MPLX (MPLX) 10-Q 4 Aug 21 2021 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Jul. 30, 2021
Document Information [Line Items]
Document Transition Report	false
Document Quarterly Report	true
Title of 12(b) Security	Common Units Representing Limited Partnership Interests
Entity Incorporation, State or Country Code	DE
Document Type	10-Q
Document Period End Date	Jun. 30, 2021
Amendment Flag	false
Entity File Number	001-35714
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	0001552000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		1,025,624,328
Entity Address, Address Line One	200 E. Hardin Street,
Entity Address, City or Town	Findlay,
Entity Address, State or Province	OH
Entity Address, Postal Zip Code	45840
City Area Code	419
Local Phone Number	421-2414
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Security Exchange Name	NYSE
Entity Tax Identification Number	27-0005456

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020
Total revenues from contracts with customers		$ 1,896		$ 1,592		$ 3,758		$ 3,373
Operating Lease, Lease Income		99		98		198		194
Income/(loss) from equity method investments(1)	[1]	66		89		136	[2]	(1,095)	[3]
Other Operating Income		3		2		4		3
Related Party Transaction, Other Revenues from Transactions with Related Party		27		25		55		51
Revenues		2,395		2,081		4,734		3,073
Related Party Transaction, Purchases from Related Party		297		280		595		556
Depreciation and amortization	[4]	318		321		647		646
Goodwill and Intangible Asset Impairment		42		0		42		2,165
General and Administrative Expense		87		96		173		193
Taxes, Miscellaneous		34		30		66		61
Costs and Expenses		1,464		1,203		2,829		4,681
Interest And Other Financial Costs From Related Parties		2		1		2		4
Other Nonoperating Income (Expense)		19		16		46		32
Interest Expense, Debt, Excluding Amortization		195		206		393		417
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent, Total		931		878		1,905		(1,608)
Provision for income taxes		0		0		1		0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total		715		655		1,464		(2,061)
Less: Net income (loss) attributable to noncontrolling interest		9		7		18		15
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total		715		655		1,463		(2,061)
Limited partners' interest in net income (loss) attributable to MPLX LP		675		617		1,383		(2,138)
Net income (loss) attributable to MPLX LP		706	[5]	648	[5]	1,445	[5]	(2,076)
Rental cost of sales
Labor and Related Expense		32		33		64		68
Limited Partners Common Units [Member]
Net income (loss) attributable to MPLX LP		$ 675	[5]	$ 617	[5]	$ 1,383	[5]	$ (2,138)
Net income (loss) attributable to MPLX LP per limited partner unit:
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax		$ 0.66		$ 0.58		$ 1.34		$ (2.02)
Common - diluted (in USD per unit)		$ 0.66		$ 0.58		$ 1.34		$ (2.02)
Weighted average limited partner units outstanding:
Common - basic (in shares)		1,029		1,059		1,033		1,059
Common - diluted (in shares)		1,029		1,059		1,033		1,059
Service [Member]
Total revenues from contracts with customers		$ 578		$ 563		$ 1,167		$ 1,175
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		907		857		1,779		1,785
Service, Other [Member]
Total revenues from contracts with customers		76		22		153		61
Product [Member]
Total revenues from contracts with customers		304		120		586		289
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		31		30		73		63
Oil and Gas, Refining and Marketing [Member]
Cost of Goods and Services Sold		293		315		566		683
Natural Gas, Midstream [Member]
Cost of Goods and Services Sold		338		87		614		222
Series A Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		21		21		41		41
Net income (loss) attributable to MPLX LP		21	[5]	21	[5]	41	[5]	41
Series B Preferred Stock [Member] | Preferred Partner [Member]
Dividends, Preferred Stock		10		10		21		21
Net income (loss) attributable to MPLX LP		10	[5]	10	[5]	21	[5]	21
Marathon Petroleum Corporation [Member]
Sales-type Lease, Revenue		136		38		173		76
Operating Lease, Lease Income		168		237		410		471
Related Party Transaction, Other Revenues from Transactions with Related Party		11		10		23		20
Related Party Transaction, Purchases from Related Party		293		276		587		547
General and Administrative Expense		63		68		120		132
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties
Labor and Related Expense		23		41		62		87
Marathon Petroleum Corporation [Member] | Service [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		907		857		1,778		1,784
Marathon Petroleum Corporation [Member] | Product [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[6]	$ 31		$ 30		$ 73		$ 63
[1]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[2]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively.
[5]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[6]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and six months ended June 30, 2021, these sales totaled $177 million and $345 million, respectively. For the three and six months ended June 30, 2020, these sales totaled $52 million and $225 million, respectively.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Equity Method Investment, Other than Temporary Impairment	$ 6	$ 1,264	$ 6
Interest Costs Capitalized	$ 5	$ 10	$ 10	$ 23

