9, 2022 1,000 1,000 6.250% senior notes due October 15, 2022 — 266 3.500% senior notes due December 1, 2022 486 486 3.375% senior notes due March 15, 2023 500 500 4.500% senior notes due July 15, 2023 989 989 6.375% senior notes due May 1, 2024 — 381 4.875% senior notes due December 1, 2024 1,149 1,149 5.250% senior notes due January 15, 2025 708 708 4.000% senior notes due February 15, 2025 500 500 4.875% senior notes due June 1, 2025 1,189 1,189 1.750% senior notes due March 1, 2026 1,500 — 4.125% senior notes due March 1, 2027 1,250 1,250 4.250% senior notes due December 1, 2027 732 732 4.000% senior notes due March 15, 2028 1,250 1,250 4.800% senior notes due February 15, 2029 750 750 2.650% senior notes due August 15, 2030 1,500 — 4.500% senior notes due April 15, 2038 1,750 1,750 5.200% senior notes due March 1, 2047 1,000 1,000 5.200% senior notes due December 1, 2047 487 487 4.700% senior notes due April 15, 2048 1,500 1,500 5.500% senior notes due February 15, 2049 1,500 1,500 4.900% senior notes due April 15, 2058 500 500 Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 23 ANDX - 3.500% - 5.250% senior notes, due 2022-2047 87 190 Financing lease obligations (1) 11 19 Total 20,536 20,119 Unamortized debt issuance costs (116) (106) Unamortized discount/premium (281) (300) Amounts due within one year (764) (9) Total long-term debt due after one year $ 19,375 $ 19,704 (1) See Note 22 for lease information. The following table shows five years of scheduled debt payments, including payments on finance lease obligations: (In millions) 2021 $ 753 2022 1,502 2023 1,502 2024 1,326 2025 $ 1,701 Credit Agreements MPLX Credit Agreement Effective July 30, 2019, in connection with the closing of the Merger, MPLX amended and restated its existing revolving credit facility (the “MPLX Credit Agreement”) to, among other things, increase borrowing capacity to up to $3.5 billion, extend its term from July 2022 to July 2024, increase the letter of credit issuing capacity to $300 million and increase the swingline capacity to $150 million. The financial covenants and the interest rate terms contained in the new credit agreement are substantially the same as those contained in the previous bank revolving credit facility. The borrowing capacity under the MPLX Credit Agreement may be increased by up to an additional $1 billion, subject to certain conditions, including the consent of lenders whose commitments would increase. In addition, the maturity date may be extended, for up to two additional one year periods, subject to, among other conditions, the approval of lenders holding the majority of the commitments then outstanding, provided that the commitments of any non-consenting lenders will terminate on the then-effective maturity date. Borrowings under the MPLX Credit Agreement bear interest at either the Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin. MPLX is charged various fees and expenses in connection with the agreement, including administrative agent fees, commitment fees on the unused portion of the facility and fees with respect to issued and outstanding letters of credit. The applicable margins to the benchmark interest rates and certain fees fluctuate based on the credit ratings in effect from time to time on MPLX’s long-term debt. The MPLX Credit Agreement contains certain representations and warranties, affirmative and restrictive covenants and events of default that MPLX considers to be usual and customary for an agreement of this type, including a financial covenant that requires MPLX to maintain a ratio of Consolidated Total Debt as of the end of each fiscal quarter to Consolidated EBITDA (both as defined in the MPLX Credit Agreement) for the prior four fiscal quarters of no greater than 5.0 to 1.0 (or 5.5 to 1.0 for up to two fiscal quarters following certain acquisitions). Consolidated EBITDA is subject to adjustments for certain acquisitions and dispositions completed and capital projects undertaken during the relevant period. Other covenants restrict MPLX and/or certain of its subsidiaries from incurring debt, creating liens on our assets and entering into transactions with affiliates. As of December 31, 2020, MPLX was in compliance with the covenants contained in the MPLX Credit Agreement. During the year ended December 31, 2020, MPLX borrowed $3,815 million under the MPLX Credit Agreement, at a weighted average interest rate of 1.490 percent, and repaid $3,640 million of these borrowings. At December 31, 2020, MPLX had $175 million outstanding borrowings under the new facility and less than $1 million in letters of credit outstanding under this facility, resulting in total availability of $3,325 million, or approximately 95.0 percent of the borrowing capacity . During the year ended December 31, 2019, MPLX borrowed $5,310 million under the MPLX Credit Agreement, at a weighted average interest rate of 3.547 percent, and repaid $5,310 million of these borrowings. At December 31, 2019, MPLX had no outstanding borrowings and less than $1 million in letters of credit outstanding under this facility, resulting in total availability of $3.5 billion, or almost 100 percent of the borrowing capacity. ANDX Credit Facilities Prior to the Merger, ANDX had revolving credit facilities (the “ANDX credit facilities”) totaling $2.1 billion in borrowing capacity, which were set to mature January 29, 2021. The ANDX credit facilities were terminated upon closing of the Merger and repaid with borrowings under the MPLX revolving credit facility. During the year ended December 31, 2019, there were borrowings of $864 million under the ANDX credit facilities, at an average interest rate of 4.129 percent, and repayments of $2.1 billion. Term Loan Agreement On September 26, 2019, MPLX entered into a Term Loan Agreement, which provides for a committed term loan facility for up to an aggregate of $1.0 billion. Borrowings under the Term Loan Agreement bear interest, at MPLX’s election, at either (i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement). The proceeds from borrowings under the Term Loan Agreement are to be used to fund the repayment of MPLX’s existing indebtedness and/or for general business purposes. On August 18, 2020 MPLX fully repaid the $1.0 billion of outstanding borrowings on the Term Loan Agreement, which resulted in the recognition of $1 million of unamortized issuance costs, which is included on the Consolidated Statements of Income as “Other financial costs.” Floating Rate Senior Notes On September 9, 2019, MPLX issued $2.0 billion aggregate principal amount of floating rate senior notes in a public offering, consisting of $1.0 billion aggregate principal amount of notes due September 2021 and $1.0 billion aggregate principal amount of notes due September 2022 (collectively, the “Floating Rate Senior Notes”). The Floating Rate Senior Notes were offered at a price to the public of 100 percent of par. The Floating Rate Senior Notes are callable, in whole or in part, at par plus accrued and unpaid interest at any time on or after September 10, 2020. The net proceeds were used to repay MPLX’s existing indebtedness and/or for general business purposes. Interest on the Floating Rate Senior Notes is payable quarterly in March, June, September and December, commencing on December 9, 2019. The interest rate applicable to the floating rate senior notes due September 2021 is LIBOR plus 0.9 percent per annum. The interest rate applicable to the floating rate senior notes due September 2022 is LIBOR plus 1.1 percent per annum. On September 14, 2020, MPLX redeemed, at par value, all of the $1.0 billion aggregate principal amount of notes due September 2021, which resulted in the recognition of $3 million of unamortized issuance costs, which is included on the Consolidated Statements of Income as “Other financial costs.” Fixed Rate Senior Notes Interest on each series of MPLX LP, MarkWest and ANDX senior notes is payable semi-annually in arrears, according to the table below. Senior Notes Interest payable semi-annually in arrears 3.500% senior notes due December 1, 2022 June 1 st and December 1 st 3.375% senior notes due March 15, 2023 March 15 th and September 15 th 4.500% senior notes due July 15, 2023 January 15 th and July 15 th 4.875% senior notes due December 1, 2024 June 1 st and December 1 st 5.250% senior notes due January 15, 2025 January 15 th and July 15 th 4.000% senior notes due February 15, 2025 February 15 th and August 15 th 4.875% senior notes due June 1, 2025 June 1 st and December 1 st 1.750% senior notes due March 1, 2026 March 1 st and September 1 st 4.125% senior notes due March 1, 2027 March 1 st and September 1 st 4.250% senior notes due December 1, 2027 June 1 st and December 1 st 4.000% senior notes due March 15, 2028 March 15 th and September 15 th 4.800% senior notes due February 15, 2029 February 15 th and August 15 th 2.650% senior notes due August 15, 2030 February 15 th and August 15 th 4.500% senior notes due April 15, 2038 April 15 th and October 15 th 5.200% senior notes due March 1, 2047 March 1 st and September 1 st 5.200% senior notes due December 1, 2047 June 1 st and December 1 st 4.700% senior notes due April 15, 2048 April 15 th and October 15 th 5.500% senior notes due February 15, 2049 February 15 th and August 15 th 4.900% senior notes due April 15, 2058 April 15 th and October 15 th On December 29, 2020, MPLX announced the redemption of all the $750 million outstanding aggregate principal amount of 5.250 percent senior notes due January 15, 2025, including approximately $42 million aggregate principal amount of senior notes issued by Andeavor Logistics LP. These amounts are included on the Consolidated Balance Sheet as “Long-term debt due within one year”. The notes were redeemed on January 15, 2021 at a price equal to 102.625 percent of the principal amount. On August 18, 2020, MPLX issued $3.0 billion aggregate principal amount of senior notes in a public offering, consisting of $1.5 billion aggregate principal amount of 1.750 percent senior notes due March 2026 and $1.5 billion aggregate principal amount of 2.650 percent senior notes due August 2030 (collectively, the “August 2020 New Senior Notes”). The August 2020 New Senior Notes were offered at a price to the public of 99.785 percent and 99.913 percent of par, respectively. Interest on each series of notes in the August 2020 New Senior Notes is payable semi-annually in arrears, commencing on March 1, 2021 for the senior notes due March 2026 and commencing on February 15, 2021 for the senior notes due August 2030. The net proceeds were used to repay the $1.0 billion of outstanding borrowings under the MPLX Term Loan Agreement; to repay the $1.0 billion aggregate principal amount of floating rate notes due September 2021; to redeem all of the $450 million aggregate principal amount of 6.375 percent senior notes due May 2024, $69 million of which was issued by ANDX; and to redeem all of the $300 million aggregate principal amount of 6.250 percent senior notes due October 2022, of which $34 million was issued by ANDX. Proceeds were also used to reduce amounts outstanding under the MPLX Credit Agreement at the time. The 6.375 percent senior notes due May 2024 were redeemed at 103.2 percent of the aggregate principal amount, which resulted in a payment of $14 million related to the note premium offset by the immediate recognition of $18 million of unamortized debt premium/discount and issuance costs, both of which are included on the Consolidated Statements of Income as “Other financial costs.” The 6.250 percent senior notes due October 2022 were redeemed at par, and resulted in the immediate recognition of $4 million of unamortized debt premium and issuance costs, which is included on the Consolidated Statements of Income as “Other financial costs.” In connection with the Merger, MPLX assumed ANDX’s outstanding senior notes, which had an aggregate principal amount of $3.75 billion, interest rates ranging from 3.5 percent to 6.375 percent and maturity dates ranging from 2019 to 2047. On September 23, 2019, $3.06 billion aggregate principal amount of ANDX’s outstanding senior notes were exchanged for an aggregate principal amount of $3.06 billion unsecured senior notes (the “Exchange Notes”) issued by MPLX in an exchange offer and consent solicitation undertaken by MPLX, leaving $690 million aggregate principal of outstanding senior notes issued by ANDX. Of this, $500 million aggregate principal amount was related to ANDX 5.5 percent senior notes due 2019. The aggregate principal amount of $500 million and accrued interest of $13.75 million was paid on October 15, 2019 using net proceeds from the Floating Rate Senior Notes and borrowings under the Term Loan Agreement discussed above and includes interest through the payoff date. The Exchange Notes consist of $266 million in aggregate principal amount of 6.25 percent senior notes due October 2022, $486 million in aggregate principal amount of 3.5 percent senior notes due December 2022, $381 million in aggregate principal amount of 6.375 percent senior notes due May 2024, $708 million in aggregate principal amount of 5.25 percent senior notes due January 2025, $732 million in aggregate principal amount of 4.25 percent senior notes due December 2027 and $487 million in aggregate principal amount of 5.2 percent senior notes due December 2047. Interest on each series of Exchange Notes is payable semi-annually in arrears according to the table above.

Revenue Revenue

Revenue Revenue	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Disaggregation of Revenue The following table represents a disaggregation of revenue for each reportable segment for the years ended December 31, 2020 and 2019: 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 333 $ 2,064 $ 2,397 Service revenue - related parties 3,556 24 3,580 Service revenue - product related — 155 155 Product sales (1) 39 597 636 Product sales - related parties 12 116 128 Total revenues from contracts with customers $ 3,940 $ 2,956 6,896 Non-ASC 606 revenue (2) 673 Total revenues and other income $ 7,569 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 346 $ 2,152 $ 2,498 Service revenue - related parties 3,419 36 3,455 Service revenue - product related — 140 140 Product sales (1) 65 741 806 Product sales - related parties 26 116 142 Total revenues from contracts with customers $ 3,856 $ 3,185 7,041 Non-ASC 606 revenue (2) 2,000 Total revenues and other income $ 9,041 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 174 $ 1,682 $ 1,856 Service revenue - related parties 2,401 3 2,404 Service revenue - product related — 220 220 Product sales (1) 12 870 882 Product sales - related parties 11 76 87 Total revenues from contracts with customers $ 2,598 $ 2,851 5,449 Non-ASC 606 revenue (2) 1,556 Total revenues and other income $ 7,005 (1) G&P “Product sales” for the year ended December 31, 2018 was adjusted in the table above by $5 million related to derivative gains and mark-to-market adjustments. There were no adjustments for the years ended December 31, 2020 and 2019. (2) Non-ASC 606 Revenue includes rental income, income 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 “Receivables, 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 table below reflects the changes in our contract balances for the years ended December 31, 2020 and 2019: (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2020 Contract assets $ 39 $ 3 $ (2) $ 40 Long-term contract asset — 2 — 2 Deferred revenue 23 22 (8) 37 Deferred revenue - related parties 53 121 (83) 91 Long-term deferred revenue 90 29 — 119 Long-term deferred revenue - related parties 55 (7) — 48 Long-term contract liability $ — $ 6 $ — $ 6 (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2019 Contract assets $ 36 $ 5 $ (2) $ 39 Deferred revenue 13 17 (7) 23 Deferred revenue - related parties 65 55 (67) 53 Long-term deferred revenue 56 34 — 90 Long-term deferred revenue - related parties $ 52 $ 3 $ — $ 55 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations for the years ended December 31, 2020 and 2019. 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 December 31, 2020, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $293 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 $ 1,891 2022 1,822 2023 1,666 2024 1,540 2025 and thereafter 4,555 Total revenue on remaining performance obligations (1)(2)(3) $ 11,474 (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	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Information [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information (In millions) 2020 2019 2018 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 821 $ 835 $ 568 Income taxes paid 2 1 1 Cash paid for amounts included in the measurement of lease liabilities Payments on operating leases 87 85 — Interest payment under finance lease obligations 1 1 — Net cash provided by financing activities included Principal payments under finance lease obligations 9 5 — Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories — 2 2 MPLX terminal lease classification change — 21 — ROU assets obtained in exchange for new operating lease obligations 17 26 — ROU assets obtained in exchange for new finance lease obligations 1 4 — Fair value of common units redeemed for Wholesale Exchange 340 — — Contribution - common units issued (1) $ — $ 7,722 $ 4,236 (1) For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. For 2019, includes limited partner units issued to MPC and public unitholders as consideration in the Merger. See Note 4. 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: (In millions) 2020 2019 2018 (Decrease)/increase in capital accruals $ (244) $ (146) $ 135

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP and Explorer to their employees. MPLX 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, 2018 through December 31, 2020: (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2018 (1) $ (14) $ (2) $ (16) Other comprehensive income - remeasurements (2) — 1 1 Balance as of December 31, 2019 (1) (14) (1) (15) Other comprehensive income/(loss) - remeasurements (2) 1 (1) — Balance as of December 31, 2020 (1) $ (13) $ (2) $ (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.”

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Equity-Based Compensation	Equity-Based Compensation Description of the Plan Effective March 15, 2018, the MPLX LP 2012 Incentive Compensation Plan (“MPLX 2012 Plan”) was replaced by the MPLX LP 2018 Incentive Compensation Plan (“MPLX 2018 Plan”). The MPLX 2018 Plan will continue in effect until February 28, 2028, unless terminated earlier. Subject to customary anti-dilution adjustments, the MPLX 2018 Plan allows for no more than 16 million common units representing limited partnership interests in MPLX to be delivered under the plan. The MPLX LP 2012 Plan allowed for no more than 2.75 million MPLX LP common limited partner units to be delivered. Consistent with the MPLX 2012 Plan, the MPLX 2018 Plan authorizes the MPLX GP board of directors (the “Board”) to grant unit options, unit appreciation rights, restricted units and phantom units, distribution equivalent rights, unit awards, profits interest units, performance units and other unit-based awards to the employees, officers and directors of the General Partner, MPLX, or any of their affiliates, including MPC. Common units delivered pursuant to an award granted under the MPLX 2018 Plan may be newly issued common units or acquired in the open market or from any other person, including an affiliate of MPLX, as determined by the Board. Unit-based Awards under the Plan MPLX expenses all unit-based payments to employees and non-employee directors based on the grant date fair value of the awards over the requisite service period, adjusted for estimated forfeitures. Phantom Units – MPLX has granted phantom units under the MPLX 2018 Plan and the MPLX 2012 Plan to non-employee directors of MPLX LP’s general partner and of MPC. Awards to non-employee directors are accounted for as non-employee awards. Phantom units granted to non-employee directors vest immediately at the time of the grant, as they are non-forfeitable, but are not issued until the director’s departure from the board of directors. Prior to issuance, non-employee directors do not have the right to vote such units and cash distribution equivalents accrue in the form of additional phantom units and will be issued when the director departs from the board of directors. MPLX has granted phantom units under the MPLX 2018 Plan and the MPLX 2012 Plan to certain officers and non-officers of MPLX, MPLX’s general partner and MPC who make significant contributions to our business. These grants are accounted for as employee awards. In general, these phantom units will vest over a requisite service period of up to three years. Prior to vesting, these phantom unit recipients will not have the right to vote such units and cash distributions declared will be accrued and paid upon vesting. The accrued distributions at December 31, 2020 and 2019 were $3 million and $6 million, respectively. The fair values of phantom units are based on the fair value of MPLX common units on the grant date. Performance Units – MPLX has granted performance units under the MPLX 2018 Plan and the MPLX 2012 Plan to certain officers of the general partner and certain eligible MPC officers who make significant contributions to our business. Performance units are designed to pay out 75 percent in cash and 25 percent in MPLX common units. The performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards. The performance units granted in 2020 are hybrid awards having a three-year performance period of January 1, 2020 through December 31, 2022. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s distributable cash flow, and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.80 per unit for the 2020 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. The performance units granted in 2019 are hybrid awards having a three-year performance period of January 1, 2019 through December 31, 2021. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX’s DCF during the performance period and a market condition based on MPLX’s total unitholder return over the performance period. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.68 per unit for the 2019 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. During the first quarter of 2018, a performance award was granted; however, due to the nature of the award terms, the grant date for this award was not established until the first quarter of 2020 and we began recognizing units and expense related to this award at that time. The performance units granted in 2018 are hybrid awards having a three-year performance period of January 1, 2018 through December 31, 2020. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on an average of MPLX LP’s distributable cash flow and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $0.45 per unit for the 2018 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. Outstanding Phantom Unit Awards The following is a summary of phantom unit award activity of MPLX common units in 2020: Phantom Units Number Weighted Aggregate Intrinsic Value (In millions) Outstanding at December 31, 2019 1,109,568 $ 35.97 Granted 238,238 19.08 Settled (686,382) 37.04 Forfeited (17,401) 33.65 Outstanding at December 31, 2020 644,023 28.65 Vested and expected to vest at December 31, 2020 640,885 28.69 $ 14 Non-forfeitable at December 31, 2020 (1) 362,682 $ 29.66 $ 8 (1) Represents a subset of phantom units held by our non-employee directors and certain of our officers and non-officer employees that are generally non-forfeitable and that would be paid out as common units upon the holder’s separation from service, or upon a predetermined date. The following is a summary of the values related to phantom units: Phantom Units Intrinsic Value of Units Issued During the Period (in millions) Weighted Average Grant Date Fair Value of Units Granted During the Period 2020 $ 12 $ 19.08 2019 14 32.62 2018 $ 18 $ 33.84 As of December 31, 2020, unrecognized compensation cost related to phantom unit awards was $2 million, which is expected to be recognized over a weighted average period of 1.8 years. Outstanding Performance Unit Awards The following table presents a summary of the 2020 activity for performance unit awards to be settled in MPLX common units: Performance Units Number of Units Weighted Outstanding at December 31, 2019 2,157,347 $ 0.84 Granted 2,147,211 0.86 Settled (1,169,354) 0.90 Forfeited (42,918) 0.84 Outstanding at December 31, 2020 3,092,286 $ 0.83 The number of common units that would be issued upon target vesting, using the closing price of our common units on December 31, 2020 would be 142,831 common units. As of December 31, 2020, unrecognized compensation cost related to equity-classified performance unit awards was $1 million which is expected to be recognized over a weighted average period of 2.0 years. Performance units paying out in MPLX common units have a grant date fair value calculated using a Monte Carlo valuation model, which requires the input of subjective assumptions. The following table provides a summary of the weighted average inputs used for these assumptions: 2020 2019 2018 Risk-free interest rate 1.29% 2.51% N/A Look-back period 1.62 years 2.84 years N/A Expected volatility 22.32% 25.01% N/A Grant date fair value of performance units granted (1) $0.86 $0.76 N/A (1) Performance units granted includes units subject to both the market condition and performance condition. The assumption for expected volatility of our unit price reflects the historical volatility of MPLX common units. The look-back period reflects the remaining performance period at the grant date. The risk-free interest rate for the remaining performance period as of the grant date is based on the U.S. Treasury yield curve in effect at the time of the grant. Total Unit-Based Compensation Expense Total unit-based compensation expense for awards settling in MPLX common units was $14 million in 2020, $22 million in 2019 and $24 million in 2018. MPC’s Stock-based Compensation Stock-based compensation expenses charged to MPLX under our employee services agreement with MPC were $13 million, $10 million and $8 million for 2020, 2019 and 2018, respectively.