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 715	$ 655	$ 1,463	$ (2,061)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax	[1]	0	0	(2)	(1)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total		715	655	1,461	(2,062)
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest [Abstract]
Less: Net income (loss) attributable to noncontrolling interest		9	7	18	15
Comprehensive Income (Loss), Net of Tax, Attributable to Parent, Total		$ 706	$ 648	$ 1,443	$ (2,077)
[1]	Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) shares in Millions, $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents		$ 8	$ 15
Receivables, net		538	452
Current Assets, Related Parties		614	677
Inventories		126	118
Other current assets		46	65
Total current assets		1,332	1,515
Equity method investments		4,033	4,036
Property, plant and equipment, net		20,352	21,218
Intangibles, net		896	959
Goodwill		7,657	7,657
Operating Lease, Right-of-Use Asset		297	309
Due from Related Parties, Noncurrent		1,144	672
Other noncurrent assets		62	48
Total assets		35,773	36,414
Current liabilities:
Accounts payable		158	152
Accrued Liabilities, Current		258	194
Due to Related Parties, Current		812	356
Accrued Property Plant and Equipment Current		54	84
Accrued interest payable		203	222
Operating Lease, Liability, Current		63	63
Other current liabilities		178	150
Total current liabilities		1,727	2,086
Long-term deferred revenue		349	314
Liabilities, Related Parties, Noncurrent		306	283
Long-term debt		19,234	19,375
Deferred income taxes		11	12
Operating Lease, Liability, Noncurrent		232	244
Debt, Current		1	764
Deferred credits and other liabilities		151	115
Total liabilities		22,010	22,429
Commitments and contingencies (see Note 21)
Equity
Total MPLX LP partners’ capital		12,552	12,772
Noncontrolling interests		243	245
Total equity		12,795	13,017
Total liabilities, preferred units and equity		35,773	36,414
Disposal Group, Including Discontinued Operation, Assets, Current		0	188
Disposal Group, Including Discontinued Operation, Liabilities, Current		0	101
Series A Convertible Preferred Units
Current liabilities:
Series A preferred units		968	968
Marathon Petroleum Corporation [Member]
Assets
Other current assets		2	1
Operating Lease, Right-of-Use Asset		230	231
Due from Related Parties, Noncurrent		32	32
Current liabilities:
Due to Related Parties, Current		689	215
Operating Lease, Liability, Current		1	1
Operating Lease, Liability, Noncurrent		$ 229	$ 229
Series B Preferred Stock [Member]
Current liabilities:
Units issued		0.6	0.6
Equity
Total MPLX LP partners’ capital		$ 611	$ 611
Units Outstanding		0.6	0.6
Limited Partners Common Units [Member] | Public [Member]
Current liabilities:
Units issued		380	391
Equity
Total MPLX LP partners’ capital		$ 9,061	$ 9,384
Units Outstanding		380	391
Limited Partners Common Units [Member] | Marathon Petroleum Corporation [Member]
Current liabilities:
Units issued		647	647
Equity
Total MPLX LP partners’ capital		$ 2,897	$ 2,792
Units Outstanding		647	647
LOOP and Explorer
Equity
Accumulated other comprehensive loss	[1]	$ (17)	$ (15)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Jun. 30, 2021	Dec. 31, 2020
Limited Partners Common Units [Member] | Public [Member]
Units issued	380	391
Units Outstanding	380	391
Limited Partners Common Units [Member] | Marathon Petroleum Corporation [Member]
Units issued	647	647
Units Outstanding	647	647
Series B Preferred Stock [Member]
Units issued	0.6	0.6
Units Outstanding	0.6	0.6