Leases Leases (Notes)

Leases Leases (Notes)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lessor, Operating Leases [Text Block]	Lessor Based on the terms of fee-based transportation and storage services agreements with MPC and third parties, MPLX is considered to be the lessor under several operating lease arrangements in accordance with GAAP. These agreements have remaining terms ranging from less than one year to 12 years with renewal options ranging from one year to 10 years, with some agreements having multiple renewal options. We are also considered to be the lessor under operating lease agreements related to certain fee-based natural gas gathering, transportation and processing agreements. MPLX’s primary natural gas lease operations relate to a natural gas gathering agreement in the Marcellus Shale for which it earns a fixed-fee for providing gathering services to a single producer using a dedicated gathering system. As the gathering system is expanded, the fixed-fee charged to the producer is adjusted to include the additional gathering assets in the lease. The primary term of the natural gas gathering arrangement expires in 2038 and will continue thereafter on a year-to-year basis until terminated by either party. Other significant natural gas implicit leases relate to a natural gas processing agreement in the Marcellus Shale and a natural gas processing agreement in the Southern Appalachia region for which MPLX earns minimum monthly fees for providing processing services to a single producer using a dedicated processing plant. The primary term of these natural gas processing agreements expires during 2027 and 2023, respectively, these contracts will continue thereafter on a year-to-year basis until terminated by either party. MPLX did not elect to use the practical expedient to combine lease and non-lease components for lessor arrangements. The tables below represent the portion of the contract allocated to the lease component based on relative standalone selling price. Lessor agreements are currently deemed operating, as we elected the practical expedient to carry forward historical classification conclusions. If and when a modification of an existing agreement occurs and the agreement is required to be assessed under ASC 842, MPLX assesses the amended agreement and makes a determination as to whether a reclassification of the lease is required. During the year ended December 31, 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 6 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. During the year ended December 31, 2019, there was a modification to MPLX terminal agreements with MPC. Based on the modification, certain terminals within the MPLX terminal agreement were reclassified from operating leases to sales-type leases. As a result, the underlying assets previously shown on the Consolidated Balance Sheets associated with the sales-type leases 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. When determining the net investment in the lease, certain variable payments were excluded from the total contract consideration, primarily related to fees for which there are no minimum volume commitments. The difference between the net book value of the underlying assets and the net investment in the lease has been recorded through equity given that the dropdown of MPLXT was a common control transaction. During the year, MPLX derecognized approximately $29 million of property, plant and equipment, derecognized approximately $3 million of existing deferred rent receivable, recorded a lease receivable of approximately $47 million, recorded an unguaranteed residual asset of approximately $6 million and equity of $21 million. Under ASC 840, MPLX’s revenue from its implicit lease arrangements, excluding executory costs, totaled approximately $1,032 million in 2018. Lease revenues included on the Consolidated Statements of Income during 2020 and 2019 were as follows: 2020 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 787 $ 268 $ 1,020 $ 257 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type rental revenue- fixed minimum) 151 — 6 — Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 $ — (1) These amounts are presented net of executory costs. The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2020: (In millions) Related Party Third Party Total 2021 $ 919 $ 186 $ 1,105 2022 913 181 1,094 2023 869 178 1,047 2024 815 174 989 2025 791 142 933 2026 and thereafter 1,300 999 2,299 Total minimum future rentals $ 5,607 $ 1,860 $ 7,467 The following is a schedule of minimum future revenue on sales-type leases as of December 31, 2020: (In millions) Related Party 2021 $ 157 2022 157 2023 158 2024 158 2025 158 2026 and thereafter 315 Total minimum future rentals 1,103 Less: present value discount 687 Lease receivable $ 416 The following schedule summarizes MPLX’s investment in assets held for operating lease by major classes as of December 31, 2020 and 2019: December 31, (In millions) 2020 2019 Pipelines $ 834 $ 745 Refining logistics 1,680 2,320 Terminals 1,276 943 Marine 129 906 Gathering and transportation 990 980 Processing and fractionation 867 855 Land, building and other 171 198 Total property, plant and equipment 5,947 6,947 Less: accumulated depreciation 2,007 2,355 Property, plant and equipment, net $ 3,940 $ 4,592 See Note 6 for additional information on where related party lease assets are recorded in the Consolidated Balance Sheets. Third-party lease assets are less than $1 million as of December 31, 2020 and are included within the “Receivables, net” and “Other noncurrent assets” captions within the Consolidated Balance Sheets.
Lessee, Finance Leases [Text Block]	Lessee We lease a wide variety of facilities and equipment under leases from third parties, including land and building space, office and field equipment, storage facilities and transportation equipment, while our related party leases primarily relate to ground leases associated with our refining logistics assets. Our remaining lease terms range from less than one one Under ASC 840, operating lease costs were $89 million in 2018. Under ASC 842, the components of lease cost were as follows: 2020 2019 (In millions) Related Party Third Party Related Party Third Party Components of lease costs: Operating lease costs $ 14 $ 78 $ 14 $ 75 Finance lease cost: Amortization of ROU assets — 3 — 5 Interest on lease liabilities — 1 — 1 Total finance lease cost — 4 — 6 Variable lease cost 1 10 1 11 Short-term lease cost — 52 — 80 Total lease cost $ 15 $ 144 $ 15 $ 172 Supplemental balance sheet data related to leases were as follows: December 31, 2020 December 31, 2019 (In millions, except % and years) Related Party Third Party Related Party Third Party Operating leases Assets Right of use assets $ 231 $ 309 $ 232 $ 365 Liabilities Operating lease liabilities 1 63 1 66 Long-term operating lease liabilities 229 244 230 302 Total operating lease liabilities $ 230 $ 307 $ 231 $ 368 Weighted average remaining lease term 46.23 years 8.04 years 47.20 years 8.59 years Weighted average discount rate 5.80 % 4.33 % 5.80 % 4.38 % Finance leases Assets Property, plant and equipment, gross $ 17 $ 46 Less: Accumulated depreciation 8 19 Property, plant and equipment, net 9 27 Liabilities Other current liabilities 2 9 Long-term debt 9 10 Total finance lease liabilities $ 11 $ 19 Weighted average remaining lease term 16.87 years 10.16 years Weighted average discount rate 5.95 % 5.87 % As of December 31, 2020, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2021 $ 14 $ 76 $ 3 2022 14 67 2 2023 14 57 2 2024 14 35 1 2025 14 19 1 2026 and thereafter 591 115 9 Gross lease payments 661 369 18 Less: Imputed interest 431 62 7 Total lease liabilities $ 230 $ 307 $ 11

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	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 December 31, 2020 and 2019, accrued liabilities for remediation totaled $17 million and $19 million, respectively. It is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, that may be imposed. At December 31, 2020 and 2019, there were no balances with MPC for indemnification of environmental costs. MPLX is involved in environmental enforcement matters arising in the ordinary course of business. While the outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on its consolidated results of operations, financial position or cash flows. MPLX is also a party to a number of other lawsuits and other proceedings arising in the ordinary course of business. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these other lawsuits and proceedings will not, individually or collectively, have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, MPLX has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require MPLX to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. MPLX is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. In connection with our approximate 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. 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 one percent redemption premium required pursuant to the indenture governing the notes) and any accrued and unpaid interest. As of December 31, 2020, our maximum potential undiscounted payments under the Contingent Equity Contribution Agreement were approximately $230 million. Other Legal Proceedings – In early July, MPLX received a Notification of Trespass Determination from the Bureau of Indian Affairs (“BIA”) relating to a portion of the Tesoro High Plains Pipeline (“THPP”) that crosses the Fort Berthold Reservation in North Dakota. The notification covered the rights of way for 23 tracts of land and demanded the immediate cessation of pipeline operations. The notification also assessed trespass damages of approximately $187 million. We appealed this determination, which triggered an automatic stay of the requested pipeline shutdown and payment. On October 29, the Assistant Secretary - Indian Affairs issued an order vacating the BIA’s trespass order and requiring the Regional Director for the BIA Great Plains Region to issue a new decision on or before December 15 covering all 34 tracts at issue. On December 15, 2020, the Regional Director of BIA issued a new trespass notice to THPP consistent with the Assistant Secretary of 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, 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. We continue to work towards a settlement of this matter with holders of the property rights at issue. Contractual Commitments and Contingencies – At December 31, 2020, MPLX’s contractual commitments to acquire property, plant and equipment totaled $156 million. These commitments were primarily related to G&P plant expansion, terminal and pipeline projects. In addition, from time to time and in the ordinary course of business, MPLX and its affiliates provide guarantees of MPLX’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require MPLX to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of December 31, 2020, management does not believe there are any indications that MPLX will not be able to meet the construction milestones, that force majeure does not apply or that such fees and charges will otherwise be triggered. Other Contractual Obligations – MPLX executed transportation and terminalling agreements that obligate us to minimum volume, throughput or payment commitments over the original terms of the agreements, which range from four (In millions) 2021 $ 103 2022 117 2023 154 2024 146 2025 124 2026 and thereafter 421 Total $ 1,065

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 12, 2021, MarkWest Energy, a wholly owned subsidiary of MPLX, closed the sale of its Javelina assets and liabilities to a third party. See Note 4 for further discussion related to this transaction. On January 15, 2021, MPLX redeemed all the $750 million outstanding aggregate principal amount of 5.250 percent senior notes due January 15, 2025. See Note 17 for further discussion of this redemption.

Select Quarterly Financial Data

Select Quarterly Financial Data	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Select Quarterly Financial Data	Select Quarterly Financial Data (Unaudited) 2020 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 992 $ 2,081 $ 2,247 $ 2,249 (Loss)/income from operations (2,486) 878 899 920 Net (loss)/income (2,716) 655 674 700 Net (loss)/income attributable to MPLX LP (2,724) 648 665 691 Net (loss)/income attributable to MPLX LP per limited partner unit: Common - basic (2.60) 0.58 0.61 0.63 Common - diluted (2.60) 0.58 0.61 0.63 Cash distributions declared per limited partner common unit 0.6875 0.6875 0.6875 0.6875 Distributions declared: Limited partner units - Public 270 270 270 269 Limited partner units - MPC 458 445 445 445 Series A preferred units 20 21 20 20 Series B preferred units 11 10 10 10 Total distributions declared $ 759 $ 746 $ 745 $ 744 2019 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 2,235 $ 2,210 $ 2,280 $ 2,316 Income/(loss) from operations 912 885 926 (346) Net income/(loss) 689 657 689 (573) Net income/(loss) attributable to MPLX LP 503 482 629 (581) Net income/(loss) attributable to MPLX LP per limited partner unit: Common - basic 0.61 0.56 0.61 (0.58) Common - diluted 0.61 0.55 0.61 (0.58) Cash distributions declared per limited partner common unit 0.6575 0.6675 0.6775 0.6875 Distributions declared: Limited partner units - Public 191 261 266 270 Limited partner units - MPC 332 431 438 446 Series A preferred units 20 21 20 20 Series B preferred units — 21 10 11 Total distributions declared $ 543 $ 734 $ 734 $ 747

Description of the Business a_2

Description of the Business and Basis of Presentation (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of presentation	Basis of Presentation – The accompanying consolidated financial statements of MPLX have been prepared in accordance with GAAP. The 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 a VIE 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 relation to the Merger described above and in Note 4, ANDX’s assets, liabilities and results of operations prior to the Merger are collectively included in what we refer to as the “Predecessor” from October 1, 2018, which was the date that MPC acquired Andeavor. MPLX’s acquisition of ANDX is considered a transfer between entities under common control due to MPC’s relationship with ANDX prior to the Merger. As an entity under common control with MPC, MPLX recorded the assets acquired and liabilities assumed on its consolidated balance sheets at MPC’s historical carrying value. For the acquiring entity, transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. Accordingly, the accompanying financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of ANDX beginning October 1, 2018.

Summary of Principal Accounti_2

Summary of Principal Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Use of estimates	Use of Estimates – The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods. Actual results could differ materially from those estimates. Estimates are subject to uncertainties due to the levels of subjectivity and judgment necessary to account for highly uncertain matters or the susceptibility of such matters to change and affect items such as valuing identified intangible assets; determining the fair value of derivative instruments; evaluating impairments of long-lived assets, goodwill and equity investments; establishing estimated useful lives for long-lived assets; acquisition accounting; recognizing share-based compensation expense; estimating revenues, expense accruals and capital expenditures; valuing AROs; and determining liabilities, if any, for environmental and legal contingencies.
Revenue recognition	Revenue Recognition – Revenue is measured based on consideration specified in a contract with a customer. MPLX recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. MPLX enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” MPLX provides services under the following types of arrangements: • Fee-based arrangements – Under fee-based arrangements, MPLX receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue MPLX earns from these arrangements is generally directly related to the volume of natural gas, NGLs, refined products or crude oil that is handled by or flows through MPLX’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, MPLX’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed. In certain instances when specifically stated in the contract terms, MPLX purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis, as MPLX takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, MPLX: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, MPLX delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes MPLX retains to third parties or related parties. Revenue is recognized on a net basis when MPLX acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, MPLX gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, MPLX must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require MPLX to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” on the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as MPLX is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by MPLX. • Purchase arrangements – Under purchase arrangements, MPLX purchases natural gas at either the wellhead or the tailgate of a plant. MPLX then gathers and delivers the natural gas to pipelines where MPLX may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs.” Often, MPLX earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements is reported as “Product sales” on the Consolidated Statements of Income and is recognized on a gross basis as MPLX purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, MPLX provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, MPLX records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of MPLX’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions, and the services offered and whether they are deemed distinct. MPLX allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. Revenue from MPLX’s service arrangements will generally be recognized over time as the performance obligation is satisfied as services are provided. MPLX has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period. In instances in which tiered pricing structures do not reflect our efforts to perform, MPLX will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue.
Revenue and expense accruals	Revenue and Expense Accruals – MPLX routinely makes accruals based on estimates for both revenues and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling MPLX’s records with those of third parties. The delayed information from third parties includes, among other things, actual volumes purchased, transported or sold, adjustments to inventory and invoices for purchases, actual natural gas and NGL deliveries and other operating expenses. MPLX makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and MPLX’s internal records have been reconciled.
Cash and cash equivalents	Cash and Cash Equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with initial maturities of three months or less.
Restricted cash	Restricted Cash – Restricted cash consists of cash and investments that must be maintained as collateral for letters of credit issued to certain third-party producer customers. The balances will be outstanding until certain capital projects are completed and the third party releases the restriction. Restricted cash also consists of cash advances to be used for the operation and maintenance of an operated pipeline system. Restricted cash is included in “Other current assets” on the Consolidated Balance Sheets.
Receivables	Receivables – Receivables primarily consist of customer accounts receivable, which are recorded at the invoiced amount and generally do not bear interest. Allowances for doubtful accounts are generally recorded when it becomes probable that the receivable will not be collected and are recorded to bad debt expense. We review the allowance quarterly with past-due balances over 90 days and other higher-risk amounts being reviewed individually for collectability. Balances that remain outstanding after reasonable collection efforts have been unsuccessful are written off through a charge to the valuation allowance and a credit to accounts receivable.
Inventories	Inventories – Inventories consist primarily of natural gas, propane, other NGLs and materials and supplies to be used in operations. Natural gas, propane, and other NGLs are valued at the lower of cost or market value. Materials and supplies are stated at the lower of cost or market value. Cost for materials and supplies are determined primarily using the weighted-average cost method.
Imbalances	Imbalances – Within our pipelines and storage assets, we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive imbalances are recorded as other current assets and negative imbalances are recorded as accounts payable. Positive and negative product imbalances are settled in cash, settled by physical delivery of gas from a different source, or tracked and settled in the future.
Property, plant and equipment	Property, Plant and Equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis over the estimated useful lives of the assets. Expenditures that extend the useful lives of assets are capitalized. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment assessment is performed and the excess of the book value over the fair value is recorded as an impairment loss. When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported on the Consolidated Statements of Income. Gains on the disposal of property, plant and equipment are recognized when they occur, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale.
Intangibles	Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined using an income and/or market approach which is compared to the carrying value of the reporting unit. The fair value under the income approach is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future operating performance, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the excess, if any, of the book value over the fair value of the reporting unit up to the amount of goodwill recorded is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. Total goodwill at December 31, 2020 was $7.7 billion and no impairment was recorded as a result of our November 30, 2020 annual goodwill impairment analysis. As a result of MPLX’s interim goodwill impairment analysis at March 31, 2020 and annual goodwill impairment analysis at November 30, 2019, we recorded impairment charges of approximately $1.8 billion and $1.2 billion, respectively. See Note 14 for further details.
Goodwill	Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined using an income and/or market approach which is compared to the carrying value of the reporting unit. The fair value under the income approach is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future operating performance, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the excess, if any, of the book value over the fair value of the reporting unit up to the amount of goodwill recorded is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. Total goodwill at December 31, 2020 was $7.7 billion and no impairment was recorded as a result of our November 30, 2020 annual goodwill impairment analysis. As a result of MPLX’s interim goodwill impairment analysis at March 31, 2020 and annual goodwill impairment analysis at November 30, 2019, we recorded impairment charges of approximately $1.8 billion and $1.2 billion, respectively. See Note 14 for further details.
Other taxes	Other Taxes – Other taxes primarily include real estate taxes.
Environmental costs	Environmental Costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. MPLX recognizes remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure and are discounted when the estimated amounts are reasonably fixed and determinable. If recoveries of remediation costs from third parties are probable, a receivable is recorded and is discounted when the estimated amount is reasonably fixed and determinable.
Asset retirement obligations	Asset Retirement Obligations – An ARO is a legal obligation associated with the retirement of tangible long-lived assets that generally result from the acquisition, construction, development or normal operation of the asset. AROs are recorded at fair value in the period in which they are incurred, if a reasonable estimate of fair value can be made, and added to the carrying amount of the associated asset. This additional carrying amount is then depreciated over the life of the asset. The liability is determined using a credit adjusted risk free interest rate and increases due to the passage of time based on the time value of money until the obligation is settled. MPLX recognizes a liability of a conditional ARO as soon as the fair value of the liability can be reasonably estimated. A conditional ARO is defined as an unconditional legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within the control of the entity. AROs have not been recognized for certain assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates.
Investment in unconsolidated affiliates	Investment in Unconsolidated Affiliates – Equity investments in which MPLX exercises significant influence, but does not control and is not the primary beneficiary, are accounted for using the equity method and are reported in “Equity method investments” on the accompanying Consolidated Balance Sheets. This includes entities in which we hold majority ownership but the minority shareholders have substantive participating rights. Differences in the basis of the investments and the separate net asset values of the investees, if any, are amortized into net income over the remaining useful lives of the underlying assets and liabilities, except for the excess related to goodwill. MPLX believes the equity method is an appropriate means for it to recognize increases or decreases measured by GAAP in the economic resources underlying the investments. Regular evaluation of these investments is appropriate to evaluate any potential need for impairment. MPLX uses evidence of a loss in value to identify if an investment has an other than a temporary decline. During the first quarter of 2020, MPLX recorded an other than temporary impairment for three joint ventures totaling $1,264 million, of which $1,251 million was related to MarkWest Utica EMG and its investment in Ohio Gathering Company, L.L.C. The impairments were recorded through “Income from equity method investments”. The
Deferred financing costs	Deferred Financing Costs – Deferred financing costs are an asset for credit facility costs and netted against debt for senior notes. These costs are amortized over the contractual term of the related obligations using the effective interest method or, in certain circumstances, accelerated if the obligation is refinanced.
Derivative instruments	Derivative Instruments – MPLX may use commodity derivatives to economically hedge a portion of its exposure to commodity price risk. All derivative instruments (including derivatives embedded in other contracts) are recorded at fair value. Certain commodity derivatives are reflected on the consolidated balance sheets on a net basis by counterparty as they are governed by master netting arrangements. MPLX discloses the fair value of all derivative instruments under the captions “Other noncurrent assets,” “Other current liabilities” and “Deferred credits and other liabilities” on the Consolidated Balance Sheets. Changes in the fair value of derivative instruments are reported on the Consolidated Statements of Income in accounts related to the item whose value or cash flows are being managed. All derivative instruments are marked to market through “Product sales,” “Purchased product costs,” or “Cost of revenues” on the Consolidated Statements of Income. Revenue gains and losses relate to contracts utilized to manage the cash flow for the sale of a product, typically NGLs. Purchased product costs gains and losses relate to contracts utilized to manage the cost of natural gas purchases, typically related to keep-whole arrangements. Cost of revenues gains and losses relate to a contract utilized to manage electricity costs. Changes in risk management for unrealized activities are reported as an adjustment to net income in computing cash flow from operating activities on the accompanying Consolidated Statements of Cash Flows. During the years ended December 31, 2020, 2019 and 2018, MPLX did not elect hedge accounting for any derivatives. MPLX has elected the normal purchases and normal sales designation for certain contracts related to the physical purchase of electric power and the sale of some commodities.
Fair value of financial instruments	Fair Value of Financial Instruments – Management believes the carrying amount of financial instruments, including cash and cash equivalents, receivables, receivables from related parties, other current assets, accounts payable, accounts payable to related parties and accrued liabilities approximate fair value because of the short-term maturity of these instruments. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximate fair value due to the variable interest rate that approximates current market rates (see Note 15). Derivative instruments are recorded at fair value, based on available market information (see Note 16).
Fair value measurement	Fair Value Measurement – Financial assets and liabilities recorded at fair value in the Consolidated Balance Sheets are categorized based upon the fair value hierarchy established by GAAP, which classifies the inputs used to measure fair value into Level 1, Level 2 or Level 3. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The methods and assumptions utilized may produce a fair value that may not be realized in future periods upon settlement. Furthermore, while MPLX believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. For further discussion, see Note 15.
Equity-based compensation arrangements	Equity-Based Compensation Arrangements – MPLX issues phantom units under its share-based compensation plan as described further in Note 21. A phantom unit entitles the grantee a right to receive a common unit upon the issuance of the phantom unit. The fair value of phantom unit awards granted to employees and non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. The fair value of the units awarded is amortized into earnings using a straight-line amortization schedule over the period of service corresponding with the vesting period. For phantom units that vest immediately and are not forfeitable, equity-based compensation expense is recognized at the time of grant. Performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards. Equity-classified performance units with a market condition use a Monte Carlo valuation model to calculate a grant date fair value of market conditions. Equity-classified performance units with a performance condition are valued based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. To satisfy common unit awards, MPLX may issue new common units, acquire common units in the open market or use common units already owned by the general partner.
Income taxes	Income Taxes – MPLX is not a taxable entity for United States federal income tax purposes or for the majority of the states that impose an income tax. Taxes on MPLX’s net income generally are borne by its partners through the allocation of taxable income. MPLX’s taxable income or loss, which may vary substantially from the net income or loss reported on the Consolidated Statements of Income, is includable in the federal income tax returns of each partner. MPLX and certain legal entities are, however, taxable entities under certain state jurisdictions. MPLX accounts for income taxes under the asset and liability method. Deferred income taxes are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, capital loss carryforwards and net operating loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect of any tax rate change on deferred taxes is recognized as tax expense/(benefit) from continuing operations in the period that includes the enactment date of the tax rate change. Realizability of deferred tax assets is assessed and, if not more likely than not, a valuation allowance is recorded to reflect the deferred tax assets at net realizable value as determined by management. All deferred tax balances are classified as long-term in the accompanying Consolidated Balance Sheets. All changes in the tax bases of assets and liabilities are allocated among operations and items charged or credited directly to equity.
Distributions	Distributions – 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, as discussed in Notes 8 and 9, and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued as a liability until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described below.
Net income per limited partner unit	Net Income Per Limited Partner Unit – MPLX uses the two-class method when calculating the net income per unit applicable to limited partners, because there is more than one class of participating security. The classes of participating securities include common units, Series A and Series B preferred units and certain equity-based compensation awards. Net income attributable to MPLX LP is allocated to the unitholders differently for preparation of the Consolidated Statements of Equity and the calculation of net income per limited partner unit. 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 and subsequently allocated to remaining unitholders in accordance with their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 7. In preparing net income per limited partner units, during periods in which a net loss attributable to MPLX is reported or periods in which the total distributions exceed the reported net income attributable to MPLX’s unitholders, the amount allocable to certain equity-based compensation awards is based on actual distributions to the equity-based compensation awards. Diluted earnings per unit is calculated by dividing net income attributable to MPLX’s common unitholders, after deducting amounts allocable to other participating securities, by the weighted average number of common units and potential common units outstanding during the period. Potential common units are excluded from the calculation of diluted earnings per unit during periods in which net income attributable to MPLX’s unitholders, after deducting amounts that are allocable to the outstanding equity-based compensation awards and preferred units, is a loss, as the impact would be anti-dilutive.
Business combinations	Business Combinations – MPLX recognizes and measures the assets acquired and liabilities assumed in a business combination based on their estimated fair values at the acquisition date, with any remaining difference recorded as goodwill or gain from a bargain purchase. Depending on the nature of the transaction, management may engage an independent valuation specialist to assist with the determination of fair value of the assets acquired, liabilities assumed, noncontrolling interests, if any, and goodwill, based on recognized business valuation methodologies. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent to the acquisition, and not later than one year from the acquisition date, MPLX will record any material adjustments to the initial estimate based on new information obtained that would have existed as of the acquisition date. An adjustment that arises from information obtained that did not exist as of the date of the acquisition will be recorded in the period of the adjustment. An income, market or cost valuation method may be utilized to estimate the fair value of the assets acquired, liabilities assumed, and noncontrolling interests, if any, in a business combination. The income valuation method represents the present value of future cash flows over the life of the asset using: (i) discrete financial forecasts, which rely on management’s estimates of volumes, certain commodity prices, revenue and operating expenses; (ii) long-term growth rates; and (iii) appropriate discount rates. The market valuation method uses prices paid for a reasonably similar asset by other purchasers in the market, with adjustments relating to any differences between the assets. The cost valuation method is based on the replacement cost of a comparable asset at prices at the time of the acquisition reduced for depreciation of the asset. Acquisition-related costs are expensed as incurred in connection with each business combination. Acquisitions in which the company or business being acquired by MPLX had an existing relationship with MPC may result in the transaction being considered a transfer between entities under common control. In this situations, MPLX records the assets acquired and liabilities assumed on its consolidated balance sheets at MPC’s historical carrying value. For the acquiring entity, transfers of businesses between entities under common control require prior periods to be retrospectively adjusted for those dates that the entity was under common control. See Note 4 for more information about the acquisitions.
Lessee, Leases	Leases – As part of the adoption of ASU No. 2016-02, Leases (“ASC 842”), we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to grandfather the historical accounting conclusions until a reassessment event is present. We also elected the practical expedient to not recognize short-term leases on the balance sheet, the practical expedient related to right of way permits and land easements which allows us to carry forward our accounting treatment for those existing agreements, and the practical expedient to combine lease and non-lease components for the majority of our underlying classes of assets except for our third-party contractor service and equipment agreements and boat and barge equipment agreements in which we are the lessee. We did not elect the practical expedient to combine lease and non-lease components for arrangements in which we are the lessor. In instances where the practical expedient was not elected, lease and non-lease consideration is allocated based on relative standalone selling price.
Lessor, Leases	Leases – As part of the adoption of ASU No. 2016-02, Leases (“ASC 842”), we elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to grandfather the historical accounting conclusions until a reassessment event is present. We also elected the practical expedient to not recognize short-term leases on the balance sheet, the practical expedient related to right of way permits and land easements which allows us to carry forward our accounting treatment for those existing agreements, and the practical expedient to combine lease and non-lease components for the majority of our underlying classes of assets except for our third-party contractor service and equipment agreements and boat and barge equipment agreements in which we are the lessee. We did not elect the practical expedient to combine lease and non-lease components for arrangements in which we are the lessor. In instances where the practical expedient was not elected, lease and non-lease consideration is allocated based on relative standalone selling price.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2020
ANDX LP [Member]
Business Acquisition [Line Items]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The fair value of assets acquired and liabilities assumed shown below represents MPC’s historical carrying values as of October 1, 2018. (In millions) As Originally Reported Adjustments (1) As Adjusted Cash and cash equivalents $ 83 $ (53) $ 30 Receivables, net 241 259 500 Inventories 21 — 21 Other current assets (2) 59 (7) 52 Equity method investments 731 (89) 642 Property, plant and equipment, net 6,709 (427) 6,282 Intangibles, net (3) 960 74 1,034 Other noncurrent assets (4) 31 (8) 23 Total assets acquired 8,835 (251) 8,584 Accounts payable 198 265 463 Other current liabilities (5) 188 (41) 147 Long-term debt 4,916 — 4,916 Deferred credits and other long-term liabilities (6) 75 1 76 Total liabilities assumed 5,377 225 5,602 Net assets acquired excluding goodwill 3,458 (476) 2,982 Goodwill 7,428 724 8,152 Total purchase price $ 10,886 $ 248 $ 11,134 (1) Inclusive of activity recorded subsequent to the acquisition of ANDX on July 30, 2019, a portion of which was recorded as a non-cash contribution from MPC. (2) Includes both related party and third party other current assets. (3) Includes approximately $4 million of favorable lease assets. In connection with the implementation of ASC 842, this balance was reclassed to “Right of use assets” on the Consolidated Balance Sheets during 2019. (4) Includes both related party and third party other noncurrent assets as well as right of use assets associated with leases. (5) Includes accrued liabilities, operating lease liabilities, long-term debt due within one year, as well as related party and third party other current liabilities. (6) Includes deferred revenue and deferred income taxes, as well as related party and third party other noncurrent liabilities.
Business Acquisition, Pro Forma Information	The following unaudited pro forma information combines the historical operations of MPLX and ANDX, giving effect to the Merger as if it had been consummated on January 1, 2018, the beginning of the earliest period presented. (In millions) 2019 2018 Total revenues and other income $ 9,041 $ 8,666 Net income attributable to MPLX LP $ 1,434 $ 2,446
Mt. Airy Terminal [Member]
Business Acquisition [Line Items]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	On September 26, 2018, MPLX acquired an eastern U.S. Gulf Coast export terminal (the “Mt. Airy Terminal”) from Pin Oak Holdings, LLC for total consideration of $451 million. At the time of the acquisition, the terminal included tanks with 4 million barrels of third-party leased storage capacity and a dock with 120 mbpd of capacity. The Mt. Airy Terminal is located on the Mississippi River between New Orleans and Baton Rouge, is in close proximity to several Gulf Coast refineries including MPC’s Garyville Refinery and is near numerous rail lines and pipelines. The Mt. Airy Terminal is accounted for within the L&S segment. In the first quarter of 2019, an adjustment to the initial purchase price was made for approximately $5 million related to the final settlement of the acquisition, which was paid in the first six months of 2019 as shown on the statement of cash flow. This reduced the total purchase price to $446 million and resulted in $336 million of property, plant and equipment, $121 million of goodwill and the remainder being attributable to net liabilities assumed.
Business Acquisition, Pro Forma Information	The amount of revenue and income from operations associated with the acquisition of the Mt. Airy Terminal included on the Consolidated Statement of Income since the September 26, 2018 acquisition date was not material to the financial statements. Assuming the acquisition had occurred on January 1, 2018, the consolidated pro forma results would not have been materially different from the reported results.

Investments and Noncontrollin_2

Investments and Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2020
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments	The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at December 31, December 31, (In millions, except ownership percentages) 2020 2020 2019 L&S MarEn Bakken Company LLC (1) 25% $ 465 $ 481 Illinois Extension Pipeline Company, L.L.C. 35% 254 265 LOOP LLC 41% 252 238 Andeavor Logistics Rio Pipeline LLC (2)(3) 67% 194 202 Minnesota Pipe Line Company, LLC (2) 17% 188 190 Whistler Pipeline LLC (3) 38% 185 134 W2W Holdings LLC (3)(4) 50% 72 — Wink to Webster Pipeline LLC (3)(4) 15% — 126 Explorer Pipeline Company 25% 72 83 Other (2)(3) 103 55 Total L&S 1,785 1,774 G&P MarkWest Utica EMG, L.L.C. (3) 57% 698 1,984 Sherwood Midstream LLC (3) 50% 557 537 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (3) 67% 307 302 Rendezvous Gas Services, L.L.C. (2)(3) 78% 159 170 Sherwood Midstream Holdings LLC (3) 51% 148 157 Centrahoma Processing LLC 40% 145 153 Other (2)(3) 237 198 Total G&P 2,251 3,501 Total $ 4,036 $ 5,275 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture 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) These investments as well as certain investments included within “Other” for both L&S and G&P are investments acquired as part of the Merger. The December 31, 2019 balance reflects all purchase accounting adjustments identified by MPC as part of its acquisition of Andeavor. (3) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (4) During the year ended December 31, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC.
Summarized Financial Information For Equity Method Investees Table	Summarized financial information for MPLX’s equity method investments for the years ended December 31, 2020, 2019 and 2018 is as follows: December 31, 2020 (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 298 $ 1,208 $ 1,506 Costs and expenses 414 531 945 (Loss)/income from operations (116) 677 561 Net (loss)/income (175) 615 440 (Loss)/income from equity method investments (2) $ (1,100) $ 164 $ (936) December 31, 2019 (1) (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 650 $ 1,417 $ 2,067 Costs and expenses 375 568 943 Income from operations 275 849 1,124 Net income 215 752 967 Income from equity method investments (2) $ 103 $ 187 $ 290 December 31, 2018 (1) (In millions) Other VIEs Non-VIEs Total Revenues and other income $ 484 $ 1,421 $ 1,905 Costs and expenses 286 738 1,024 Income from operations 198 683 881 Net income 197 606 803 Income from equity method investments (2) $ 67 $ 180 $ 247 (1) The financial information for equity method investments for 2019 includes financial information of equity method investments acquired as part of the Merger. The financial information for equity method investments for 2018 includes financial information of equity method investments acquired as part of the Merger for the last three months of 2018. See Note 1 for additional information. (2) “Income from equity method investments” includes the impact of any basis differential amortization or accretion. The 2020 and 2019 amounts include impairment of $1,264 million and $42 million, respectively. Summarized balance sheet information for MPLX’s equity method investments as of December 31, 2020 and 2019 is as follows: December 31, 2020 (In millions) Other 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 December 31, 2019 (In millions) Other VIEs Non-VIEs Total Current assets $ 534 $ 330 $ 864 Noncurrent assets 5,862 5,134 10,996 Current liabilities 192 245 437 Noncurrent liabilities $ 305 $ 822 $ 1,127

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	12 Months Ended
	Dec. 31, 2020
Related Party Transaction [Line Items]
Schedule Of Entity Wide Information Allocated Related Party Employee Benefit Costs By Income Statement Line [Table Text Block]	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 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 rent and lease agreements with MPC. MPC has also been advancing certain strategic priorities to lay a foundation for long-term success, including plans to optimize its assets and structurally lower costs in 2021 and beyond, which included an involuntary workforce reduction plan. The workforce reduction plan, together with employee reductions resulting from MPC's indefinite idling of its Martinez, California and Gallup, New Mexico refineries, affected approximately 2,050 employees. All of the employees that conduct MPLX’s business are directly employed by affiliates of MPC, and certain of those employees were affected by MPC’s workforce reductions. During 2020, MPLX reimbursed MPC for $37 million related to severance and employee benefits related expenses that MPC recorded in connection with its workforce reductions. These costs are shown on the Consolidated Statements of Income as “Restructuring expenses.” 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: (In millions) 2020 2019 2018 Rental cost of sales - related parties $ 160 $ 165 $ 31 Purchases - related parties MPC 1,099 1,210 919 Other 17 21 6 Total Purchases - related parties 1,116 1,231 925 General and administrative expenses MPC 254 243 199 Restructuring expenses MPC $ 37 $ — $ — Some charges incurred under the omnibus and employee service 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 2020, 2019 and 2018, these charges totaled $97 million, $169 million and $152 million, respectively.
Related Party Transactions Disclosure [Text Block]	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. In addition, 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 as discussed below. The commercial agreements with MPC include: • MPLX has a fuels distribution agreement with MPC under which MPC pays MPLX a tiered monthly volume-based fee for marketing and selling MPC’s products. This agreement is subject to a minimum quarterly volume and has an initial term of 10 years, subject to a 5-year renewal period under terms to be renegotiated at that time. • MPLX has various pipeline transportation agreements under which MPC pays MPLX fees for transporting crude and refined products on MPLX’s pipeline systems. These agreements are subject to minimum throughput volumes under which MPC will pay MPLX deficiency payments for any period in which they do not ship the minimum committed volume. These deficiency payments can be applied as credits to future periods in which MPC ships volumes in excess of the minimum volume, subject to a limited period of time. These agreements are subject to various terms and renewal periods. • MPLX has a marine transportation agreement with an initial term of 6 years under which MPC pays MPLX fees for providing marine transportation of crude oil, feedstock and refined petroleum products, and related services. This agreement is subject to two automatic renewal periods of 5 years each. • MPLX has various trucking transportation services agreements with terms ranging from month-to-month to 10 years, under which MPC pays MPLX fees for gathering barrels and providing trucking, dispatch, delivery and data services. Most of these agreements are subject to minimum volume commitments and have various terms regarding carry-forward of deficiency payments as credits towards excess volumes shipped in future periods. These agreements are subject to various terms and renewal periods. • MPLX has numerous storage services agreements governing storage services at various types of facilities including terminals, pipeline tank farms, caverns and refineries, under which MPC pays MPLX per-barrel fees for providing storage services. Some of these agreements provide MPC with exclusive access to storage at certain locations, such as storage located at MPC’s refineries or storage in certain caverns. Under these agreements, MPC pays MPLX a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. Many of the refinery storage agreements also contain provisions for logistical services to be provided by MPLX, for which MPC pays monthly fees. These agreements are subject to various terms and renewal periods. • MPLX has a 10-year terminal services agreement governing certain terminals under which MPC pays MPLX fees for terminal storage for refined petroleum products. Under this agreement MPC pays MPLX agreed upon fees relating to MPC product deliveries as well as any viscosity surcharges, loading, handling, transfers or other related charges. This agreement is subject to minimum volume throughput commitments under which MPC pays a deficiency payment for any period in which they do not meet the minimum committed volume. The terminal services agreement with MPC includes automatic renewal terms ranging from one one • MPLX has a year to year 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. The pricing structure under this agreement provides for a base volume subject to a base rate and incremental volumes subject to variable rates which are calculated with reference to certain of our costs incurred as processor of the volumes. The pricing for both the base and incremental volumes are subject to revision each year. In many cases, agreements are location-based hybrid agreements, containing provisions relating to multiple of the types of agreements and services described above. Operating Agreements MPLX operates various pipelines owned by MPC under operating services agreements. Under these operating services agreements, MPLX receives an operating fee for operating the assets and is reimbursed for all direct and indirect costs associated with operating the assets. Most of these agreements are indexed for inflation. These agreements range from one Co-location Services Agreements MPLX is party to co-location services agreements with MPC’s refineries, under which MPC provides management, operational and other services to the subsidiaries of Refining Logistics. Refining Logistics pays MPC monthly fixed fees and direct reimbursements for such services calculated as set forth in the agreements. These agreements have initial terms of 50 years. Ground Lease Agreements MPLX is party to ground lease agreements with certain of MPC’s refineries under which MPLX is the lessor of certain sections of property which contain facilities owned by Refining Logistics and are within the premises of MPC’s refineries. Refining Logistics pays MPC monthly fixed fees under these ground leases. These agreements have initial terms of 50 years. Marine Services Agreements with MPC MPLX has a management services agreement and a loss control agreement with MPC under which it provides management and loss control services to assist MPC in the oversight and management of the marine business. MPLX receives fixed annual fees for providing the required services, which are subject to predetermined annual escalation rates. These agreements are subject to an initial terms of five years and automatically renew for one additional five Omnibus Agreements MPLX has omnibus agreements with MPC that address MPLX’s payment of fixed annual fees to MPC for the provision of executive management services by certain executive officers of the general partner and MPLX’s reimbursement of MPC for the provision of certain general and administrative services to it. They also provide for MPC’s indemnification to MPLX for certain matters, including environmental, title and tax matters; as well as our indemnification of MPC for certain matters under these agreements. Employee Services Agreements MPLX has various employee services agreements and secondment agreements with MPC under which MPLX reimburses MPC for employee benefit expenses, along with the provision of operational and management services in support of both our L&S and G&P segments’ operations. Loan Agreement MPLX is party to a loan agreement with 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. On April 27, 2018, MPLX and MPC Investment entered into an amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion. In connection with the Merger, on July 31, 2019, MPLX and MPC Investment entered into a second amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement to $1.5 billion in aggregate principal amount of all loans outstanding at any one time. The 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 the maturity date. Borrowings under the MPC Loan Agreement prior to July 31, 2019 bore interest at LIBOR plus 1.50 percent, while borrowings as of and after July 31, 2019 will bear interest at the one-month 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 17. Activity on the MPC Loan Agreement was as follows: (In millions, except %) December 31, 2020 December 31, 2019 Borrowings $ 6,264 $ 8,540 Average interest rate of borrowings 2.278 % 3.441 % Repayments $ 6,858 $ 7,946 Outstanding balance at end of period $ — $ 594 Prior to the Merger, ANDX was also party to a loan agreement with MPC (“ANDX-MPC Loan Agreement”). This facility was entered into on December 21, 2018, with a borrowing capacity of $500 million. In connection with the Merger, on July 31, 2019, MPLX repaid the entire outstanding balance and terminated the ANDX-MPC Loan Agreement. There was no activity on the ANDX-MPC Loan Agreement in 2018. Activity on the agreement during 2019 prior to the Merger was as follows: (In millions, except %) December 31, 2019 Borrowings $ 773 Average interest rate of borrowings 4.249 % Repayments $ 773 Outstanding balance at end of period $ — Related Party Revenue Related party sales to MPC consist of crude oil and refined products pipeline and trucking transportation services based on tariff/contracted rates; storage, terminal and fuels distribution services based on contracted rates; and marine transportation services. Related party sales to MPC also consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: (In millions) 2020 2019 2018 Service revenue MPC $ 3,578 $ 3,455 $ 2,404 Other 2 — — Total Service revenue - related parties 3,580 3,455 2,404 Rental income MPC 952 1,196 846 Product sales (1) MPC 128 140 87 Other — 2 — Total Product sales - related parties 128 142 87 Other income MPC 192 47 41 Other 62 67 58 Total Other income - related parties $ 254 $ 114 $ 99 (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 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 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 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 rent and lease agreements with MPC. MPC has also been advancing certain strategic priorities to lay a foundation for long-term success, including plans to optimize its assets and structurally lower costs in 2021 and beyond, which included an involuntary workforce reduction plan. The workforce reduction plan, together with employee reductions resulting from MPC's indefinite idling of its Martinez, California and Gallup, New Mexico refineries, affected approximately 2,050 employees. All of the employees that conduct MPLX’s business are directly employed by affiliates of MPC, and certain of those employees were affected by MPC’s workforce reductions. During 2020, MPLX reimbursed MPC for $37 million related to severance and employee benefits related expenses that MPC recorded in connection with its workforce reductions. These costs are shown on the Consolidated Statements of Income as “Restructuring expenses.” 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: (In millions) 2020 2019 2018 Rental cost of sales - related parties $ 160 $ 165 $ 31 Purchases - related parties MPC 1,099 1,210 919 Other 17 21 6 Total Purchases - related parties 1,116 1,231 925 General and administrative expenses MPC 254 243 199 Restructuring expenses MPC $ 37 $ — $ — Some charges incurred under the omnibus and employee service 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 2020, 2019 and 2018, these charges totaled $97 million, $169 million and $152 million, respectively. Related Party Assets and Liabilities December 31, (In millions) 2020 2019 Current assets - related parties Receivables - MPC $ 615 $ 621 Receivables - Other 27 22 Prepaid - MPC 4 9 Other - MPC 1 — Lease Receivables - MPC 30 4 Total 677 656 Noncurrent assets - related parties Long-term receivables - MPC 32 21 Right of use assets - MPC 231 232 Long-term lease receivables - MPC 386 43 Unguaranteed residual asset - MPC 23 7 Total 672 303 Current liabilities - related parties Payables - MPC 215 911 Payables - Other 43 37 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 66 42 Deferred revenue - Project reimbursements - MPC 30 16 Deferred revenue - Project reimbursements - Other 1 1 Total 356 1,008 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 230 Long-term deferred revenue - Project reimbursements - MPC 47 53 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 283 $ 290
Schedule of Long-term Debt Instruments [Table Text Block]	Loan Agreement MPLX is party to a loan agreement with 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. On April 27, 2018, MPLX and MPC Investment entered into an amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion. In connection with the Merger, on July 31, 2019, MPLX and MPC Investment entered into a second amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement to $1.5 billion in aggregate principal amount of all loans outstanding at any one time. The 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 the maturity date. Borrowings under the MPC Loan Agreement prior to July 31, 2019 bore interest at LIBOR plus 1.50 percent, while borrowings as of and after July 31, 2019 will bear interest at the one-month 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 17. Activity on the MPC Loan Agreement was as follows: (In millions, except %) December 31, 2020 December 31, 2019 Borrowings $ 6,264 $ 8,540 Average interest rate of borrowings 2.278 % 3.441 % Repayments $ 6,858 $ 7,946 Outstanding balance at end of period $ — $ 594 Prior to the Merger, ANDX was also party to a loan agreement with MPC (“ANDX-MPC Loan Agreement”). This facility was entered into on December 21, 2018, with a borrowing capacity of $500 million. In connection with the Merger, on July 31, 2019, MPLX repaid the entire outstanding balance and terminated the ANDX-MPC Loan Agreement. There was no activity on the ANDX-MPC Loan Agreement in 2018. Activity on the agreement during 2019 prior to the Merger was as follows: (In millions, except %) December 31, 2019 Borrowings $ 773 Average interest rate of borrowings 4.249 % Repayments $ 773 Outstanding balance at end of period $ —
Schedule of Entity Wide Information Sales to Related Parties [Table Text Block]	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: (In millions) 2020 2019 2018 Service revenue MPC $ 3,578 $ 3,455 $ 2,404 Other 2 — — Total Service revenue - related parties 3,580 3,455 2,404 Rental income MPC 952 1,196 846 Product sales (1) MPC 128 140 87 Other — 2 — Total Product sales - related parties 128 142 87 Other income MPC 192 47 41 Other 62 67 58 Total Other income - related parties $ 254 $ 114 $ 99 (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 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 million, respectively.
Schedule of Related Party Transactions [Table Text Block]	Related Party Assets and Liabilities December 31, (In millions) 2020 2019 Current assets - related parties Receivables - MPC $ 615 $ 621 Receivables - Other 27 22 Prepaid - MPC 4 9 Other - MPC 1 — Lease Receivables - MPC 30 4 Total 677 656 Noncurrent assets - related parties Long-term receivables - MPC 32 21 Right of use assets - MPC 231 232 Long-term lease receivables - MPC 386 43 Unguaranteed residual asset - MPC 23 7 Total 672 303 Current liabilities - related parties Payables - MPC 215 911 Payables - Other 43 37 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 66 42 Deferred revenue - Project reimbursements - MPC 30 16 Deferred revenue - Project reimbursements - Other 1 1 Total 356 1,008 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 230 Long-term deferred revenue - Project reimbursements - MPC 47 53 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 283 $ 290

Net Income (Loss) Per Limited_2

Net Income (Loss) Per Limited Partner Unit (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class [Table Text Block]	(In millions) 2020 2019 2018 Net (loss)/income attributable to MPLX LP $ (720) $ 1,033 $ 1,818 Less: Distributions declared on Series A preferred units (1) 81 81 75 Distributions declared on Series B preferred units (1) 41 42 — Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1)(2) 2,872 2,635 1,985 Undistributed net loss attributable to MPLX LP $ (3,714) $ (1,725) $ (242) (1) See Note 8 for distribution information. (2) Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates. 2020 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net (loss)/income attributable to MPLX LP per unit: Net (loss)/income attributable to MPLX LP: Distributions declared $ 2,872 $ 81 $ 41 $ 2,994 Undistributed net loss attributable to MPLX LP (3,714) — — (3,714) Net (loss)/income attributable to MPLX LP (1) $ (842) $ 81 $ 41 $ (720) Weighted average units outstanding: Basic 1,051 1,051 Diluted 1,051 1,051 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.80) Diluted $ (0.80) 2019 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared $ 2,635 $ 81 $ 42 $ 2,758 Undistributed net loss attributable to MPLX LP (1,725) — — (1,725) Net income attributable to MPLX LP (1) $ 910 $ 81 $ 42 $ 1,033 Weighted average units outstanding: Basic (2) 906 906 Diluted (2) 907 907 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.00 Diluted $ 1.00 2018 (In millions, except per unit data) Limited Series A Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distribution declared $ 1,985 $ 75 $ 2,060 Undistributed net loss attributable to MPLX LP (242) — (242) Net income attributable to MPLX LP (1) $ 1,743 $ 75 $ 1,818 Weighted average units outstanding: Basic 761 761 Diluted 761 761 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.29 Diluted $ 2.29
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	2020 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net (loss)/income attributable to MPLX LP per unit: Net (loss)/income attributable to MPLX LP: Distributions declared $ 2,872 $ 81 $ 41 $ 2,994 Undistributed net loss attributable to MPLX LP (3,714) — — (3,714) Net (loss)/income attributable to MPLX LP (1) $ (842) $ 81 $ 41 $ (720) Weighted average units outstanding: Basic 1,051 1,051 Diluted 1,051 1,051 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.80) Diluted $ (0.80) 2019 (In millions, except per unit data) Limited Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared $ 2,635 $ 81 $ 42 $ 2,758 Undistributed net loss attributable to MPLX LP (1,725) — — (1,725) Net income attributable to MPLX LP (1) $ 910 $ 81 $ 42 $ 1,033 Weighted average units outstanding: Basic (2) 906 906 Diluted (2) 907 907 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.00 Diluted $ 1.00 2018 (In millions, except per unit data) Limited Series A Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distribution declared $ 1,985 $ 75 $ 2,060 Undistributed net loss attributable to MPLX LP (242) — (242) Net income attributable to MPLX LP (1) $ 1,743 $ 75 $ 1,818 Weighted average units outstanding: Basic 761 761 Diluted 761 761 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.29 Diluted $ 2.29 (1) Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. (2) The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire year. See Notes 4 and 8 for additional information about the treatment of these units.