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		3 Months Ended		6 Months Ended				12 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2020
Operating activities:
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 715	$ 655	$ 1,463		$ (2,061)
Adjustments to reconcile net income/(loss) to net cash provided by operating activities:
Amortization of deferred financing costs				35		29
Depreciation and amortization	[1]	318	321	647		646
Goodwill and Intangible Asset Impairment		42	0	42		2,165
Deferred income taxes				(1)		(1)
(Income)/loss from equity method investments(1)	[2]	(66)	(89)	(136)	[3]	1,095	[4]
Distributions from unconsolidated affiliates				239		226
Changes in:
Current receivables				(83)		31
Inventories				(8)		(7)
Fair value of derivatives				39		(9)
Current accounts payable and accrued liabilities				77		(102)
Current assets/current liabilities - related parties				101		27
Increase (Decrease) in Other Operating Assets and Liabilities, Net				1		(1)
Deferred revenue				43		49
All other, net				29		26
Net cash provided by operating activities				2,489		2,114
Net Cash Provided by (Used in) Investing Activities [Abstract]
Additions to property, plant and equipment				(235)		(708)
Disposal of assets				74		43
Investments in unconsolidated affiliates				(84)		(222)
Distributions from unconsolidated affiliates - return of capital				0		110
Net cash used in investing activities				(245)		(777)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Long-term debt - borrowings				2,800		2,500
Long-term debt - repayments				(3,746)		(1,682)
Payments for Repurchase of Common Stock				(310)		0
Distributions to noncontrolling interests				(20)		(17)
Distributions to unitholders and general partner				(1,421)		(1,445)
Proceeds from Contributions from Parent				17		20
All other, net				(2)		(5)
Net cash used in financing activities				(2,251)		(1,285)
Net (decrease)/increase in cash, cash equivalents and restricted cash				(7)		52
Cash, cash equivalents and restricted cash at beginning of period				15		15		$ 15
Cash, cash equivalents and restricted cash at end of period		$ 8	$ 67	8		67		15
Loss on disposal of assets				1		1
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Related party debt - borrowings				4,435		2,708		6,264
Repayments of Related Party Debt				(3,942)		(3,302)		$ (6,858)
Series A Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders				(41)		(41)
Series B Preferred Stock [Member] | Preferred Partner [Member]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Distributions to preferred unitholders				$ (21)		$ (21)
[1]	Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively.
[2]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[3]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.