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Stockholders Equity	The table below summarizes the changes in the number of units outstanding for the years ended December 31, 2018, 2019, and 2020: (In units) Common General Partner (1) Total Balance at December 31, 2017 407,130,020 8,308,773 415,438,793 Unit-based compensation awards 348,387 140 348,527 Contribution of Refining Logistics and Fuels Distribution (See Note 4) 111,611,111 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 (10,586,691) 264,413,309 Balance at December 31, 2018 794,089,518 — 794,089,518 Unit-based compensation awards 288,031 — 288,031 Issuance of units in connection with the Merger (See Note 4) 262,829,592 — 262,829,592 Conversion of Series A preferred units 1,148,330 — 1,148,330 Balance at December 31, 2019 1,058,355,471 — 1,058,355,471 Unit-based compensation awards 478,438 — 478,438 Wholesale Exchange (18,582,088) — (18,582,088) Units redeemed in unit repurchase program (1,473,843) — (1,473,843) Balance at December 31, 2020 1,038,777,978 — 1,038,777,978 (1) Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisition of Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two GP Interest.
Distributions Made to Limited Partner, by Distribution	The allocation of total quarterly cash distributions to general, limited, and preferred unitholders is as follows for the years ended December 31, 2020, 2019 and 2018. 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. (In millions) 2020 2019 2018 Limited partners' distributions: Common unitholders, includes common units of general partner $ 2,872 $ 2,635 $ 1,985 Series A preferred unit distributions 81 81 75 Series B preferred unit distribution 41 42 — Total cash distributions declared $ 2,994 $ 2,758 $ 2,060

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	12 Months Ended
	Dec. 31, 2020
Redeemable Preferred Units Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance for 2020 and 2019 are summarized below: (In millions) 2020 2019 Balance at beginning of period $ 968 $ 1,004 Net income allocated 81 81 Distributions received by preferred unitholders (81) (81) Conversion of preferred units to common units — (36) Balance at end of period $ 968 $ 968

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The table below provides a reconciliation between “Net income” and Segment Adjusted EBITDA. (In millions) 2020 2019 2018 Reconciliation to Net (loss)/income: L&S Segment Adjusted EBITDA $ 3,488 $ 2,748 $ 2,057 G&P Segment Adjusted EBITDA 1,723 1,586 1,418 Total reportable segments 5,211 4,334 3,475 Depreciation and amortization (1) (1,377) (1,254) (867) Provision for income taxes (2) — (8) Amortization of deferred financing costs (61) (42) (55) Gain/(loss) on extinguishment of debt 19 — (46) Non-cash equity-based compensation (14) (22) (23) Impairment expense (2,165) (1,197) — Net interest and other financial costs (854) (873) (613) (Loss)/income from equity method investments (936) 290 247 Distributions/adjustments related to equity method investments (499) (562) (458) Unrealized derivative (losses)/gains (2) (3) 1 5 Acquisition costs — (14) (4) Restructuring expenses (37) — — Other (6) (1) — Adjusted EBITDA attributable to noncontrolling interests 37 32 18 Adjusted EBITDA attributable to Predecessor (3) — 770 335 Net (loss)/income $ (687) $ 1,462 $ 2,006 (1) Depreciation and amortization attributable to L&S was $633 million, $503 million and $308 million for the years ended 2020, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $744 million, $751 million and $559 million for 2020, 2019 and 2018, respectively. (2) MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (3) The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	December 31, (In millions) 2020 2019 Segment Assets Cash and cash equivalents $ 15 $ 15 L&S 20,938 20,810 G&P 15,461 19,605 Total assets $ 36,414 $ 40,430
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 for the years ended December 31, 2020, 2019 and 2018 as well as total assets for our reportable segments as of December 31, 2020 and 2019: (In millions) 2020 2019 2018 L&S Service revenue $ 3,889 $ 3,765 $ 2,575 Rental income 985 1,235 856 Product related revenue 51 91 23 Income from equity method investments 154 200 171 Other income 206 61 47 Total segment revenues and other income (1) 5,285 5,352 3,672 Segment Adjusted EBITDA (2) 3,488 2,748 2,057 Restructuring expenses 29 — — Capital expenditures (3) 498 1,060 708 Investments in unconsolidated affiliates 141 289 3 G&P Service revenue 2,088 2,188 1,685 Rental income 365 349 342 Product related revenue 868 997 1,171 (Loss)/income from equity method investments (1,090) 90 76 Other income 53 65 59 Total segment revenues and other income (1) 2,284 3,689 3,333 Segment Adjusted EBITDA (2) 1,723 1,586 1,418 Restructuring expenses 8 — — Capital expenditures (3) 441 1,203 1,545 Investments in unconsolidated affiliates $ 125 $ 424 $ 338 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $567 million, $660 million and $371 million for 2020, 2019 and 2018, respectively. Third party revenues for the G&P segment were $2,088 million, $3,474 million and $3,198 million for 2020, 2019 and 2018, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”

Inventories (Tables)

Inventories (Tables)	12 Months Ended
	Dec. 31, 2020
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: December 31, (In millions) 2020 2019 NGLs $ 5 $ 5 Line fill 13 10 Spare parts, materials and supplies 100 95 Total inventories $ 118 $ 110

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: Estimated December 31, (In millions) 2020 2019 L&S Pipelines 2-51 years $ 6,026 $ 5,572 Refining logistics 13-40 years 2,333 2,870 Terminals 4-40 years 1,643 1,109 Marine 15-20 years 965 906 Land, building and other 1-61 years 1,584 1,817 Construction-in progress 262 660 Total L&S property, plant and equipment 12,813 12,934 G&P Gathering and transportation 5-40 years 7,547 7,159 Processing and fractionation 10-40 years 5,721 5,545 Land, building and other 3-40 years 507 484 Construction-in-progress 287 745 Total G&P property, plant and equipment 14,062 13,933 Total property, plant and equipment 26,875 26,867 Less accumulated depreciation (1) 5,657 4,722 Property, plant and equipment, net $ 21,218 $ 22,145

Goodwill and Intangibles (Table

Goodwill and Intangibles (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The changes in carrying amount of goodwill were as follows for the periods presented: (In millions) L&S G&P Total Gross goodwill as of December 31, 2018 $ 7,234 $ 2,912 $ 10,146 Accumulated impairment losses — (130) (130) Balance as of December 31, 2018 7,234 2,782 10,016 Impairment losses — (1,197) (1,197) Acquisitions (1) 488 229 717 Balance as of December 31, 2019 7,722 1,814 9,536 Impairment losses — (1,814) (1,814) Wholesale Exchange (Note 4) (65) — (65) Balance as of December 31, 2020 7,657 — 7,657 Gross goodwill as of December 31, 2020 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of December 31, 2020 $ 7,657 $ — $ 7,657
Schedule of Acquired Finite-Lived Intangible Assets by Major Class	MPLX’s intangible assets are comprised of customer contracts and relationships. The weighted average amortization period for intangible assets acquired during 2019 was approximately nine years. Gross intangible assets with accumulated amortization as of December 31, 2020 and 2019 is shown below: December 31, 2020 December 31, 2019 (In millions) Useful Life Gross Accumulated Amortization (1)(2) Net Gross Accumulated Amortization (1) Net L&S 6 - 8 years $ 283 $ (81) $ 202 $ 283 $ (45) $ 238 G&P 6 - 25 years 1,288 (531) 757 1,288 (256) 1,032 $ 1,571 $ (612) $ 959 $ 1,571 $ (301) $ 1,270 (1) Amortization expense attributable to the G&P segment for the years ended December 31, 2020 and 2019 was $98 million and $127 million, respectively. Amortization expense attributable to the L&S segment for the year ended December 31, 2020 and 2019 was $36 million and $31 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	Estimated future amortization expense related to the intangible assets at December 31, 2020 is as follows: (In millions) 2021 $ 127 2022 127 2023 127 2024 127 2025 113 Thereafter 338 Total $ 959

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value on a recurring basis as of December 31, 2020 and 2019 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. December 31, 2020 2019 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (63) $ — $ (60) Total carrying value on Consolidated Balance Sheets $ — $ (63) $ — $ (60) five
Schedule of changes in Level 3 fair value measurements [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. 2020 2019 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (60) $ — $ (61) Total losses (realized and unrealized) included in earnings (1) — (9) — (5) Settlements — 6 — 6 Fair value at end of period — (63) — (60) The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (4) $ — $ (5) (1) Gains and losses on commodity derivatives 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, by Balance Sheet Grouping [Table Text Block]	December 31, 2020 2019 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 22,951 $ 20,244 $ 21,054 $ 19,800

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of December 31, 2020, MPLX had no outstanding commodity contracts.
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: December 31, (In millions) 2020 2019 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets /Other current liabilities $ — $ (7) $ — $ (5) Other noncurrent assets /Deferred credits and other liabilities — (56) — (55) Total $ — $ (63) $ — $ (60) (1) Includes embedded derivatives in commodity contracts as discussed above.
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: (In millions) 2020 2019 2018 Product sales Realized gains $ — $ — $ 4 Unrealized gains — — 2 Total derivative gains related to product sales — — 6 Purchased product costs Realized losses (6) (6) (12) Unrealized (losses)/gains (3) 1 3 Total derivative loss related to purchased product costs (9) (5) (9) Total derivative losses $ (9) $ (5) $ (3)

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Extinguishment of Debt [Line Items]
Summary of Outstanding Borrowings	MPLX’s outstanding borrowings at December 31, 2020 and 2019 consisted of the following: December 31, (In millions) 2020 2019 MPLX LP: Bank revolving credit facility due July 30, 2024 $ 175 $ — Term loan facility due September 26, 2021 — 1,000 Floating rate senior notes due September 9, 2021 — 1,000 Floating rate senior notes due September 9, 2022 1,000 1,000 6.250% senior notes due October 15, 2022 — 266 3.500% senior notes due December 1, 2022 486 486 3.375% senior notes due March 15, 2023 500 500 4.500% senior notes due July 15, 2023 989 989 6.375% senior notes due May 1, 2024 — 381 4.875% senior notes due December 1, 2024 1,149 1,149 5.250% senior notes due January 15, 2025 708 708 4.000% senior notes due February 15, 2025 500 500 4.875% senior notes due June 1, 2025 1,189 1,189 1.750% senior notes due March 1, 2026 1,500 — 4.125% senior notes due March 1, 2027 1,250 1,250 4.250% senior notes due December 1, 2027 732 732 4.000% senior notes due March 15, 2028 1,250 1,250 4.800% senior notes due February 15, 2029 750 750 2.650% senior notes due August 15, 2030 1,500 — 4.500% senior notes due April 15, 2038 1,750 1,750 5.200% senior notes due March 1, 2047 1,000 1,000 5.200% senior notes due December 1, 2047 487 487 4.700% senior notes due April 15, 2048 1,500 1,500 5.500% senior notes due February 15, 2049 1,500 1,500 4.900% senior notes due April 15, 2058 500 500 Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 23 ANDX - 3.500% - 5.250% senior notes, due 2022-2047 87 190 Financing lease obligations (1) 11 19 Total 20,536 20,119 Unamortized debt issuance costs (116) (106) Unamortized discount/premium (281) (300) Amounts due within one year (764) (9) Total long-term debt due after one year $ 19,375 $ 19,704
Schedule of Debt Payments	The following table shows five years of scheduled debt payments, including payments on finance lease obligations: (In millions) 2021 $ 753 2022 1,502 2023 1,502 2024 1,326 2025 $ 1,701
Schedule of interest payable dates	Interest on each series of MPLX LP, MarkWest and ANDX senior notes is payable semi-annually in arrears, according to the table below. Senior Notes Interest payable semi-annually in arrears 3.500% senior notes due December 1, 2022 June 1 st and December 1 st 3.375% senior notes due March 15, 2023 March 15 th and September 15 th 4.500% senior notes due July 15, 2023 January 15 th and July 15 th 4.875% senior notes due December 1, 2024 June 1 st and December 1 st 5.250% senior notes due January 15, 2025 January 15 th and July 15 th 4.000% senior notes due February 15, 2025 February 15 th and August 15 th 4.875% senior notes due June 1, 2025 June 1 st and December 1 st 1.750% senior notes due March 1, 2026 March 1 st and September 1 st 4.125% senior notes due March 1, 2027 March 1 st and September 1 st 4.250% senior notes due December 1, 2027 June 1 st and December 1 st 4.000% senior notes due March 15, 2028 March 15 th and September 15 th 4.800% senior notes due February 15, 2029 February 15 th and August 15 th 2.650% senior notes due August 15, 2030 February 15 th and August 15 th 4.500% senior notes due April 15, 2038 April 15 th and October 15 th 5.200% senior notes due March 1, 2047 March 1 st and September 1 st 5.200% senior notes due December 1, 2047 June 1 st and December 1 st 4.700% senior notes due April 15, 2048 April 15 th and October 15 th 5.500% senior notes due February 15, 2049 February 15 th and August 15 th 4.900% senior notes due April 15, 2058 April 15 th and October 15 th

Revenue Revenue (Tables)

Revenue Revenue (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	The following table represents a disaggregation of revenue for each reportable segment for the years ended December 31, 2020 and 2019: 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 333 $ 2,064 $ 2,397 Service revenue - related parties 3,556 24 3,580 Service revenue - product related — 155 155 Product sales (1) 39 597 636 Product sales - related parties 12 116 128 Total revenues from contracts with customers $ 3,940 $ 2,956 6,896 Non-ASC 606 revenue (2) 673 Total revenues and other income $ 7,569 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 346 $ 2,152 $ 2,498 Service revenue - related parties 3,419 36 3,455 Service revenue - product related — 140 140 Product sales (1) 65 741 806 Product sales - related parties 26 116 142 Total revenues from contracts with customers $ 3,856 $ 3,185 7,041 Non-ASC 606 revenue (2) 2,000 Total revenues and other income $ 9,041 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 174 $ 1,682 $ 1,856 Service revenue - related parties 2,401 3 2,404 Service revenue - product related — 220 220 Product sales (1) 12 870 882 Product sales - related parties 11 76 87 Total revenues from contracts with customers $ 2,598 $ 2,851 5,449 Non-ASC 606 revenue (2) 1,556 Total revenues and other income $ 7,005 (1) G&P “Product sales” for the year ended December 31, 2018 was adjusted in the table above by $5 million related to derivative gains and mark-to-market adjustments. There were no adjustments for the years ended December 31, 2020 and 2019.
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block]	The table below reflects the changes in our contract balances for the years ended December 31, 2020 and 2019: (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2020 Contract assets $ 39 $ 3 $ (2) $ 40 Long-term contract asset — 2 — 2 Deferred revenue 23 22 (8) 37 Deferred revenue - related parties 53 121 (83) 91 Long-term deferred revenue 90 29 — 119 Long-term deferred revenue - related parties 55 (7) — 48 Long-term contract liability $ — $ 6 $ — $ 6 (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2019 Contract assets $ 36 $ 5 $ (2) $ 39 Deferred revenue 13 17 (7) 23 Deferred revenue - related parties 65 55 (67) 53 Long-term deferred revenue 56 34 — 90 Long-term deferred revenue - related parties $ 52 $ 3 $ — $ 55 (1) Balance represents ASC 606 portion of each respective line item.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	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 December 31, 2020, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $293 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 $ 1,891 2022 1,822 2023 1,666 2024 1,540 2025 and thereafter 4,555 Total revenue on remaining performance obligations (1)(2)(3) $ 11,474 (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 Inform_2

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Information [Abstract]
Summary of Supplemental Cash Flow Information	(In millions) 2020 2019 2018 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 821 $ 835 $ 568 Income taxes paid 2 1 1 Cash paid for amounts included in the measurement of lease liabilities Payments on operating leases 87 85 — Interest payment under finance lease obligations 1 1 — Net cash provided by financing activities included Principal payments under finance lease obligations 9 5 — Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories — 2 2 MPLX terminal lease classification change — 21 — ROU assets obtained in exchange for new operating lease obligations 17 26 — ROU assets obtained in exchange for new finance lease obligations 1 4 — Fair value of common units redeemed for Wholesale Exchange 340 — — Contribution - common units issued (1) $ — $ 7,722 $ 4,236 (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: (In millions) 2020 2019 2018 (Decrease)/increase in capital accruals $ (244) $ (146) $ 135

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2018 through December 31, 2020: (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2018 (1) $ (14) $ (2) $ (16) Other comprehensive income - remeasurements (2) — 1 1 Balance as of December 31, 2019 (1) (14) (1) (15) Other comprehensive income/(loss) - remeasurements (2) 1 (1) — Balance as of December 31, 2020 (1) $ (13) $ (2) $ (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.”

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Restricted Stock Unit, Activity	The following is a summary of phantom unit award activity of MPLX common units in 2020: Phantom Units Number Weighted Aggregate Intrinsic Value (In millions) Outstanding at December 31, 2019 1,109,568 $ 35.97 Granted 238,238 19.08 Settled (686,382) 37.04 Forfeited (17,401) 33.65 Outstanding at December 31, 2020 644,023 28.65 Vested and expected to vest at December 31, 2020 640,885 28.69 $ 14 Non-forfeitable at December 31, 2020 (1) 362,682 $ 29.66 $ 8 (1) Represents a subset of phantom units held by our non-employee directors and certain of our officers and non-officer employees that are generally non-forfeitable and that would be paid out as common units upon the holder’s separation from service, or upon a predetermined date.
Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested and Expected to Vest	The following is a summary of the values related to phantom units: Phantom Units Intrinsic Value of Units Issued During the Period (in millions) Weighted Average Grant Date Fair Value of Units Granted During the Period 2020 $ 12 $ 19.08 2019 14 32.62 2018 $ 18 $ 33.84
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Performance Units, Activity	The following table presents a summary of the 2020 activity for performance unit awards to be settled in MPLX common units: Performance Units Number of Units Weighted Outstanding at December 31, 2019 2,157,347 $ 0.84 Granted 2,147,211 0.86 Settled (1,169,354) 0.90 Forfeited (42,918) 0.84 Outstanding at December 31, 2020 3,092,286 $ 0.83
Schedule of Share-based Compensation, Performance Unit Awards, Valuation Assumptions	The following table provides a summary of the weighted average inputs used for these assumptions: 2020 2019 2018 Risk-free interest rate 1.29% 2.51% N/A Look-back period 1.62 years 2.84 years N/A Expected volatility 22.32% 25.01% N/A Grant date fair value of performance units granted (1) $0.86 $0.76 N/A (1) Performance units granted includes units subject to both the market condition and performance condition.

Leases Lessee Disclosure (Table

Leases Lessee Disclosure (Tables)	12 Months Ended
	Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Lessee, Finance Leases [Text Block]	Lessee We lease a wide variety of facilities and equipment under leases from third parties, including land and building space, office and field equipment, storage facilities and transportation equipment, while our related party leases primarily relate to ground leases associated with our refining logistics assets. Our remaining lease terms range from less than one one Under ASC 840, operating lease costs were $89 million in 2018. Under ASC 842, the components of lease cost were as follows: 2020 2019 (In millions) Related Party Third Party Related Party Third Party Components of lease costs: Operating lease costs $ 14 $ 78 $ 14 $ 75 Finance lease cost: Amortization of ROU assets — 3 — 5 Interest on lease liabilities — 1 — 1 Total finance lease cost — 4 — 6 Variable lease cost 1 10 1 11 Short-term lease cost — 52 — 80 Total lease cost $ 15 $ 144 $ 15 $ 172 Supplemental balance sheet data related to leases were as follows: December 31, 2020 December 31, 2019 (In millions, except % and years) Related Party Third Party Related Party Third Party Operating leases Assets Right of use assets $ 231 $ 309 $ 232 $ 365 Liabilities Operating lease liabilities 1 63 1 66 Long-term operating lease liabilities 229 244 230 302 Total operating lease liabilities $ 230 $ 307 $ 231 $ 368 Weighted average remaining lease term 46.23 years 8.04 years 47.20 years 8.59 years Weighted average discount rate 5.80 % 4.33 % 5.80 % 4.38 % Finance leases Assets Property, plant and equipment, gross $ 17 $ 46 Less: Accumulated depreciation 8 19 Property, plant and equipment, net 9 27 Liabilities Other current liabilities 2 9 Long-term debt 9 10 Total finance lease liabilities $ 11 $ 19 Weighted average remaining lease term 16.87 years 10.16 years Weighted average discount rate 5.95 % 5.87 % As of December 31, 2020, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2021 $ 14 $ 76 $ 3 2022 14 67 2 2023 14 57 2 2024 14 35 1 2025 14 19 1 2026 and thereafter 591 115 9 Gross lease payments 661 369 18 Less: Imputed interest 431 62 7 Total lease liabilities $ 230 $ 307 $ 11
Lease, Cost [Table Text Block]	Under ASC 840, operating lease costs were $89 million in 2018. Under ASC 842, the components of lease cost were as follows: 2020 2019 (In millions) Related Party Third Party Related Party Third Party Components of lease costs: Operating lease costs $ 14 $ 78 $ 14 $ 75 Finance lease cost: Amortization of ROU assets — 3 — 5 Interest on lease liabilities — 1 — 1 Total finance lease cost — 4 — 6 Variable lease cost 1 10 1 11 Short-term lease cost — 52 — 80 Total lease cost $ 15 $ 144 $ 15 $ 172
Supplemental Balance Sheet Disclosures [Text Block]	Supplemental balance sheet data related to leases were as follows: December 31, 2020 December 31, 2019 (In millions, except % and years) Related Party Third Party Related Party Third Party Operating leases Assets Right of use assets $ 231 $ 309 $ 232 $ 365 Liabilities Operating lease liabilities 1 63 1 66 Long-term operating lease liabilities 229 244 230 302 Total operating lease liabilities $ 230 $ 307 $ 231 $ 368 Weighted average remaining lease term 46.23 years 8.04 years 47.20 years 8.59 years Weighted average discount rate 5.80 % 4.33 % 5.80 % 4.38 % Finance leases Assets Property, plant and equipment, gross $ 17 $ 46 Less: Accumulated depreciation 8 19 Property, plant and equipment, net 9 27 Liabilities Other current liabilities 2 9 Long-term debt 9 10 Total finance lease liabilities $ 11 $ 19 Weighted average remaining lease term 16.87 years 10.16 years Weighted average discount rate 5.95 % 5.87 %
Schedule of Future Minimum Lease Payments for Operating and Finance Leases [Table Text Block]	As of December 31, 2020, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2021 $ 14 $ 76 $ 3 2022 14 67 2 2023 14 57 2 2024 14 35 1 2025 14 19 1 2026 and thereafter 591 115 9 Gross lease payments 661 369 18 Less: Imputed interest 431 62 7 Total lease liabilities $ 230 $ 307 $ 11
Lessor, Operating Lease, Payment to be Received, Fiscal Year Maturity [Table Text Block]	The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2020: (In millions) Related Party Third Party Total 2021 $ 919 $ 186 $ 1,105 2022 913 181 1,094 2023 869 178 1,047 2024 815 174 989 2025 791 142 933 2026 and thereafter 1,300 999 2,299 Total minimum future rentals $ 5,607 $ 1,860 $ 7,467
Sales-type and Direct Financing Leases, Lease Receivable, Maturity [Table Text Block]	The following is a schedule of minimum future revenue on sales-type leases as of December 31, 2020: (In millions) Related Party 2021 $ 157 2022 157 2023 158 2024 158 2025 158 2026 and thereafter 315 Total minimum future rentals 1,103 Less: present value discount 687 Lease receivable $ 416
Operating Lease, Lease Income	Under ASC 840, MPLX’s revenue from its implicit lease arrangements, excluding executory costs, totaled approximately $1,032 million in 2018. Lease revenues included on the Consolidated Statements of Income during 2020 and 2019 were as follows: 2020 2019 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 787 $ 268 $ 1,020 $ 257 Sales-type leases: Profit/(loss) recognized at the commencement date — — — — Interest income (Sales-type rental revenue- fixed minimum) 151 — 6 — Interest income (Revenue from variable lease payments) $ 1 $ — $ 1 $ — (1) These amounts are presented net of executory costs.

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Contractual Obligation, Fiscal Year Maturity Schedule	MPLX executed transportation and terminalling agreements that obligate us to minimum volume, throughput or payment commitments over the original terms of the agreements, which range from four (In millions) 2021 $ 103 2022 117 2023 154 2024 146 2025 124 2026 and thereafter 421 Total $ 1,065

Select Quarterly Financial Da_2

Select Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Data	2020 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 992 $ 2,081 $ 2,247 $ 2,249 (Loss)/income from operations (2,486) 878 899 920 Net (loss)/income (2,716) 655 674 700 Net (loss)/income attributable to MPLX LP (2,724) 648 665 691 Net (loss)/income attributable to MPLX LP per limited partner unit: Common - basic (2.60) 0.58 0.61 0.63 Common - diluted (2.60) 0.58 0.61 0.63 Cash distributions declared per limited partner common unit 0.6875 0.6875 0.6875 0.6875 Distributions declared: Limited partner units - Public 270 270 270 269 Limited partner units - MPC 458 445 445 445 Series A preferred units 20 21 20 20 Series B preferred units 11 10 10 10 Total distributions declared $ 759 $ 746 $ 745 $ 744 2019 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 2,235 $ 2,210 $ 2,280 $ 2,316 Income/(loss) from operations 912 885 926 (346) Net income/(loss) 689 657 689 (573) Net income/(loss) attributable to MPLX LP 503 482 629 (581) Net income/(loss) attributable to MPLX LP per limited partner unit: Common - basic 0.61 0.56 0.61 (0.58) Common - diluted 0.61 0.55 0.61 (0.58) Cash distributions declared per limited partner common unit 0.6575 0.6675 0.6775 0.6875 Distributions declared: Limited partner units - Public 191 261 266 270 Limited partner units - MPC 332 431 438 446 Series A preferred units 20 21 20 20 Series B preferred units — 21 10 11 Total distributions declared $ 543 $ 734 $ 734 $ 747

Description of the Business a_3

Description of the Business and Basis of Presentation (Detail) $ in Millions	Jul. 31, 2020shares	Jul. 30, 2019shares	Oct. 31, 2012	Mar. 27, 2012	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Date of partnership formation				Mar. 27, 2012
Date for initial public offering completed			Oct. 31, 2012
Number of reportable segments					2
Equity Method Investment, Other than Temporary Impairment					$ 1,264	$ 14
Impairment of Intangible Assets, Finite-lived					177
Impairment of Long-Lived Assets Held-for-use					174
Impairment Charges					3,429
Goodwill, Impairment Loss					$ 1,814	$ 1,197
ANDX LP [Member] | Nonpublic [Member]
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.0328
ANDX LP [Member] | Public
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.135
Western Refining Wholesale & Western Refining Product Transport [Member]
Description Of Business And Basis Of Presentation [Line Items]
Partners' Capital Account, Units, Redeemed | shares	18,582,088				18,582,088

Summary of Principal Accounti_3

Summary of Principal Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Impairment expense	$ 2,165	$ 1,197	$ 0
Goodwill	7,657	9,536	10,016
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment	1,264	14
Goodwill and Intangible Asset Impairment	2,165	1,197	0
Goodwill, Impairment Loss	1,814	1,197
Goodwill	7,657	9,536	10,016
Gathering and Processing [Member]
Accounting Policies [Abstract]
Goodwill	0	1,814	2,782
Schedule of Equity Method Investments [Line Items]
Goodwill, Impairment Loss	1,814	1,197
Goodwill	0	$ 1,814	$ 2,782
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment	$ 1,251

Accounting Standards ASU 2016-0

Accounting Standards ASU 2016-02 Leases and related updates (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable, Allowance for Credit Loss, Current	$ 1
Receivables, net	$ 452	$ 593

Acquisitions Acquisitions (Ande

Acquisitions Acquisitions (Andeavor Logistics LP) (Details) - USD ($) $ in Millions		Jul. 30, 2019	Oct. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018		Jul. 31, 2020	Sep. 09, 2019	Jul. 29, 2019	Dec. 31, 2017
Business Acquisition [Line Items]
Cash and cash equivalents				$ 15				$ 15				$ 15		$ 15
Total revenues and other income				2,249	$ 2,247	$ 2,081	$ 992	2,316	$ 2,280	$ 2,210	$ 2,235	7,569		9,041		$ 7,005
Goodwill, Purchase Accounting Adjustments											(5)
Goodwill				7,657				9,536				7,657		9,536		10,016
Net (loss)/income attributable to MPLX LP				691	665	648	(2,724)	(581)	629	482	503	(720)	[1]	1,033	[1]	1,818	[1]
Receivables, net				452				593				452		593
Current Assets, Related Parties				677				656				677		656
Inventory, Gross				118				110				118		110
Other current assets				65				110				65		110
Assets, Current				1,515				1,484				1,515		1,484
Equity method investments				4,036				5,275				4,036		5,275
Property, plant and equipment, net				21,218				22,145				21,218		22,145
Intangibles, net				959				1,270				959		1,270
Other Assets, Related Parties, Noncurrent				672				303				672		303
Other noncurrent assets				48				52				48		52
Total assets				36,414				40,430				36,414		40,430
Accounts payable				152				242				152		242
Accrued liabilities				194				187				194		187
Current Liabilities, Related Parties				356				1,008				356		1,008
Accrued property, plant and equipment				84				283				84		283
Accrued interest payable				222				210				222		210
Other current liabilities				150				127				150		127
Liabilities, Current				2,086				2,132				2,086		2,132
Long-term deferred revenue				314				217				314		217
Liabilities, Related Parties, Noncurrent				283				290				283		290
Long-term debt				19,375				19,704				19,375		19,704
Deferred credits and other liabilities				115				192				115		192
Liabilities				22,429				22,849				22,429		22,849
Series A preferred units				968				968				968		968
Total MPLX LP partners’ capital				12,772				16,364				12,772		16,364
Partners' Capital, Including Portion Attributable to Noncontrolling Interest				13,017				16,613				13,017		16,613		17,731					$ 9,973
Accumulated other comprehensive loss	[2]			(15)				(15)				(15)		(15)		(16)
Noncontrolling interests				245				249				245		249
Liabilities and Equity				36,414				40,430				36,414		40,430
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest				700	$ 674	$ 655	$ (2,716)	(573)	$ 689	$ 657	$ 689	(687)		1,462		2,006
Andeavor Logistics [Member]
Business Acquisition [Line Items]
Total revenues and other income														2,400		580
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			$ 30
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables			500
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory			21
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	[3]		52
Business Combination, Recognized Identifiable Assets Acquired, Equity Method Investments			642
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment			6,282
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			1,034
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	[4]		23
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			8,584
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable			463
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	[5]		147
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt			4,916
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	[6]		76
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			5,602
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			2,982
Goodwill			8,152
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest			11,134
Net (loss)/income attributable to MPLX LP														1,434		2,446
Acquisition Costs, Period Cost														14
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest														266		172
ANDX LP [Member] | Public
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.135
ANDX LP [Member] | Nonpublic [Member]
Business Acquisition [Line Items]
Common Units Conversion Ratio - ANDX to MPLX		1.0328
Western Refining Wholesale & Western Refining Product Transport [Member]
Business Acquisition [Line Items]
Goodwill																		$ 65
Excess of Fair Value over Book Value																		$ 250
Operating Lease Right Of Use Asset [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	[7],[8]		4
Logistics and Storage [Member]
Business Acquisition [Line Items]
Goodwill				7,657				7,722				7,657		7,722		7,234
Intangibles, net				202				238				202		238
Logistics and Storage [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Goodwill			7,200
Gathering and Processing [Member]
Business Acquisition [Line Items]
Goodwill				0				1,814				0		1,814		2,782
Intangibles, net				757				1,032				$ 757		1,032
Gathering and Processing [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Goodwill			1,000
Previously Reported [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			83
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables			241
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory			21
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	[3]		59
Business Combination, Recognized Identifiable Assets Acquired, Equity Method Investments			731
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment			6,709
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			960
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	[4]		31
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			8,835
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable			198
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	[5]		188
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt			4,916
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	[6]		75
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			5,377
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			3,458
Goodwill			7,428
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest			10,886
Revision of Prior Period, Adjustment [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	[8]		(53)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	[8]		259
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory	[8]		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	[3],[8]		(7)
Business Combination, Recognized Identifiable Assets Acquired, Equity Method Investments	[8]		(89)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	[8]		(427)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	[7],[8]		74
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	[4],[8]		(8)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	[8]		(251)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	[8]		265
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	[5],[8]		(41)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	[8]		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	[6],[8]		1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	[8]		225
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	[8]		(476)
Goodwill	[8]		724
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest	[8]		248
Customer Contracts [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			$ 950
Minimum | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life			2 years
Minimum | Logistics and Storage [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life												6 years
Minimum | Gathering and Processing [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life												6 years
Maximum | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life			10 years
Maximum | Logistics and Storage [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life												8 years
Maximum | Gathering and Processing [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life												25 years
Senior Notes
Business Acquisition [Line Items]
Long-term debt, gross																			$ 2,000
Senior Notes | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Long-term debt, gross			$ 1,100
Andeavor Logistics [Member] | ANDX Bank Revolving Credit Facilities [Member]
Business Acquisition [Line Items]
Line of Credit Facility, Current Borrowing Capacity																				$ 2,100
Limited Partners Common Units
Business Acquisition [Line Items]
Net (loss)/income attributable to MPLX LP	[1]											$ (842)		910		1,743
Limited Partners Common Units | Public
Business Acquisition [Line Items]
Total MPLX LP partners’ capital				9,384				10,800				9,384		10,800
Partners' Capital, Including Portion Attributable to Noncontrolling Interest				9,384				10,800				9,384		10,800		8,336					8,379
Marathon Petroleum Corporation [Member]
Business Acquisition [Line Items]
Other current assets				1				0				1		0
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Total MPLX LP partners’ capital				2,792				4,968				2,792		4,968
Partners' Capital, Including Portion Attributable to Noncontrolling Interest				2,792				4,968				2,792		4,968		(1,612)					2,099
Retained Earnings [Member] | Marathon Petroleum Corporation [Member]
Business Acquisition [Line Items]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest				0				0				0		0		10,867					$ 0
LOOP and Explorer
Business Acquisition [Line Items]
Accumulated other comprehensive loss				$ (15)				$ (15)				$ (15)		(15)
Pro Forma [Member] | Andeavor Logistics [Member]
Business Acquisition [Line Items]
Total revenues and other income														$ 9,041		$ 8,666
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[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.”
[3]	Includes both related party and third party other current assets.
[4]	Includes both related party and third party other noncurrent assets as well as right of use assets associated with leases.
[5]	Includes accrued liabilities, operating lease liabilities, long-term debt due within one year, as well as related party and third party other current liabilities.
[6]	Includes deferred revenue and deferred income taxes, as well as related party and third party other noncurrent liabilities.
[7]	Includes approximately $4 million of favorable lease assets. In connection with the implementation of ASC 842, this balance was reclassed to “Right of use assets” on the Consolidated Balance Sheets during 2019.
[8]	Inclusive of activity recorded subsequent to the acquisition of ANDX on July 30, 2019, a portion of which was recorded as a non-cash contribution from MPC.

Acquisitions Acquisitions (Acqu

Acquisitions Acquisitions (Acquisition Mt. Airy Terminal) (Details) bbl / d in Thousands, bbl in Millions, $ in Millions	Sep. 26, 2018USD ($)bbl / dbbl	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2018USD ($)
Business Acquisition [Line Items]
Intangibles, net			$ 1,270	$ 959
Goodwill			9,536	$ 7,657	$ 10,016
Goodwill, Purchase Accounting Adjustments		$ (5)
Mt. Airy Terminal [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment			336
Goodwill			121
Payments to Acquire Businesses, Gross	$ 451		$ 446
Storage Capacity | bbl	4
Barrels Handled | bbl / d	120

Acquisitions Acquisitions (Refi

Acquisitions Acquisitions (Refining Logistics and Fuels Distribution) (Details) bbl in Millions, $ in Millions		Feb. 01, 2018USD ($)Tanksharesbbl	Dec. 31, 2018USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)shares
Business Acquisition [Line Items]
Contribution from MPC				$ 261	$ 346	$ 11,980
Property, plant and equipment, net				21,218	22,145
Goodwill			$ 10,016	$ 7,657	9,536	$ 10,016
Number of storage tanks | Tank		619
Number of rail and truck racks		32
Number of docks and gasoline blenders		18
Marathon Petroleum Corporation [Member]
Business Acquisition [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%		2.00%
Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual			1,359
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual			$ 874
Payments to Acquire Businesses, Gross		$ 4,100
Partners' Capital Account, Units, Acquisitions | shares						113,888,889
Equity interest issued or issuable, fair value assigned		4,300
Business Acquisition, Total Consideration, Fair Value Assigned		8,400
Property, plant and equipment, net		830
Refining Logistics [Member]
Business Acquisition [Line Items]
Goodwill		85
Fuels Distribution [Member]
Business Acquisition [Line Items]
Goodwill		$ 130
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares						111,611,111
Limited Partners Common Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		111,611,111
Limited Partners Common Units | MPLX Logistics LLC | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		18,176,666
Limited Partners Common Units | MPLX Holdings Inc | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		7,824,167
General Partner Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]					2,277,778
General Partner Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		2,277,778
General Partner Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		85,610,278
Marathon Petroleum Corporation [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Contribution from MPC		$ 23.7
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Contribution from MPC				$ 261	$ 315	$ 0
Crude Oil
Business Acquisition [Line Items]
Storage Capacity | bbl		56
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisition of Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two GP Interest.Series B Preferred Units - Prior to the Merger, ANDX issu

Acquisitions (Details)

Acquisitions (Details) - USD ($) $ in Millions	Jul. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill		$ 7,657	$ 9,536	$ 10,016
Western Refining Wholesale & Western Refining Product Transport [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Redeemed	18,582,088	18,582,088
Partner Capital, Units, contributed, fair value assigned	$ 340
Fair Value, Net Asset (Liability)	90
Goodwill	$ 65

Wholesale Exchange (Details)

Wholesale Exchange (Details) - USD ($) $ in Millions	Jul. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill		$ 7,657	$ 9,536	$ 10,016
Western Refining Wholesale & Western Refining Product Transport [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Redeemed	18,582,088	18,582,088
Partner Capital, Units, contributed, fair value assigned	$ 340
Fair Value, Net Asset (Liability)	90
Excess of Fair Value over Book Value	250
Goodwill	$ 65

Investments and Noncontrollin_3

Investments and Noncontrolling Interest (Equity Method Investments) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 4,036	$ 5,275
Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	38.00%
Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1],[2]	15.00%
Centrahoma Processing LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%
Explorer
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%
MarEn Bakken
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[3]	25.00%
MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	67.00%
Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	78.00%
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	57.00%
Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	50.00%
Sherwood Midstream Holdings | Direct Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	51.00%
Sherwood Midstream Holdings | Indirect Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		24.50%
Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	67.00%
Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		17.00%
W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1],[2]	50.00%
Bakken Pipeline System | Indirect Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		9.19%
Logistics and Storage [Member] | Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	$ 185	134
Logistics and Storage [Member] | Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1],[2]	0	126
Logistics and Storage [Member] | Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investments		72	83
Logistics and Storage [Member] | Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investments		254	265
Logistics and Storage [Member] | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments		252	238
Logistics and Storage [Member] | MarEn Bakken
Schedule of Equity Method Investments [Line Items]
Equity method investments	[3]	465	481
Logistics and Storage [Member] | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	103	55	[4]
Logistics and Storage [Member] | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	194	202	[4]
Logistics and Storage [Member] | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		188	190	[4]
Logistics and Storage [Member] | W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1],[2]	72	0
Gathering and Processing [Member] | Centrahoma Processing LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments		145	153
Gathering and Processing [Member] | MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	307	302
Gathering and Processing [Member] | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	159	170	[4]
Gathering and Processing [Member] | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	698	1,984
Gathering and Processing [Member] | Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	557	537
Gathering and Processing [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	148	157
Gathering and Processing [Member] | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	237	198	[4]
Operating Segments | Logistics and Storage [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		1,785	1,774
Operating Segments | Gathering and Processing [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 2,251	$ 3,501
[1]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.
[2]	During the year ended December 31, 2020, we contributed our ownership in Wink to Webster Pipeline LLC to W2W Holdings LLC.
[3]	The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture 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.
[4]	These investments as well as certain investments included within “Other” for both L&S and G&P are investments acquired as part of the Merger. The December 31, 2019 balance reflects all purchase accounting adjustments identified by MPC as part of its acquisition of Andeavor.

Investments and Noncontrollin_4

Investments and Noncontrolling Interests (Summary of Equity Method Investment Financial Information) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total revenues and other income		$ 2,249	$ 2,247	$ 2,081	$ 992	$ 2,316	$ 2,280	$ 2,210	$ 2,235	$ 7,569	$ 9,041		$ 7,005
Total costs and expenses										7,358	6,664		4,277
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		700	$ 674	$ 655	$ (2,716)	(573)	$ 689	$ 657	$ 689	(687)	1,462		2,006
(Loss)/income from equity method investments										(936)	290		247
Income (Loss) from Equity Method Investments from Asset Impairment and Elimination of Basis Differential											42
Assets, Current		1,515				1,484				1,515	1,484
Liabilities, Current		2,086				2,132				2,086	2,132
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Total revenues and other income										1,506	2,067	[1]	1,905	[1]
Total costs and expenses										945	943	[1]	1,024	[1]
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest										561	1,124	[1]	881	[1]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest										440	967	[1]	803	[1]
(Loss)/income from equity method investments	[2]									(936)	290	[1]	247	[1]
Assets, Current		848				864				848	864
Assets, Noncurrent		11,886				10,996				11,886	10,996
Liabilities, Current		510				437				510	437
Liabilities, Noncurrent		2,734				1,127				2,734	1,127
Non-VIEs
Schedule of Equity Method Investments [Line Items]
Total revenues and other income										1,208	1,417	[1]	1,421	[1]
Total costs and expenses										531	568	[1]	738	[1]
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest										677	849	[1]	683	[1]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest										615	752	[1]	606	[1]
(Loss)/income from equity method investments	[2]									164	187	[1]	180	[1]
Assets, Current		318				330				318	330
Assets, Noncurrent		4,997				5,134				4,997	5,134
Liabilities, Current		187				245				187	245
Liabilities, Noncurrent		830				822				830	822
Other VIEs
Schedule of Equity Method Investments [Line Items]
Total revenues and other income										298	650	[1]	484	[1]
Total costs and expenses										414	375	[1]	286	[1]
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest										(116)	275	[1]	198	[1]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest										(175)	215	[1]	197	[1]
(Loss)/income from equity method investments	[2]									(1,100)	103	[1]	$ 67	[1]
Assets, Current		530				534				530	534
Assets, Noncurrent		6,889				5,862				6,889	5,862
Liabilities, Current		323				192				323	192
Liabilities, Noncurrent		$ 1,904				$ 305				$ 1,904	$ 305
[1]	The financial information for equity method investments for 2019 includes financial information of equity method investments acquired as part of the Merger. The financial information for equity method investments for 2018 includes financial information of equity method investments acquired as part of the Merger for the last three months of 2018. See Note 1 for additional information
[2]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion. The 2020 and 2019 amounts include impairment of $1,264 million and $42 million, respectively.

Investments and Noncontrollin_5

Investments and Noncontrolling Interests (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments from Asset Impairment			$ 28
Equity Method Investment, Other than Temporary Impairment		$ 1,264	14
Income (Loss) from Equity Method Investments from Asset Impairment and Elimination of Basis Differential			42
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment		$ 1,264
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	57.00%
Equity Method Investment, Other than Temporary Impairment		$ 1,251
Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	[1]	50.00%
Sherwood Midstream Holdings | Indirect Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		24.50%
Gathering and Processing [Member]
Schedule of Equity Method Investments [Line Items]
Basis difference		$ (57)	1,000
Basis difference, not amortized		31	498
Logistics and Storage [Member]
Schedule of Equity Method Investments [Line Items]
Basis difference		331	329
Basis difference, not amortized		$ 167	$ 167
[1]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.

Related Party Agreements and _3

Related Party Agreements and Transactions (Narrative) (Detail) - Marathon Petroleum Corporation [Member]	12 Months Ended
	Dec. 31, 2020
Fuels Distribution [Member]
Commercial Agreements
Renewal Term Agreement	5 years
Term Of Agreements	10 years
Marine Services Equipment [Member]
Commercial Agreements
Term Of Agreements	6 years
Automatic renewal term agreement	5 years
Number of renewals	2
Commercial Agreements | Co-location Agreements [Member]
Commercial Agreements
Term Of Agreements	50 years
Commercial Agreements | Marine Management Services [Member]
Commercial Agreements
Term Of Agreements	5 years
Automatic renewal term agreement	5 years
Number of renewals	1
Commercial Agreements | Ground lease Agreements [Member]
Commercial Agreements
Term Of Agreements	50 years
Operating Agreements [Member] | Minimum
Commercial Agreements
Term Of Agreements	1 year
Operating Agreements [Member] | Maximum
Commercial Agreements
Term Of Agreements	5 years
Terminal Services Agreements [Member] | Commercial Agreements | Minimum | Terminal Services Agreements [Member]
Commercial Agreements
Term Of Agreements	1 year
Automatic renewal term agreement	1 year
Terminal Services Agreements [Member] | Commercial Agreements | Maximum | Terminal Services Agreements [Member]
Commercial Agreements
Term Of Agreements	10 years
Automatic renewal term agreement	5 years
Trucking Services Agreement [Member] | Commercial Agreements | Trucking Services Agreement [Member]
Commercial Agreements
Term Of Agreements	10 years

Related Party Agreements and _4

Related Party Agreements and Transactions (Intercompany Loans with Related Parties) (Detail) - MPC Investment - Related Party Revolving Credit Agreement - USD ($) $ in Millions	Jul. 31, 2019	Apr. 27, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 21, 2018	Apr. 26, 2018
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,500	$ 1,000				$ 500
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 1.25 percent	LIBOR plus 1.50 percent
Proceeds from lines of credit			$ 6,264	$ 8,540
Interest rate during period			2.278%	3.441%
Repayments of lines of credit			$ 6,858	$ 7,946
Outstanding balance			$ 0	594
ANDX LP [Member]
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity					$ 500
Proceeds from lines of credit				$ 773
Interest rate during period				4.249%
Repayments of lines of credit				$ 773
Outstanding balance				$ 0

Related Party Agreements and _5

Related Party Agreements and Transactions (Summary of Revenue by Income Statement RP Line Item) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Rental income - related parties		$ 952	$ 1,196	$ 846
Related Party Transaction, Other Revenues from Transactions with Related Party		254	114	99
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Rental income - related parties		952	1,196	846
Sales Revenue, Goods, Related Party, Net Zero		462	1,120	607
Related Party Transaction, Other Revenues from Transactions with Related Party		192	47	41
Other Affiliates [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Other Revenues from Transactions with Related Party		62	67	58
Service [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3,580	3,455	2,404
Service [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3,578	3,455	2,404
Service [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		2	0	0
Product [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[1]	128	142	87
Product [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[1]	128	140	87
Product [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		$ 0	$ 2	$ 0
[1]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 million, respectively.

Related Party Agreements and _6

Related Party Agreements and Transactions (Summary of Expenses by Income Statement RP Line Item) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)employees	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Related Party Transaction [Line Items]
Restructuring Charges	$ 37	$ 0	$ 0
Entity Number of Employees | employees	2,050
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Labor and Related Expense	$ 1,116	1,231	925
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Restructuring Charges	37	0	0
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties [Member]
Related Party Transaction [Line Items]
Labor and Related Expense	160	165	31
Marathon Petroleum Corporation [Member] | Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Labor and Related Expense	1,099	1,210	919
Marathon Petroleum Corporation [Member] | General and Administrative Expense - Related Party [Member]
Related Party Transaction [Line Items]
Labor and Related Expense	254	243	199
Other Affiliates [Member] | Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Labor and Related Expense	17	21	6
Construction-in-progress | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 97	$ 169	$ 152

Related Party Agreements and _7

Related Party Agreements and Transactions (Summary of Balance Sheet by RP Line Item) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Related Party Transaction [Line Items]
Other current assets	$ 65		$ 110
Current Assets, Related Parties	677		656
Operating Lease, Right-of-Use Asset	309		365
Sales-type Lease, Unguaranteed Residual Asset		$ 10
Other Assets, Related Parties, Noncurrent	672		303
Operating Lease, Liability, Current	63		66
Current Liabilities, Related Parties	356		1,008
Operating Lease, Liability, Noncurrent	244		302
Liabilities, Related Parties, Noncurrent	283		290
Other Affiliates [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	27		22
Due to Related Parties, Current	43		37
Other Affiliates [Member] | Other current liabilities
Related Party Transaction [Line Items]
Deferred revenue related parties	1		1
Other Affiliates [Member] | Other Noncurrent Liabilities [Member]
Related Party Transaction [Line Items]
Deferred Revenue, Noncurrent, Related Parties	7		7
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	615		621
Prepaid insurance	4		9
Other current assets	1		0
Net Investment in Lease, Current	30		4
Due from Related Parties, Noncurrent	32		21
Operating Lease, Right-of-Use Asset	231		232
Net Investment in Lease, Noncurrent	386		43
Sales-type Lease, Unguaranteed Residual Asset	23		7	$ 6
Due to Related Parties, Current	215		911
Operating Lease, Liability, Current	1		1
Operating Lease, Liability, Noncurrent	229		230
Marathon Petroleum Corporation [Member] | Minimum Committed Volume Contracts [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	66		42
Marathon Petroleum Corporation [Member] | Reimbursable Projects
Related Party Transaction [Line Items]
Deferred revenue related parties	30		16
Deferred Revenue, Noncurrent, Related Parties	$ 47		$ 53

Net Income (Loss) Per Limited_3

Net Income (Loss) Per Limited Partner Unit (Schedule of Distributions by Partner Class) (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended								9 Months Ended	12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Net Income Per Share [Line Items]
Potentially dilutive securities											1		1		1
Net (loss)/income attributable to MPLX LP		$ 691	$ 665	$ 648	$ (2,724)	$ (581)	$ 629	$ 482	$ 503		$ (720)	[1]	$ 1,033	[1]	$ 1,818	[1]
Total cash distributions declared											2,994		2,758		2,060
Undistributed net loss attributable to MPLX LP											(3,714)		(1,725)		(242)
Contribution from MPC											261		346		11,980
ANDX LP [Member] | Marathon Petroleum Corporation [Member]
Net Income Per Share [Line Items]
Contribution from MPC						12.5				$ 37.5
Limited Partners Common Units
Net Income Per Share [Line Items]
Net (loss)/income attributable to MPLX LP	[1]										(842)		910		1,743
Total cash distributions declared	[2]										2,872		2,635	[3]	1,985
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Net Income Per Share [Line Items]
Total cash distributions declared		$ 445	$ 445	$ 445	$ 458	$ 446	$ 438	$ 431	$ 332
Contribution from MPC											261		315		0
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net Income Per Share [Line Items]
Net (loss)/income attributable to MPLX LP	[1]										81		81		75
Total cash distributions declared	[2]										81		81		75
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net Income Per Share [Line Items]
Net (loss)/income attributable to MPLX LP	[1]										41		42
Total cash distributions declared	[2]										$ 41		$ 42		$ 0
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	See Note 8 for distribution information.
[3]	Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates.

Net Income (Loss) Per Limited_4

Net Income (Loss) Per Limited Partner Unit (Basic and Diluted Earnings Per Unit) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Net income attributable to MPLX LP:
Total cash distributions declared										$ 2,994		$ 2,758		$ 2,060
Undistributed net income loss attributable to MPLX LP										(3,714)		(1,725)		(242)
Net (loss)/income attributable to MPLX LP		$ 691	$ 665	$ 648	$ (2,724)	$ (581)	$ 629	$ 482	$ 503	$ (720)	[1]	$ 1,033	[1]	$ 1,818	[1]
Weighted average units outstanding:
Basic (shares)										1,051		906	[2]	761
Diluted (shares)										1,051		907	[2]	761
Limited Partners Common Units
Net income attributable to MPLX LP:
Total cash distributions declared	[3]									$ 2,872		$ 2,635	[4]	$ 1,985
Undistributed net income loss attributable to MPLX LP										(3,714)		(1,725)		(242)
Net (loss)/income attributable to MPLX LP	[1]									$ (842)		$ 910		$ 1,743
Weighted average units outstanding:
Basic (shares)										1,051		906	[2]	761
Diluted (shares)										1,051		907	[2]	761
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)		$ 0.63	$ 0.61	$ 0.58	$ (2.60)	$ (0.58)	$ 0.61	$ 0.56	$ 0.61	$ (0.80)		$ 1		$ 2.29
Diluted (in USD per unit)		$ 0.63	$ 0.61	$ 0.58	$ (2.60)	$ (0.58)	$ 0.61	$ 0.55	$ 0.61	$ (0.80)		$ 1		$ 2.29
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net income attributable to MPLX LP:
Total cash distributions declared	[3]									$ 81		$ 81		$ 75
Undistributed net income loss attributable to MPLX LP										0		0		0
Net (loss)/income attributable to MPLX LP	[1]									81		81		75
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net income attributable to MPLX LP:
Total cash distributions declared	[3]									41		42		$ 0
Undistributed net income loss attributable to MPLX LP										0		0
Net (loss)/income attributable to MPLX LP	[1]									$ 41		$ 42
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	The Series B preferred units and the MPLX common units issued in connection with the Merger were not outstanding during the entire year. See Notes 4 and 8 for additional information about the treatment of these units.
[3]	See Note 8 for distribution information.
[4]	Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates.

Repurchase Program (Details)

Repurchase Program (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 04, 2021	Dec. 31, 2020
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Shares		1,473,843
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 967
Stock Repurchased and Retired During Period, Value		33
Stock Repurchase Program, Authorized Amount		$ 1,000
Treasury Stock Acquired, Average Cost Per Share		$ 22.29
Subsequent Event
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	99,406
Stock Repurchased and Retired During Period, Value	$ 2

Equity (Changes in Partners Cap

Equity (Changes in Partners Capital, Unit Rollforward) (Details) - shares		Jul. 31, 2020	Feb. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stockholders Equity [Line Items]
Beginning balance, number of units				1,058,355,471	794,089,518	415,438,793
Unit-based compensation awards, in units				478,438	288,031	348,527
Unit conversion					1,148,330	264,413,309
Ending balance, number of units				1,038,777,978	1,058,355,471	794,089,518
Stock Repurchased and Retired During Period, Shares				(1,473,843)
Marathon Petroleum Corporation [Member]
Stockholders Equity [Line Items]
Aggregate percentage of general partner interest			2.00%	2.00%
Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Redeemed		(18,582,088)		(18,582,088)
Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions						113,888,889
ANDX LP [Member]
Stockholders Equity [Line Items]
Units, acquisitions					262,829,592
Limited Partners Common Units
Stockholders Equity [Line Items]
Beginning balance, number of units				1,058,355,471	794,089,518	407,130,020
Unit-based compensation awards, in units				478,438	288,031	348,387
Unit conversion					1,148,330	275,000,000
Ending balance, number of units				1,038,777,978	1,058,355,471	794,089,518
Stock Repurchased and Retired During Period, Shares				(1,473,843)
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Stockholders Equity [Line Items]
Units outstanding				647,415,452	666,000,000
Limited Partners Common Units | Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Redeemed				(18,582,088)
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions						111,611,111
Limited Partners Common Units | ANDX LP [Member]
Stockholders Equity [Line Items]
Units, acquisitions					262,829,592
General Partner Units
Stockholders Equity [Line Items]
Beginning balance, number of units	[1]			0	0	8,308,773
Unit-based compensation awards, in units	[1]			0	0	140
Unit conversion	[1]				0	10,586,691
Ending balance, number of units	[1]			0	0	0
Stock Repurchased and Retired During Period, Shares	[1]			0
General Partner Units | Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Redeemed	[1]			0
General Partner Units | Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions	[1]					2,277,778
General Partner Units | ANDX LP [Member]
Stockholders Equity [Line Items]
Units, acquisitions	[1]				0
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisition of Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two GP Interest.Series B Preferred Units - Prior to the Merger, ANDX issu

Equity (Cash Distributions) (De

Equity (Cash Distributions) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		Feb. 16, 2021	Feb. 12, 2021	Feb. 08, 2021	Jan. 28, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	Jul. 30, 2019
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions to preferred units															$ 2,994	$ 2,758		$ 2,060
Contribution from MPC															261	346		11,980
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders															37	30		17
Marathon Petroleum Corporation [Member] | ANDX LP [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Contribution from MPC										$ 12.5				$ 37.5
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions															2,872	2,635		1,985
Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions						$ 10	$ 10	$ 10	$ 11	11	$ 10	$ 21	$ 0		41	42		0
Preferred Class A [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions						$ 20	$ 20	$ 21	$ 20	$ 20	$ 20	$ 21	$ 20		81	81		75
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution made to limited partner, declaration date					Jan. 28, 2021
Distributions to preferred units					$ 714
Cash distributions declared per limited partner common unit					$ 0.6875
Distribution made to limited partner, date of record				Feb. 8, 2021
Subsequent Event | Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution made to limited partner, distribution date			Feb. 12, 2021
Subsequent Event | Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions to preferred units		$ 21
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions to preferred units	[1]														$ 2,872	$ 2,635	[2]	1,985
Cash distributions declared per limited partner common unit						$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6775	$ 0.6675	$ 0.6575		$ 2.7500	$ 2.6900
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions to preferred units						$ 445	$ 445	$ 445	$ 458	$ 446	$ 438	$ 431	$ 332
Limited Partners' Capital Account, Units Issued						647				666				666	647	666			161
Contribution from MPC															$ 261	$ 315		0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders															$ 0	0		$ 0
Preferred Partner [Member] | Preferred Class B [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders																$ (21)
[1]	See Note 8 for distribution information.
[2]	Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates.

Wholesale Exchange and Merger (

Wholesale Exchange and Merger (Details) - shares	Jul. 31, 2020	Jul. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Limited Partners Common Units | Marathon Petroleum Corporation [Member]
Limited Partners' Capital Account, Units Issued		161,000,000	647,000,000	666,000,000
Public | Limited Partners Common Units
Limited Partners' Capital Account, Units Issued		102,000,000	391,000,000	392,000,000
Western Refining Wholesale & Western Refining Product Transport [Member]
Partners' Capital Account, Units, Redeemed	18,582,088		18,582,088
Western Refining Wholesale & Western Refining Product Transport [Member] | Limited Partners Common Units
Partners' Capital Account, Units, Redeemed			18,582,088
ANDX LP [Member] | Public
Common Units Conversion Ratio - ANDX to MPLX		1.135
ANDX LP [Member] | Nonpublic [Member]
Common Units Conversion Ratio - ANDX to MPLX		1.0328

Series A Preferred Units (Detai

Series A Preferred Units (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Partners' Capital Account, Units, Converted	1,148,330	264,413,309
Series A Convertible Preferred Units
Partners' Capital Account, Units, Converted	1,200,000

Series B Preferred Units (Detai

Series B Preferred Units (Details) - USD ($) $ / shares in Units, $ in Millions	Jul. 29, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 30, 2019	Dec. 31, 2017
Partners' Capital, Including Portion Attributable to Noncontrolling Interest		$ 13,017	$ 16,613	$ 17,731		$ 9,973
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity		(768)	1,381	1,931
Noncontrolling interests		(2,922)	(2,959)	(2,058)
Preferred Class B [Member]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest		611	611	0	$ 615	$ 0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity		41	17	0
Noncontrolling interests		$ (41)	$ (21)	$ 0
Andeavor Logistics [Member]
Preferred Units, Outstanding	600,000
ANDX LP [Member]
Preferred Stock, Dividend Rate, Percentage	6.875%
Units issued, price per unit	$ 1,000
MPLX LP [Member] | Series B Preferred Stock [Member]
Preferred Units, Outstanding		600,000
Preferred Stock, Dividend Rate, Percentage		465.20%
Preferred Stock, Dividend Payment Rate, Variable		LIBOR plus

Units Outstanding (Details)

Units Outstanding (Details) - shares		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Partners' Capital Account, Units		1,038,777,978	1,058,355,471	794,089,518	415,438,793
Series B Preferred Stock [Member] | MPLX LP [Member]
Preferred Units, Outstanding		600,000
Limited Partners Common Units
Partners' Capital Account, Units		1,038,777,978	1,058,355,471	794,089,518	407,130,020
General Partner Units
Partners' Capital Account, Units	[1]	0	0	0	8,308,773
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisition of Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two GP Interest.Series B Preferred Units - Prior to the Merger, ANDX issu

ATM Program (Details)

ATM Program (Details) $ in Billions	Dec. 31, 2020USD ($)
Stockholders Equity [Line Items]
Common Stock, Monetary Value of Shares Authorized	$ 1.7

Redeemable Preferred Units (Nar

Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 28, 2021	May 13, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2017
Redeemable Noncontrolling Interest [Line Items]
Partners' Capital Account, Units, Converted			1,148,330	264,413,309
Partners' Capital Account, Units			1,058,355,471	794,089,518	1,038,777,978	415,438,793
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units		30,800,000
Preferred Stock, Dividend Rate, Percentage		6.50%
Units issued, price per unit		$ 32.50
Issuance of redeemable preferred units		$ 984
Preferred units, dividend rate, per-dollar-amount		$ 0.528125
Preferred units, 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 applicable change of control conversion rate.
Partners' Capital Account, Units, Converted			1,200,000
Partners' Capital Account, Units					29,600,000
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution made to limited partner, declaration date	Jan. 28, 2021
Cash distributions declared per limited partner common unit	$ 0.6875

Redeemable Preferred Units (Rol

Redeemable Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	May 13, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Redeemable Noncontrolling Interest [Line Items]
Redeemable preferred units, beginning balance		$ 968
Distributions to unitholders and general partner		(2,884)	$ (2,435)	$ (1,819)
Redeemable preferred units, ending balance		968	968
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Redeemable preferred units, beginning balance		968	1,004
Issuance of redeemable preferred units	$ 984
Net income allocated		81	81
Distributions to unitholders and general partner		(81)	(81)
Redeemable preferred units, ending balance		968	968	$ 1,004
Convertible Preferred Stock Converted to Other Securities		$ 0	$ (36)

Segment Information (Details)

Segment Information (Details)	12 Months Ended
	Dec. 31, 2020
Segment Reporting Information [Line Items]
Number of reportable segments	2

Segment Information - Revenues

Segment Information - Revenues and Adjusted EBITDA (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income		$ 2,249	$ 2,247	$ 2,081	$ 992	$ 2,316	$ 2,280	$ 2,210	$ 2,235	$ 7,569	$ 9,041	$ 7,005
Revenue from Contract with Customer, Excluding Assessed Tax										6,896	7,041	5,449
Rental income										398	388	352
(Loss)/income from equity method investments										(936)	290	247
Other income										5	12	7
Investments in unconsolidated affiliates										(266)	(713)	(341)
Restructuring Charges										37	0	0
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA										5,211	4,334	3,475
Segment Reconciling Items
Segment Reporting, Revenue Reconciling Item [Line Items]
(Loss)/income from equity method investments										(936)	290	247
Restructuring Charges										(37)	0	0
Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										3,940	3,856	2,598
Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income	[1]									5,285	5,352	3,672
Rental income										985	1,235	856
(Loss)/income from equity method investments										154	200	171
Other income										206	61	47
Adjusted EBITDA										3,488	2,748	2,057
Capital Expenditure	[2]									498	1,060	708
Investments in unconsolidated affiliates										(141)	(289)	(3)
Restructuring Charges										29	0	0
Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,956	3,185	2,851
Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income	[1]									2,284	3,689	3,333
Rental income										365	349	342
(Loss)/income from equity method investments										(1,090)	90	76
Other income										53	65	59
Adjusted EBITDA										1,723	1,586	1,418
Capital Expenditure	[2]									441	1,203	1,545
Investments in unconsolidated affiliates										(125)	(424)	(338)
Restructuring Charges										8	0	0
Service [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,397	2,498	1,856
Service [Member] | Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										333	346	174
Service [Member] | Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										3,889	3,765	2,575
Service [Member] | Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,064	2,152	1,682
Service [Member] | Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,088	2,188	1,685
Product [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										636	806	887
Product [Member] | Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										51	91	23
Product [Member] | Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										868	997	1,171
Third Party | Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income										567	660	371
Third Party | Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income										$ 2,088	$ 3,474	$ 3,198
[1]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $567 million, $660 million and $371 million for 2020, 2019 and 2018, respectively. Third party revenues for the G&P segment were $2,088 million, $3,474 million and $3,198 million for 2020, 2019 and 2018, respectively.
[2]	Capital expenditures do not include adjustments for asset retirement expenditures.

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Cash and cash equivalents	$ 15	$ 15
Total assets	36,414	40,430
Operating Segments | Logistics and Storage [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	20,938	20,810
Operating Segments | Gathering and Processing [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 15,461	$ 19,605

Segment Information - Reconcili

Segment Information - Reconciliation Adjusted EBITDA to Net income (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Depreciation and amortization										$ 1,377	$ 1,254	$ 867
Provision for income taxes										2	0	8
Amortization of deferred financing costs										61	42	55
Total compensation expense										14	22	24
Goodwill and Intangible Asset Impairment										(2,165)	(1,197)	0
(Loss)/income from equity method investments										(936)	290	247
Distributions from unconsolidated affiliates										459	525	412
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 700	$ 674	$ 655	$ (2,716)	$ (573)	$ 689	$ 657	$ 689	(687)	1,462	2,006
Restructuring Charges										37	0	0
Logistics and Storage [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization										633	503	308
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization										744	751	559
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA										5,211	4,334	3,475
Operating Segments | Logistics and Storage [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA										3,488	2,748	2,057
(Loss)/income from equity method investments										154	200	171
Restructuring Charges										29	0	0
Operating Segments | Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA										1,723	1,586	1,418
(Loss)/income from equity method investments										(1,090)	90	76
Restructuring Charges										8	0	0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]									1,377	1,254	867
Provision for income taxes										2	0	8
Amortization of deferred financing costs										61	42	55
Extinguishment of Debt, Gain (Loss), Net of Tax										19	0	(46)
Total compensation expense										(14)	(22)	(23)
Goodwill and Intangible Asset Impairment										(2,165)	(1,197)	0
Interest and Other Financial Costs										854	873	613
(Loss)/income from equity method investments										(936)	290	247
Distributions from unconsolidated affiliates										499	562	458
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[2]									(3)	1	5
Acquisition Costs, Period Cost										0	(14)	(4)
Other Cost and Expense, Operating										(6)	(1)	0
Adjusted EBITDA attributable to noncontrolling interests										37	32	18
Adjusted EBITDA attributable to Predecessor	[3]									0	770	335
Restructuring Charges										$ (37)	$ 0	$ 0
[1]	Depreciation and amortization attributable to L&S was $633 million, $503 million and $308 million for the years ended 2020, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $744 million, $751 million and $559 million for 2020, 2019 and 2018, respectively.
[2]	MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.
[3]	The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.

Major Customers and Concentra_2

Major Customers and Concentration of Credit Risk (Details) - First Major Customer - Marathon Petroleum Corporation [Member]		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues And Other Income]
Concentration Risk [Line Items]
Concentration risk, percentage	[1],[2]	55.00%	53.00%	48.00%
Revenue Benchmark [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	[2],[3]	56.00%	56.00%	50.00%
Logistics and Storage [Member] | Revenues And Other Income]
Concentration Risk [Line Items]
Concentration risk, percentage	[1],[2]	89.00%	88.00%	90.00%
Logistics and Storage [Member] | Revenue Benchmark [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	[2],[3]	92.00%	91.00%	94.00%
Gathering and Processing [Member] | Revenues And Other Income]
Concentration Risk [Line Items]
Concentration risk, percentage	[1],[2]	4.00%	4.00%	2.00%
Gathering and Processing [Member] | Revenue Benchmark [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	[2],[3]	4.00%	4.00%	3.00%
[1]	The percent calculations exclude losses attributable to the impairment of equity method investments.
[2]	The percent calculations exclude revenues attributable to volumes shipped by MPC under joint tariffs with third parties, which are treated as third-party revenue for accounting purposes.
[3]	Operating revenues consist of service revenue, service revenue - product related, rental income and product sales.

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Inventory Disclosure [Abstract]
NGLs	$ 5	$ 5
Line fill	13	10
Spare parts, materials and supplies	100	95
Total inventories	$ 118	$ 110

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 26,875		$ 26,867
Total property, plant and equipment	5,657	[1]	4,722
Less accumulated depreciation(1)	21,218		22,145
Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	12,813		12,934
Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	14,062		13,933
Gas Gathering And Transmission Equipment And Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	7,547		7,159
Processing, Fractionation And Storage Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,721		5,545
Pipelines And Related Assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	6,026		5,572
Barges and towing vessels [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	965		906
Terminals and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,643		1,109
Refineries and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,333		2,870
Land Building Office Equipment And Other [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,584		1,817
Land Building Office Equipment And Other [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	507		484
Construction-in-progress | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	262		660
Construction-in-progress | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 287		$ 745
Minimum | Gas Gathering And Transmission Equipment And Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Minimum | Processing, Fractionation And Storage Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years
Minimum | Pipelines And Related Assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Minimum | Barges and towing vessels [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	15 years
Minimum | Terminals and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	4 years
Minimum | Refineries and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	13 years
Minimum | Land Building Office Equipment And Other [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	1 year
Minimum | Land Building Office Equipment And Other [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Maximum | Gas Gathering And Transmission Equipment And Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Maximum | Processing, Fractionation And Storage Facilities [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Maximum | Pipelines And Related Assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	51 years
Maximum | Barges and towing vessels [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years
Maximum | Terminals and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Maximum | Refineries and related assets [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Maximum | Land Building Office Equipment And Other [Member] | Logistics and Storage [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	61 years
Maximum | Land Building Office Equipment And Other [Member] | Gathering and Processing [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
[1]	The December 31, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below

Property Plant and Equipment (N

Property Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 5,657	[1]	$ 4,722
Impairment of Long-Lived Assets Held-for-use	$ 174
[1]	The December 31, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below

Goodwill and Intangibles (Goodw

Goodwill and Intangibles (Goodwill) (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Goodwill [Line Items]
Goodwill, Impairment Loss	$ 1,814	$ 1,197
Number of reporting units	3
Goodwill	$ 7,657	9,536	$ 10,016
Impairment of Intangible Assets, Finite-lived	$ 177
Total Number of Reporting Units	5
Gathering and Processing [Member]
Goodwill [Line Items]
Goodwill, Impairment Loss	$ 1,814	1,197
Goodwill	$ 0	$ 1,814	$ 2,782
Minimum
Goodwill [Line Items]
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount	9.00%
Maximum
Goodwill [Line Items]
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount	42.00%
Measurement Input, Discount Rate [Member] | Goodwill | Income Approach Valuation Technique | Minimum
Goodwill [Line Items]
Fair Value Inputs	9.50%
Measurement Input, Discount Rate [Member] | Goodwill | Income Approach Valuation Technique | Maximum
Goodwill [Line Items]
Fair Value Inputs	11.50%
Crude Gathering [Member]
Goodwill [Line Items]
Goodwill	$ 1,100
Crude Gathering [Member] | Measurement Input, Discount Rate [Member] | Goodwill | Income Approach Valuation Technique
Goodwill [Line Items]
Fair Value Inputs	7.80%

Goodwill and Intangibles (Recon

Goodwill and Intangibles (Reconciliation of Goodwill) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Gross goodwill		$ 10,798		$ 10,146
Accumulated impairment losses		(3,141)		(130)
Goodwill, beginning balance		9,536	$ 10,016
Impairment losses		(1,814)	(1,197)
Acquisitions	[1]		717
Goodwill, ending balance		7,657	9,536
Western Refining Wholesale & Western Refining Product Transport [Member]
Goodwill [Line Items]
Goodwill, Written off Related to Sale of Business Unit		65
Logistics and Storage [Member]
Goodwill [Line Items]
Gross goodwill		7,657		7,234
Accumulated impairment losses		0		0
Goodwill, beginning balance		7,722	7,234
Impairment losses		0	0
Acquisitions	[1]		488
Goodwill, ending balance		7,657	7,722
Logistics and Storage [Member] | Western Refining Wholesale & Western Refining Product Transport [Member]
Goodwill [Line Items]
Goodwill, Written off Related to Sale of Business Unit		65
Gathering and Processing [Member]
Goodwill [Line Items]
Gross goodwill		3,141		2,912
Accumulated impairment losses		(3,141)		$ (130)
Goodwill, beginning balance		1,814	2,782
Impairment losses		(1,814)	(1,197)
Acquisitions	[1]		229
Goodwill, ending balance		0	$ 1,814
Gathering and Processing [Member] | Western Refining Wholesale & Western Refining Product Transport [Member]
Goodwill [Line Items]
Goodwill, Written off Related to Sale of Business Unit		$ 0
[1]	Acquisitions in 2019 are inclusive of measurement period adjustments related to the Merger and Mt. Airy Terminal acquisition.

Goodwill and Intangibles (Intan

Goodwill and Intangibles (Intangible Assets) (Details) - USD ($) $ in Millions		Oct. 01, 2018	Dec. 31, 2020		Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross			$ 1,571		$ 1,571
Accumulated amortization	[1]		(612)		(301)
Intangibles, net			959		1,270
Logistics and Storage [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets			36		31
Gross			283		283
Accumulated amortization	[1]		(81)		(45)
Intangibles, net			$ 202		238
Logistics and Storage [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			6 years
Logistics and Storage [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			8 years
Gathering and Processing [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets			$ 98		127
Gross			1,288		1,288
Accumulated amortization	[1]		(531)	[2]	(256)
Intangibles, net			$ 757		$ 1,032
Gathering and Processing [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			6 years
Gathering and Processing [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			25 years
Andeavor Logistics [Member]
Finite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			9 years
Andeavor Logistics [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		2 years
Andeavor Logistics [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		10 years
[1]	Amortization expense attributable to the G&P segment for the years ended December 31, 2020 and 2019 was $98 million and $127 million, respectively. Amortization expense attributable to the L&S segment for the year ended December 31, 2020 and 2019 was $36 million and $31 million, respectively.
[2]	Impairment charge of $177 million is included within the G&P accumulated amortization.

Goodwill and Intangibles (Futur

Goodwill and Intangibles (Future Amortization Expense) (Details) $ in Millions	Dec. 31, 2020USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2018	$ 127
2019	127
2020	127
2021	127
2022	113
Thereafter	338
Total future intangibles amortization expenses	$ 959

Fair Value Measurements - Recur

Fair Value Measurements - Recurring (Financial Instruments by Valuation Hierarchy) (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 0	$ 0
Liabilities	(63)	(60)
Embedded derivatives in commodity contracts | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Liabilities	$ (63)	$ (60)

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring (Significant Unobservable Inputs in Level 3 Valuation) (Details)	12 Months Ended
	Dec. 31, 2020USD ($)$ / gal
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, Average Forward Price | $ / gal	0.59
Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Embedded Derivative Renewal Term	5 years
Embedded Derivative Second Renewal Term	5 years
Significant unobservable inputs (Level 3) | Commodity contracts | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value inputs commodity price	$ 0.47
Significant unobservable inputs (Level 3) | Commodity contracts | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value inputs commodity price	$ 1.09
Significant unobservable inputs (Level 3) | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Probability of Renewal	100.00%
Fair Value Inputs Probability of Renewal Second Term	100.00%

Fair Value Measurements - Rec_3

Fair Value Measurements - Recurring (Changes in Level 3 Measurements) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Commodity Derivative Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value at beginning of period		$ 0	$ 0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	[1]	0	0
Settlements		0	0
Fair value at end of period		0	0
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period		0	0
Embedded Derivatives in Commodity Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value at beginning of period		(60)	(61)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	[1]	(9)	(5)
Settlements		6	6
Fair value at end of period		(63)	(60)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period		$ (4)	$ (5)
[1]	Gains and losses on commodity derivatives classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
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)	$ 22,951	$ 21,054
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including amounts due within one year)	$ 20,244	$ 19,800

Derivative Financial Instrume_3

Derivative Financial Instruments (Embedded Derivatives in Commodity Contracts) (Details) - Embedded derivative in natural gas processing and purchase contract [Member] - Embedded derivatives in commodity contracts $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Derivative [Line Items]
Number of renewals	2
Term of counterparty option to renew gas purchase agreement	5 years
Embedded derivative fair value of embedded derivative liability including inception value allocable to host contract	$ 63	$ 60
Embedded Derivative Second Renewal Term	5 years

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivatives Balance Sheet Location) (Details) - Commodity contracts - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	[1]	$ 0	$ 0
Derivative liability, fair value, gross liability	[1]	(63)	(60)
Not Designated as Hedging Instrument | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset		0	0
Not Designated as Hedging Instrument | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability		(7)	(5)
Not Designated as Hedging Instrument | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset		0	0
Not Designated as Hedging Instrument | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability		$ (56)	$ (55)
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume_5

Derivative Financial Instruments (Derivatives Income Statement Location) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative loss	$ (9)	$ (5)	$ (3)
Product sales
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains	0	0	4
Total derivative loss	0	0	6
Purchased product costs
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains	(6)	(6)	(12)
Total derivative loss	(9)	(5)	(9)
Not Designated as Hedging Instrument | Product sales
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	0	0	2
Not Designated as Hedging Instrument | Purchased product costs
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	$ (3)	$ 1	$ 3

Debt (Summary of Outstanding Bo

Debt (Summary of Outstanding Borrowings) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Oct. 15, 2020	Aug. 31, 2020	Aug. 28, 2020	Aug. 18, 2020	Dec. 31, 2019	Sep. 24, 2019	Sep. 23, 2019	Sep. 09, 2019	Jul. 30, 2019
Debt Instrument [Line Items]
Finance Lease, Liability		$ 11
Debt and capital lease obligations		20,536					$ 20,119
Unamortized debt issuance costs		116					106
Unamortized discount		281					300
Amounts due within one year		764					9
Total long-term debt due after one year		19,375					19,704
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Outstanding balance		175					0
Term loan facility due 2019
Debt Instrument [Line Items]
Outstanding balance		0				$ 1,000	1,000
Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross										$ 2,000
Senior Notes | Floating Rate Senior Notes Due September 2021 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		0					1,000			1,000
Senior Notes | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		1,000					1,000			$ 1,000
Finance Lease [Member]
Debt Instrument [Line Items]
Finance Lease, Liability	[1]	$ 11					19
MPLX LP | Line of Credit | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Outstanding balance							0
MPLX LP | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross						$ 3,000			$ 3,060
MPLX LP | Senior Notes | Floating Rate Senior Notes Due September 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Sep. 9, 2021
MPLX LP | Senior Notes | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Sep. 9, 2022
MPLX LP | Senior Notes | Senior Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	$ 300				266		$ 266
Debt Instrument, Maturity Date		Oct. 15, 2022
Interest rate, stated percentage		6.25%	6.25%						6.25%
MPLX LP | Senior Notes | Senior Notes Due December 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 486					486		$ 486
Debt Instrument, Maturity Date		Dec. 1, 2022
Interest rate, stated percentage									3.50%
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 500					500
Debt Instrument, Maturity Date		Mar. 15, 2023
Interest rate, stated percentage		3.375%
MPLX LP | Senior Notes | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 989					989
Debt Instrument, Maturity Date		Jul. 15, 2023
Interest rate, stated percentage		4.50%
MPLX LP | Senior Notes | Senior Notes Due May 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		$ 450			381		$ 381
Debt Instrument, Maturity Date		May 1, 2024
Interest rate, stated percentage		6.375%			6.375%				6.375%
MPLX LP | Senior Notes | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,149					1,149
Debt Instrument, Maturity Date		Dec. 1, 2024
Interest rate, stated percentage		4.875%
MPLX LP | Senior Notes | Senior Notes Due January 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 708					708		$ 708
Debt Instrument, Maturity Date		Jan. 15, 2025
Interest rate, stated percentage		5.25%							5.25%
MPLX LP | Senior Notes | Senior Notes Due February 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 500					500
Debt Instrument, Maturity Date		Feb. 15, 2025
Interest rate, stated percentage		4.00%
MPLX LP | Senior Notes | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,189					1,189
Debt Instrument, Maturity Date		Jun. 1, 2025
Interest rate, stated percentage		4.875%
MPLX LP | Senior Notes | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,250					1,250
Debt Instrument, Maturity Date		Mar. 1, 2027
Interest rate, stated percentage		4.125%
MPLX LP | Senior Notes | Senior Notes Due December 2027 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 732					732		$ 732
Debt Instrument, Maturity Date		Dec. 1, 2027
Interest rate, stated percentage		4.25%							4.25%
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,250					1,250
Debt Instrument, Maturity Date		Mar. 15, 2028
Interest rate, stated percentage		4.00%
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 750					750
Debt Instrument, Maturity Date		Feb. 15, 2029
Interest rate, stated percentage		4.80%
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,750					1,750
Debt Instrument, Maturity Date		Apr. 15, 2038
Interest rate, stated percentage		4.50%
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,000					1,000
Debt Instrument, Maturity Date		Mar. 1, 2047
Interest rate, stated percentage		5.20%
MPLX LP | Senior Notes | Senior Notes Due December 2047 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 487					487		$ 487
Debt Instrument, Maturity Date		Dec. 1, 2047
Interest rate, stated percentage		5.20%							5.20%
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,500					1,500
Debt Instrument, Maturity Date		Apr. 15, 2048
Interest rate, stated percentage		4.70%
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,500					1,500
Debt Instrument, Maturity Date		Feb. 15, 2049
Interest rate, stated percentage		5.50%
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 500					500
Debt Instrument, Maturity Date		Apr. 15, 2058
Interest rate, stated percentage		4.90%
MPLX LP | Senior Notes | Senior Notes Due August 2030 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,500					0
Debt Instrument, Maturity Date		Aug. 15, 2030
Interest rate, stated percentage		2.65%
MPLX LP | Senior Notes | Senior Notes Due March 2026 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,500					0
Debt Instrument, Maturity Date		Mar. 1, 2026
Interest rate, stated percentage		1.75%
MarkWest | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 23					23
ANDX LP [Member] | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 87					$ 190	$ 690	$ 3,060		$ 3,750
ANDX LP [Member] | Senior Notes | Senior Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 34
ANDX LP [Member] | Senior Notes | Senior Notes Due May 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, gross					$ 69
[1]	See Note 22 for lease information.

Debt (Summary of Outstanding _2

Debt (Summary of Outstanding Borrowings Interest Rates and Table Due Dates) (Details)	12 Months Ended
	Dec. 31, 2020	Oct. 15, 2020	Aug. 28, 2020	Sep. 23, 2019	Jul. 30, 2019
Term loan facility due 2019
Debt Instrument [Line Items]
Long-term Debt, Maturity Date	Sep. 26, 2021
MPLX LP | Line of Credit | MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Expiration date	Jul. 30, 2024
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 9, 2021
MPLX LP | Senior Notes | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Sep. 9, 2022
MPLX LP | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	6.25%	6.25%		6.25%
Debt Instrument, Maturity Date	Oct. 15, 2022
MPLX LP | Senior Notes | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Interest rate, stated percentage				3.50%
Debt Instrument, Maturity Date	Dec. 1, 2022
MPLX LP | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%
Debt Instrument, Maturity Date	Mar. 15, 2023
MPLX LP | Senior Notes | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Debt Instrument, Maturity Date	Mar. 15, 2028
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.80%
Debt Instrument, Maturity Date	Feb. 15, 2029
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Debt Instrument, Maturity Date	Apr. 15, 2038
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Interest rate, stated percentage	5.20%
Debt Instrument, Maturity Date	Mar. 1, 2047
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.70%
Debt Instrument, Maturity Date	Apr. 15, 2048
MPLX LP | Senior Notes | Senior Notes Due November 2049 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2049
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.90%
Debt Instrument, Maturity Date	Apr. 15, 2058
MPLX LP | Senior Notes | Senior Notes Due May 2024 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	6.375%		6.375%	6.375%
Debt Instrument, Maturity Date	May 1, 2024
MPLX LP | Senior Notes | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MPLX LP | Senior Notes | Senior Notes Due January 2025 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.25%			5.25%
Debt Instrument, Maturity Date	Jan. 15, 2025
MPLX LP | Senior Notes | Senior Notes Due February 2025 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
MPLX LP | Senior Notes | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MPLX LP | Senior Notes | Senior Notes Due March 2027 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.125%
Debt Instrument, Maturity Date	Mar. 1, 2027
MPLX LP | Senior Notes | Senior Notes Due December 2027 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.25%			4.25%
Debt Instrument, Maturity Date	Dec. 1, 2027
MPLX LP | Senior Notes | Senior Notes Due December 2047 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.20%			5.20%
Debt Instrument, Maturity Date	Dec. 1, 2047
MPLX LP | Senior Notes | Senior Notes Due March 2026 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	1.75%
Debt Instrument, Maturity Date	Mar. 1, 2026
MPLX LP | Senior Notes | Senior Notes Due August 2030 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	2.65%
Debt Instrument, Maturity Date	Aug. 15, 2030
MarkWest | Senior Notes | Senior Notes Due July 2023 [Member] | Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest | Senior Notes | Senior Notes Due June 2025 [Member] | Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
ANDX LP [Member] | Senior Notes | Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage					3.50%
ANDX LP [Member] | Senior Notes | Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage					6.375%
ANDX LP [Member] | Senior Notes | 4.000% senior notes due February 2025 | Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.50%
Debt Instrument, Maturity Date	Dec. 1, 2022
ANDX LP [Member] | Senior Notes | Senior Notes Due January 2025 [Member] | Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	5.25%
ANDX LP [Member] | Senior Notes | Senior Notes Due December 2047 [Member] | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Dec. 1, 2047
ANDX LP [Member] | Senior Notes | Senior Notes Due October 2019 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage				5.50%

Debt (Schedule of Debt Payments

Debt (Schedule of Debt Payments) (Detail) $ in Millions	Dec. 31, 2020USD ($)
Debt Disclosure [Abstract]
2020	$ 753
2021	1,502
2022	1,502
2023	1,326
2024	$ 1,701

Debt (Credit Agreements) (Detai

Debt (Credit Agreements) (Detail) $ in Millions	Sep. 14, 2020USD ($)	Sep. 26, 2019USD ($)	Sep. 09, 2019USD ($)	Jul. 30, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Aug. 18, 2020USD ($)	Sep. 24, 2019USD ($)	Sep. 23, 2019USD ($)	Jul. 29, 2019USD ($)
Debt Instrument [Line Items]
Proceeds from long-term lines of credit					$ 6,810	$ 9,174	$ 13,476
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					175	0
MPLX Revolving Credit Facility due July 2024 [Member] | Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity				300
MPLX Revolving Credit Facility due July 2024 [Member] | Swingline Loan
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 150
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP
Debt Instrument [Line Items]
Proceeds from long-term lines of credit					$ 3,815
Interest rate, effective percentage					1.49%
Debt Instrument, Description of Variable Rate Basis				Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA				4
Repayments of long-term lines of credit					$ 3,640
Letters of credit outstanding, amount					1
Remaining borrowing capacity					$ 3,325
Remaining borrowing capacity, percentage					95.00%
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP | Maximum
Debt Instrument [Line Items]
Additional borrowing capacity				$ 1,000
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)				5
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)				5.5
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP | Minimum
Debt Instrument [Line Items]
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)				1
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)				1
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP | Letter of Credit
Debt Instrument [Line Items]
Number of renewals				2
Debt Instrument, Renewal Term				1 year
Debt Instrument, Second Renewal Term [Line Items]				1 year
Term loan facility due 2019
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 1,000
Long-term debt - repayments					$ 1,000
Debt Instrument, Description of Variable Rate Basis		(i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement)
Long-term Line of Credit					0	1,000		$ 1,000
Extinguishment of Debt, Gain (Loss), Net of Tax					1
MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Proceeds from long-term lines of credit						$ 5,310
Interest rate, effective percentage						3.547%
Repayments of long-term lines of credit						$ 5,310
Letters of credit outstanding, amount						1
Remaining borrowing capacity						$ 3,500
Remaining borrowing capacity, percentage						100.00%
ANDX Bank Revolving Credit Facilities [Member] | Andeavor Logistics [Member]
Debt Instrument [Line Items]
Proceeds from long-term lines of credit						$ 864
Interest rate, effective percentage						4.129%
Repayments of long-term lines of credit						$ 2,100
Line of Credit Facility, Current Borrowing Capacity											$ 2,100
Line of Credit | MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Long-term Line of Credit						0
Senior Notes
Debt Instrument [Line Items]
Long-term debt, outstanding			$ 2,000
Senior Notes | MPLX LP
Debt Instrument [Line Items]
Long-term debt, outstanding								$ 3,000		$ 3,060
Senior Notes | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term debt, outstanding				$ 3,750	87	190			$ 690	$ 3,060
Senior Notes | Floating Rate Senior Notes Due September 2021 [Member]
Debt Instrument [Line Items]
Long-term debt - repayments	$ 1,000
Debt Instrument, Description of Variable Rate Basis			LIBOR plus 0.9 percent per annum
Long-term debt, outstanding			$ 1,000		0	$ 1,000
Extinguishment of Debt, Gain (Loss), Net of Tax					$ 3

Debt (Senior Notes) (Details)

Debt (Senior Notes) (Details) - USD ($) $ in Thousands

Sep. 14, 2020

Oct. 15, 2019

Sep. 09, 2019

Jul. 30, 2019

Dec. 31, 2020

Jan. 15, 2021

Oct. 15, 2020

Aug. 31, 2020

Aug. 28, 2020

Aug. 18, 2020

Dec. 31, 2019

Sep. 24, 2019

Sep. 23, 2019

Senior Notes

Debt Instrument [Line Items]

Long-term debt, gross

$ 2,000,000

Percent of par

100.00%

Senior Notes | Floating Rate Senior Notes Due September 2021 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,000,000

$ 0

$ 1,000,000

Extinguishment of Debt, Gain (Loss), Net of Tax

(3,000)

Debt Instrument, Description of Variable Rate Basis

LIBOR plus 0.9 percent per annum

Long-term debt - repayments

$ 1,000,000

Senior Notes | Floating Rate Senior Notes Due September 2022 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,000,000

1,000,000

1,000,000

Debt Instrument, Description of Variable Rate Basis

LIBOR plus 1.1 percent per annum

Senior Notes | Senior Notes Due May 2024 [Member]

Debt Instrument [Line Items]

Extinguishment of Debt, Gain (Loss), Net of Tax

(18,000)

MPLX LP | MPLX Revolving Credit Facility due July 2024 [Member]

Debt Instrument [Line Items]

Debt Instrument, Description of Variable Rate Basis

Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin

MPLX LP | Senior Notes

Debt Instrument [Line Items]

Long-term debt, gross

$ 3,000,000

$ 3,060,000

MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 750,000

750,000

Interest rate, stated percentage

4.80%

MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,500,000

1,500,000

Interest rate, stated percentage

5.50%

MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 500,000

500,000

Interest rate, stated percentage

3.375%

MPLX LP | Senior Notes | 4.125% senior notes due March 2027

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,250,000

1,250,000

Interest rate, stated percentage

4.125%

MPLX LP | Senior Notes | 5.200% senior notes due March 2047

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,000,000

1,000,000

Interest rate, stated percentage

5.20%

MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,250,000

1,250,000

Interest rate, stated percentage

4.00%

MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,750,000

1,750,000

Interest rate, stated percentage

4.50%

MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,500,000

1,500,000

Interest rate, stated percentage

4.70%

MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 500,000

500,000

Interest rate, stated percentage

4.90%

MPLX LP | Senior Notes | Senior Notes Due October 2022 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 0

$ 300,000

266,000

$ 266,000

Interest rate, stated percentage

6.25%

6.25%

6.25%

MPLX LP | Senior Notes | Senior Notes Due December 2022 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 486,000

486,000

$ 486,000

Interest rate, stated percentage

3.50%

MPLX LP | Senior Notes | Senior Notes Due May 2024 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 0

$ 450,000

381,000

$ 381,000

Interest rate, stated percentage

6.375%

6.375%

6.375%

Percent of par

103.20%

MPLX LP | Senior Notes | Senior Notes Due January 2025 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 708,000

708,000

$ 708,000

Interest rate, stated percentage

5.25%

5.25%

MPLX LP | Senior Notes | Senior Notes Due January 2025 [Member] | Subsequent Event

Debt Instrument [Line Items]

Long-term debt, gross

$ 750,000

Interest rate, stated percentage

525.00%

Percent of par

10262.50%

MPLX LP | Senior Notes | Senior Notes Due December 2027 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 732,000

732,000

$ 732,000

Interest rate, stated percentage

4.25%

4.25%

MPLX LP | Senior Notes | Senior Notes Due December 2047 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 487,000

487,000

$ 487,000

Interest rate, stated percentage

5.20%

5.20%

MPLX LP | Senior Notes | Senior Notes Due March 2026 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,500,000

0

Interest rate, stated percentage

1.75%

Percent of par

99.785%

MPLX LP | Senior Notes | Senior Notes Due August 2030 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 1,500,000

0

Interest rate, stated percentage

2.65%

Percent of par

99.913%

ANDX LP [Member] | Senior Notes

Debt Instrument [Line Items]

Long-term debt, gross

$ 3,750,000

$ 87,000

190,000

$ 690,000

$ 3,060,000

ANDX LP [Member] | Senior Notes | Senior Notes Due October 2019 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 500,000

Interest rate, stated percentage

5.50%

Long-term debt - repayments

$ 500,000

Interest Costs Incurred

$ 13,750

ANDX LP [Member] | Senior Notes | Senior Notes Due October 2022 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 34,000

ANDX LP [Member] | Senior Notes | Senior Notes Due May 2024 [Member]

Debt Instrument [Line Items]

Long-term debt, gross

$ 69,000

ANDX LP [Member] | Senior Notes | Senior Notes Due January 2025 [Member] | Subsequent Event

Debt Instrument [Line Items]

Long-term debt, gross

$ 42,000

MarkWest | Senior Notes

Debt Instrument [Line Items]

Long-term debt, gross

23,000

$ 23,000

Debt Premium [Member] | Senior Notes | Senior Notes Due October 2022 [Member]

Debt Instrument [Line Items]

Extinguishment of Debt, Gain (Loss), Net of Tax

4,000

Debt Premium [Member] | Senior Notes | Senior Notes Due May 2024 [Member]

Debt Instrument [Line Items]

Extinguishment of Debt, Gain (Loss), Net of Tax

$ (14,000)

Minimum | ANDX LP [Member] | Senior Notes

Debt Instrument [Line Items]

Interest rate, stated percentage

3.50%

Minimum | ANDX LP [Member] | Senior Notes | Senior Notes Due December 2022 [Member]

Debt Instrument [Line Items]

Interest rate, stated percentage

3.50%

Maximum | ANDX LP [Member] | Senior Notes

Debt Instrument [Line Items]

Interest rate, stated percentage

6.375%

Maximum | ANDX LP [Member] | Senior Notes | Senior Notes Due January 2025 [Member]

Debt Instrument [Line Items]

Interest rate, stated percentage

5.25%

Letter of Credit | MPLX LP | MPLX Revolving Credit Facility due July 2024 [Member]

Debt Instrument [Line Items]

Debt Instrument, Renewal Term

1 year

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										$ 6,896	$ 7,041	$ 5,449
Other Operating Income										(9)	(5)	(3)
Revenue Not from Contract with Customer, Other	[1]									673	2,000	1,556
Total revenues and other income		$ 2,249	$ 2,247	$ 2,081	$ 992	$ 2,316	$ 2,280	$ 2,210	$ 2,235	7,569	9,041	7,005
Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,397	2,498	1,856
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										3,580	3,455	2,404
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										155	140	220
Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										636	806	887
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[2]									636	806	882
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[3]									128	142	87
Logistics and Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										3,940	3,856	2,598
Logistics and Storage [Member] | Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										333	346	174
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										3,556	3,419	2,401
Logistics and Storage [Member] | Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										0	0	0
Logistics and Storage [Member] | Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[2]									39	65	12
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										12	26	11
Gathering and Processing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,956	3,185	2,851
Gathering and Processing [Member] | Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,064	2,152	1,682
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										24	36	3
Gathering and Processing [Member] | Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										155	140	220
Gathering and Processing [Member] | Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[2]									597	741	870
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										116	116	76
Other Income [Member]
Disaggregation of Revenue [Line Items]
Other Operating Income										$ 0	$ 0	$ 5
[1]	Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
[2]	G&P “Product sales” for the year ended December 31, 2018 was adjusted in the table above by $5 million related to derivative gains and mark-to-market adjustments. There were no adjustments for the years ended December 31, 2020 and 2019
[3]	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 2020, 2019 and 2018, these sales totaled $462 million, $1,120 million and $607 million, respectively.

Revenue Contract Balance Rollfo

Revenue Contract Balance Rollforward (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Beginning Balance Deferred Revenue, Noncurrent		$ 217
Ending Balance Deferred Revenue, Noncurrent		314	$ 217
Contract Liability, Noncurrent, Period Increase (Decrease) [Line Items]		6
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized		0	0
Accounting Standards Update 2014-09 [Member]
Beginning Balance Contract with Customer, Asset, Gross	[1]	39	36
Contract with Customer, Asset Increase (Decrease)		3	5
Contract with Customer, Asset, Reclassified to Receivable	[2]	(2)	(2)
Ending Balance Contract with Customer, Asset, Gross		40	39	[1]
Contract with Customer, Asset Increase (Decrease), Noncurrent		2
Contract with Customer, Asset, before Allowance for Credit Loss, Noncurrent		2	0	[1]
Beginning Balance Deferred Revenue, Current	[1]	23	13
Deferred Revenue, Period Increase (Decrease)		22	17
Deferred Revenue, Revenue Recognized	[2]	(8)	(7)
Ending Balance Deferred Revenue, Current		37	23	[1]
Beginning Balance Deferred revenue - related parties	[1]	53	65
Deferred Revenue - Related Party, Period Increase (Decrease)		121	55
Deferred Revenue - Related Parties, Revenue Recognized	[2]	(83)	(67)
Ending Balance Deferred Revenue, Current, Related Parties		91	53	[1]
Beginning Balance Deferred Revenue, Noncurrent	[1]	90	56
Deferred Revenue, Noncurrent, Period Increase (Decrease)		29	34
Deferred Revenue, Noncurrent, Revenue Recognized	[2]	0	0
Ending Balance Deferred Revenue, Noncurrent		119	90	[1]
Beginning Balance Deferred Revenue, Noncurrent, Related Parties	[1]	55	52
Deferred Revenue, Noncurrent, Related Party, Period Increase (Decrease)		(7)	3
Deferred Revenue, Noncurrent, Related Parties, Revenue Recognized	[2]	0	0
Ending Balance Deferred Revenue, Noncurrent, Related Parties		48	55	[1]
Contract with Customer, Liability, Noncurrent		$ 6	$ 0	[1]
[1]	Balance represents ASC 606 portion of each respective line item.
[2]	No significant revenue was recognized related to past performance obligations for the years ended December 31, 2020 and 2019.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	Dec. 31, 2020USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with Customer, Liability	$ 293
Revenue, Remaining Performance Obligation, Amount	11,474	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,891
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,822
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,666
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,540
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	4 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 4,555
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	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	9 Months Ended	12 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Supplemental Cash Flow Information [Abstract]
Interest paid (net of amounts capitalized)			$ 821	$ 835		$ 568
Income taxes paid			2	1		1
Operating Lease, Payments			87	85		0
Finance Lease, Interest Expense			1	1		0
Finance Lease, Principal Payments			9	5		0
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories			0	2		2
Sales-type Lease, Selling Profit (Loss)	$ 209	$ 21	0	21		0
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability			17	26		0
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability			1	4		0
Fair Value of Units Redeemed			340	0		0
Contribution - common units issued			$ 0	$ 7,722	[1]	$ 4,236	[1]
[1]	For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. For 2019, includes limited partner units issued to MPC and public unitholders as consideration in the Merger. See Note 4.

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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Information [Abstract]
(Decrease)/increase in capital accruals	$ (244)	$ (146)	$ 135

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	$ (15)	$ (15)	$ (16)
Other Comprehensive Income (Loss), Net of Tax	[2]	0	1
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(13)	(14)	(14)
Other Comprehensive Income (Loss), Net of Tax	[2]	1	0
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(2)	(1)	$ (2)
Other Comprehensive Income (Loss), Net of Tax	[2]	$ (1)	$ 1
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”
[2]	Components of other comprehensive loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation (Narr

Equity-Based Compensation (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation Expense
Total compensation expense		$ 14	$ 22	$ 24
Phantom Units
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 19.08	$ 32.62	$ 33.84
Outstanding Phantom and Performance Unit Awards
Non-forfeitable at December 31, 2020(1)	[1]	362,682
Unrecognized compensation cost		$ 2
Cost not yet recognized, period for recognition		1 year 9 months 18 days
Vested and expected to vest at December 31, 2020		640,885
Performance Shares
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 0.86	0.76
Outstanding Phantom and Performance Unit Awards
Unrecognized compensation cost		$ 1
Cost not yet recognized, period for recognition		2 years
Vested and expected to vest at December 31, 2020		142,831
Performance Units Market Condition [Member] | 2020 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 0.80
Performance Units Market Condition [Member] | 2019 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)			0.68
Performance Units Market Condition [Member] | 2018 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 0.45
Mplx Two Thousand Eighteen Incentive Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		16,000,000
Mplx 2012 Incentive Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		2,750,000
Maximum | Performance Unit Performance Condition [Member] | 2020 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 2
Maximum | Performance Unit Performance Condition [Member] | 2019 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)			$ 2
Maximum | Performance Unit Performance Condition [Member] | 2018 [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted(1)		$ 2
Officer | Phantom Units
Unit-based awards under the Plan
Award requisite service period		3 years
Accrued distributions		$ 3	$ 6
Officer | Performance Shares
Unit-based awards under the Plan
Award percentage paid out in cash		75.00%
Award percentage paid out in stock		25.00%
General and administrative expenses
Compensation Expense
Stock-based compensation plans expenses		$ 13	$ 10	$ 8
[1]	Represents a subset of phantom units held by our non-employee directors and certain of our officers and non-officer employees that are generally non-forfeitable and that would be paid out as common units upon the holder’s separation from service, or upon a predetermined date.

Equity-Based Compensation (Summ

Equity-Based Compensation (Summary of Phantom Unit Award Activity) (Detail) - Phantom Units - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Units
Outstanding at December 31, 2019		1,109,568
Granted		238,238
Settled		(686,382)
Forfeited		(17,401)
Outstanding at December 31, 2020		644,023	1,109,568
Vested and expected to vest at December 31, 2020		640,885
Non-forfeitable at December 31, 2020(1)	[1]	362,682
Weighted Average Fair Value
Outstanding at December 31, 2016 (usd per share)		$ 35.97
Grant date fair value of performance units granted(1)		19.08	$ 32.62	$ 33.84
Settled (usd per share)		37.04
Forfeited (usd per share)		33.65
Outstanding at December 31, 2017 (usd per share)		28.65	$ 35.97
Vested and expected to vest at December 31, 2017 (usd per share)		28.69
Convertible at December 31, 2017 (usd per share)	[1]	$ 29.66
Aggregate Intrinsic Value
Vested and expected to vest at December 31, 2020		$ 14
Non-forfeitable at December 31, 2020(1)	[1]	$ 8
[1]	Represents a subset of phantom units held by our non-employee directors and certain of our officers and non-officer employees that are generally non-forfeitable and that would be paid out as common units upon the holder’s separation from service, or upon a predetermined date.

Equity-Based Compensation (Su_2

Equity-Based Compensation (Summary of Values Related To Vested and Unvested Restricted Stock Awards) (Details) - Phantom Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of units issued during the period	$ 12	$ 14	$ 18
Grant date fair value of performance units granted(1)	$ 19.08	$ 32.62	$ 33.84

Equity-Based Compensation (Su_3

Equity-Based Compensation (Summary of Performance Unit Award Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 19.08	$ 32.62	$ 33.84
Granted	238,238
Weighted Average Fair Value
Outstanding at December 31, 2016 (usd per share)	$ 35.97
Settled (usd per share)	37.04
Forfeited (usd per share)	33.65
Outstanding at December 31, 2017 (usd per share)	28.65	35.97
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 0.86	$ 0.76
Number of Units
Outstanding at December 31, 2019	2,157,347
Granted	2,147,211
Settled	(1,169,354)
Forfeited	(42,918)
Outstanding at December 31, 2020	3,092,286	2,157,347
Weighted Average Fair Value
Outstanding at December 31, 2016 (usd per share)	$ 0.84
Granted (usd per share)	0.86
Settled (usd per share)	0.90
Forfeited (usd per share)	0.84
Outstanding at December 31, 2017 (usd per share)	$ 0.83	$ 0.84

Equity-Based Compensation (Assu

Equity-Based Compensation (Assumptions) (Details) - Performance Shares - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.29%	2.51%
Look-back period	1.62 years	2.84 years
Expected volatility	22.32%	25.01%
Grant date fair value of performance units granted(1)	$ 0.86	$ 0.76

Leases Lessee Disclosures (Deta

Leases Lessee Disclosures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Leases, Rent Expense, Net			$ 89
Finance Lease, Liability, to be Paid, Year One	$ 3
Finance Lease, Liability, to be Paid, Year Two	2
Finance Lease, Liability, to be Paid, Year Three	2
Finance Lease, Liability, to be Paid, Year Four	1
Finance Lease, Liability, to be Paid, Year Five	1
Finance Lease, Liability, to be Paid, after Year Five	9
Finance Lease, Liability, Payment, Due	18
Finance Lease, Liability, Undiscounted Excess Amount	7
Operating Lease, Right-of-Use Asset	309		365
Operating Lease, Liability, Current	63		66
Finance Lease, Interest Expense	1		1	$ 0
Operating Lease, Liability, Noncurrent	244		302
Property, plant and equipment, gross	26,875		26,867
Accumulated depreciation	5,657	[1]	4,722
Property, plant and equipment, net	21,218		22,145
Finance Lease, Liability	11
Third Party
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Liability, to be Paid, Year One	76
Lessee, Operating Lease, Liability, to be Paid, Year Two	67
Lessee, Operating Lease, Liability, to be Paid, Year Three	57
Lessee, Operating Lease, Liability, to be Paid, Year Four	35
Lessee, Operating Lease, Liability, to be Paid, Year Five	19
Lessee, Operating Lease, Liability, to be Paid, after Year Five	115
Lessee, Operating Lease, Liability, to be Paid	369
Operating Lease, Right-of-Use Asset	309		365
Operating Lease, Liability, Current	63		66
Operating Lease, Cost	78		75
Finance Lease, Right-of-Use Asset, Amortization	3		5
Finance Lease, Interest Expense	1		1
Finance Lease Cost	4		6
Variable Lease, Cost	10		11
Short-term Lease, Cost	52		80
Lease, Cost	144		172
Operating Lease, Liability, Noncurrent	$ 244		$ 302
Operating Lease, Weighted Average Remaining Lease Term	8 years 14 days		8 years 7 months 2 days
Operating Lease, Weighted Average Discount Rate, Percent	4.33%		4.38%
Other current liabilities	$ 2		$ 9
Long-term debt	$ 9		$ 10
Finance Lease, Weighted Average Remaining Lease Term	16 years 10 months 13 days		10 years 1 month 28 days
Finance Lease, Weighted Average Discount Rate, Percent	5.95%		5.87%
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	$ 62
Operating Lease, Liability	307		$ 368
Finance Lease, Liability	11		19
Marathon Petroleum Corporation [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Liability, to be Paid, Year One	14
Lessee, Operating Lease, Liability, to be Paid, Year Two	14
Lessee, Operating Lease, Liability, to be Paid, Year Three	14
Lessee, Operating Lease, Liability, to be Paid, Year Four	14
Lessee, Operating Lease, Liability, to be Paid, Year Five	14
Lessee, Operating Lease, Liability, to be Paid, after Year Five	591
Lessee, Operating Lease, Liability, to be Paid	661
Operating Lease, Right-of-Use Asset	231		232
Operating Lease, Liability, Current	1		1
Operating Lease, Cost	14		14
Finance Lease, Right-of-Use Asset, Amortization	0		0
Finance Lease, Interest Expense	0		0
Finance Lease Cost	0		0
Variable Lease, Cost	1		1
Short-term Lease, Cost	0		0
Lease, Cost	15		15
Operating Lease, Liability, Noncurrent	$ 229		$ 230
Operating Lease, Weighted Average Remaining Lease Term	46 years 2 months 23 days		47 years 2 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent	5.80%		5.80%
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	$ 431
Operating Lease, Liability	$ 230		$ 231
Lease Agreements, Lessee [Member] | Maximum
Lessee, Lease, Description [Line Items]
Term Of Agreements	58 years
Renewal Term Agreement	50 years
Lease Agreements, Lessee [Member] | Minimum
Lessee, Lease, Description [Line Items]
Term Of Agreements	1 year
Renewal Term Agreement	1 year
Finance Lease [Member] | Third Party
Lessee, Lease, Description [Line Items]
Property, plant and equipment, gross	$ 17		46
Accumulated depreciation	8		19
Property, plant and equipment, net	$ 9		$ 27
[1]	The December 31, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below

Leases Lessor Disclosures (Deta

Leases Lessor Disclosures (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Lessor, Lease, Description [Line Items]
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount		$ 171	$ 29
Deferred Rent Receivables, Net			3
Lessor, Operating Lease, Payment to be Received, Year One				$ 1,105
Sales-type Lease, Selling Profit (Loss)		209	21	0		$ 21	$ 0
Lessor, Operating Lease, Payment to be Received, Year Two				1,094
Lessor, Operating Lease, Payment to be Received, Year Three				1,047
Lessor, Operating Lease, Payment to be Received, Year Four				989
Lessor, Operating Lease, Payment to be Received, Year Five				933
Lessor, Operating Lease, Payment to be Received, after Year Five				2,299
Operating Leases, Future Minimum Payments Receivable				7,467
Sales-type Lease, Lease Receivable		370	47
Sales-type Lease, Unguaranteed Residual Asset		$ 10
Operating Leases, Income Statement, Minimum Lease Revenue							$ 1,032
Property, plant and equipment, gross				26,875		26,867
Accumulated depreciation				5,657	[1]	4,722
Property, plant and equipment, net				21,218		22,145
Equipment Leased to Other Party
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				5,947		6,947
Accumulated depreciation				2,007		2,355
Property, plant and equipment, net				3,940		4,592
Equipment Leased to Other Party | Pipelines And Related Assets [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				834		745
Equipment Leased to Other Party | Refineries and related assets [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				1,680		2,320
Equipment Leased to Other Party | Terminals and related assets [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				1,276		943
Equipment Leased to Other Party | Barges and towing vessels [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				129		906
Equipment Leased to Other Party | Gas Gathering And Transmission Equipment And Facilities [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				990		980
Equipment Leased to Other Party | Processing, Fractionation And Storage Facilities [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				867		855
Equipment Leased to Other Party | Land Building Office Equipment And Other [Member]
Lessor, Lease, Description [Line Items]
Property, plant and equipment, gross				171		198
Third Party
Lessor, Lease, Description [Line Items]
Lessor, Operating Lease, Payment to be Received, Year One				186
Operating Lease, Lease Income	[2]			268		257
Sales-type Lease, Selling Profit (Loss)				0		0
Sales-type Lease, Interest Income				0		0
Sales-type Lease, Variable Lease Income				0		0
Lessor, Operating Lease, Payment to be Received, Year Two				181
Lessor, Operating Lease, Payment to be Received, Year Three				178
Lessor, Operating Lease, Payment to be Received, Year Four				174
Lessor, Operating Lease, Payment to be Received, Year Five				142
Lessor, Operating Lease, Payment to be Received, after Year Five				999
Operating Leases, Future Minimum Payments Receivable				1,860
Sales-type Lease, Lease Receivable				1
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year One				157
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Two				157
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Three				158
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Four				158
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Five				158
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, after Year Five				315
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received				1,103
Lessor, Operating Lease, Payment to be Received, Year One				919
Operating Lease, Lease Income	[2]			787		1,020
Sales-type Lease, Selling Profit (Loss)				0		0
Sales-type Lease, Interest Income				151		6
Sales-type Lease, Variable Lease Income				1		1
Lessor, Operating Lease, Payment to be Received, Year Two				913
Lessor, Operating Lease, Payment to be Received, Year Three				869
Lessor, Operating Lease, Payment to be Received, Year Four				815
Lessor, Operating Lease, Payment to be Received, Year Five				791
Lessor, Operating Lease, Payment to be Received, after Year Five				1,300
Operating Leases, Future Minimum Payments Receivable				5,607
Sales-type and Direct Financing Leases, Lease Receivable, Undiscounted Excess Amount				687
Sales-type Lease, Lease Receivable				416
Sales-type Lease, Unguaranteed Residual Asset			$ 6	$ 23		$ 7
Maximum | Lease Agreements, Lessor [Member]
Lessor, Lease, Description [Line Items]
Term Of Agreements				12 years
Renewal Term Agreement				10 years
Minimum | Lease Agreements, Lessor [Member]
Lessor, Lease, Description [Line Items]
Term Of Agreements				1 year
Renewal Term Agreement				1 year
[1]	The December 31, 2020 balance includes property, plant and equipment impairment charges recorded during the first quarter of 2020 as discussed below
[2]	These amounts are presented net of executory costs.

Commitments and Contingencies_2

Commitments and Contingencies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Commitments And Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 17	$ 19
Contractual commitments to acquire property, plant and equipment	156
Loss Contingency, Damages Sought, Value	187
Loss Contingency, Damages Paid, Value	4
Environmental Loss Contingency [Member] | Marathon Petroleum Corporation [Member]
Commitments And Contingencies [Line Items]
Receivable from MPC for indemnification of environmental costs	$ 0	$ 0
Indirect Ownership Interest | Bakken Pipeline System
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
Minimum | Minimum Committed Volume Contracts [Member]
Commitments And Contingencies [Line Items]
Term Of Agreements	4 years

Commitments and Contingencies_3

Commitments and Contingencies (Minimum Future Payments) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Transportation And Terminalling Agreements
Other Commitments [Line Items]
2020	$ 103
2021	117
2022	154
2023	146
2024	124
2025 and thereafter	421
Total	$ 1,065
Minimum Committed Volume Contracts [Member] | Minimum
Other Commitments [Line Items]
Term Of Agreements	4 years
Minimum Committed Volume Contracts [Member] | Maximum
Other Commitments [Line Items]
Term Of Agreements	12 years

Subsequent Events (Details)

Subsequent Events (Details) - Senior Notes - USD ($) $ in Millions	Jan. 15, 2021	Dec. 31, 2020	Aug. 18, 2020	Dec. 31, 2019	Sep. 23, 2019	Sep. 09, 2019
Subsequent Event [Line Items]
Long-term debt, gross						$ 2,000
MPLX LP [Member]
Subsequent Event [Line Items]
Long-term debt, gross			$ 3,000		$ 3,060
Senior Notes Due January 2025 [Member] | MPLX LP [Member]
Subsequent Event [Line Items]
Interest rate, stated percentage		5.25%			5.25%
Long-term debt, gross		$ 708		$ 708	$ 708
Senior Notes Due January 2025 [Member] | MPLX LP [Member] | Subsequent Event
Subsequent Event [Line Items]
Interest rate, stated percentage	525.00%
Long-term debt, gross	$ 750

Select Quarterly Financial Da_3

Select Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Quarterly Financial Data [Line Items]
Total revenues and other income		$ 2,249	$ 2,247	$ 2,081	$ 992	$ 2,316	$ 2,280	$ 2,210	$ 2,235	$ 7,569		$ 9,041		$ 7,005
(Loss)/income from operations		920	899	878	(2,486)	(346)	926	885	912	211		2,377		2,728
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		700	674	655	(2,716)	(573)	689	657	689	(687)		1,462		2,006
Net income (loss) attributable to MPLX LP		691	665	648	(2,724)	(581)	629	482	503	(720)	[1]	1,033	[1]	1,818	[1]
Distributions declared:
Total cash distributions declared										2,994		2,758		2,060
Total distributions declared		$ 744	$ 745	$ 746	$ 759	$ 747	$ 734	$ 734	$ 543
Limited Partners Common Units
Quarterly Financial Data [Line Items]
Net income (loss) attributable to MPLX LP	[1]									$ (842)		$ 910		$ 1,743
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.63	$ 0.61	$ 0.58	$ (2.60)	$ (0.58)	$ 0.61	$ 0.56	$ 0.61	$ (0.80)		$ 1		$ 2.29
Common - diluted (in USD per unit)		0.63	0.61	0.58	(2.60)	(0.58)	0.61	0.55	0.61	(0.80)		1		$ 2.29
Cash distributions declared per limited partner common unit		$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6875	$ 0.6775	$ 0.6675	$ 0.6575	$ 2.7500		$ 2.6900
Distributions declared:
Total cash distributions declared	[2]									$ 2,872		$ 2,635	[3]	$ 1,985
Public | Limited Partners Common Units
Distributions declared:
Total cash distributions declared		$ 269	$ 270	$ 270	$ 270	$ 270	$ 266	$ 261	$ 191
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Distributions declared:
Total cash distributions declared		445	445	445	458	446	438	431	332
Preferred Class A [Member]
Distributions declared:
Limited partners' distributions		20	20	21	20	20	20	21	20	81		81		75
Preferred Class B [Member]
Distributions declared:
Limited partners' distributions		$ 10	$ 10	$ 10	$ 11	$ 11	$ 10	$ 21	$ 0	$ 41		$ 42		$ 0
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	See Note 8 for distribution information.
[3]	Distributions on common units exclude $37.5 million of waived distributions for the year ended December 31, 2019, with respect to units held by MPC and its affiliates.

Uncategorized Items - mplx-2020

Label	Element	Value
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	$ 9,000,000