Consolidated Statements of Ca_2

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

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions		Total	LOOP and Explorer	Series B Preferred Stock [Member]	Public [Member]Limited Partners Common Units [Member]	Marathon Petroleum Corporation [Member]Limited Partners Common Units [Member]	AOCI Attributable to Parent [Member]	Noncontrolling Interest [Member]
Noncontrolling interests		$ 249
Total MPLX LP partners’ capital				$ 611	$ 10,800	$ 4,968
Accumulated other comprehensive loss			$ (15)
Beginning Balance at Dec. 31, 2019		16,613
Distributions to:		2,736		(11)	(1,022)	(1,733)	$ 0	$ (8)
Contributions from:		(225)		0	0	(225)	0	0
Unitholders and general partner		(738)		(21)	(271)	(446)	0	0
Noncontrolling interests		(9)		0	0	0	0	(9)
Other		1		0	2	0	(1)	0
Ending Balance at Mar. 31, 2020		13,356
Noncontrolling interests		248
Total MPLX LP partners’ capital				601	9,509	3,014
Accumulated other comprehensive loss			(16)
Distributions to:		(634)		(10)	229	388	0	(7)
Contributions from:		(6)		0	0	(6)	0	0
Unitholders and general partner		(728)		0	(270)	(458)	0	0
Noncontrolling interests		(8)		0	0	0	0	(8)
Other		2		0	1	1	0	0
Ending Balance at Jun. 30, 2020		13,262
Noncontrolling interests		247
Total MPLX LP partners’ capital				611	9,469	2,951
Accumulated other comprehensive loss	[1]		(16)
Noncontrolling interests		245
Total MPLX LP partners’ capital		12,772		611	9,384	2,792
Accumulated other comprehensive loss	[1]		(15)
Beginning Balance at Dec. 31, 2020		13,017
Distributions to:		(729)		(11)	266	443	0	(9)
Stock Repurchased and Retired During Period, Value		(155)		0	(155)	0	0	0
Contributions from:		(7)		0	0	(7)	0	0
Unitholders and general partner		(735)		(21)	(269)	(445)	0	0
Noncontrolling interests		(10)		0	0	0	0	(10)
Other		(3)		0	0	(1)	(2)	0
Ending Balance at Mar. 31, 2021		12,850
Beginning Balance at Dec. 31, 2020		13,017
Distributions to:				(21)
Stock Repurchased and Retired During Period, Value		(310)
Unitholders and general partner				(21)
Ending Balance at Jun. 30, 2021		12,795
Noncontrolling interests		244
Total MPLX LP partners’ capital				601	9,226	2,796
Accumulated other comprehensive loss			(17)
Beginning Balance at Mar. 31, 2021		12,850
Distributions to:		(693)		(10)	251	423	0	(9)
Stock Repurchased and Retired During Period, Value		(155)		0	(155)	0	0	0
Contributions from:		(122)		0	0	(122)	0	0
Unitholders and general partner		(707)		0	(262)	(445)	0	0
Noncontrolling interests		(10)		0	0	0	0	(10)
Other		2		0	1	1	$ 0	$ 0
Ending Balance at Jun. 30, 2021		12,795
Noncontrolling interests		243
Total MPLX LP partners’ capital		$ 12,552		$ 611	$ 9,061	$ 2,897
Accumulated other comprehensive loss	[1]		$ (17)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Description of the Business and

Description of the Business and Basis of Presentation	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (“WRW”) to Western Refining Southwest, Inc. (now known as Western Refining Southwest LLC) (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – During the first half of 2021, we continued to see improvements in the environment in which our business operates as COVID-19 impacts continue to subside. The increased availability of vaccinations and easing of COVID-19 restrictions has been followed by an increase in economic activity, however, we are unable to predict the potential effects a resurgence of COVID-19 may have on our financial position and results. In the second quarter of 2021, we recognized impairment expense of $42 million within our G&P segment related to our continued emphasis on portfolio optimization with the anticipated divestiture of several non-core assets and the closure of other non-core assets. During the first quarter of 2020, the overall deterioration in the economy and the environment in which MPLX and its customers operate, as well as a sustained decrease in unit price, were considered triggering events at that time resulting in impairments of the carrying value of certain assets. We recognized impairments related to goodwill, certain equity method investments and certain long-lived assets (including intangibles), within our G&P segment. Many of our producer customers refined and updated production forecasts in response to the environment at that time, which impacted their expected future demand for our services, including the future utilization of our assets. Additionally, certain of our contracts have commodity price exposure, including NGL prices, which experienced increased volatility as noted above. The table below provides information related to the impairments recognized during the first quarter of 2020 as well as the corresponding footnote where additional information can be found. (In millions) Impairment Footnote Reference Goodwill $ 1,814 12 Equity method investments 1,264 4 Intangibles, net 177 12 Property, plant and equipment, net 174 11 Total impairments $ 3,429 Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results to be expected for the full year. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.

Accounting Standards Accounting

Accounting Standards Accounting Standards	6 Months Ended
	Jun. 30, 2021
Accounting Changes and Error Corrections [Abstract]
Accounting Standards Update and Change in Accounting Principle [Text Block]	Accounting StandardsRecently AdoptedWe did not adopt any ASUs during the first six months of 2021 that are expected to have a material impact to our financial statements or financial statement disclosures

Acquisitions

Acquisitions	6 Months Ended
	Jun. 30, 2021
Business Combinations [Abstract]
Business Combination Disclosure [Text Block]	Acquisitions and Dispositions Sale of Javelina Assets and Liabilities On February 12, 2021, MarkWest Energy Operating Company, L.L.C., (“MarkWest Energy”) a wholly owned subsidiary of MPLX, completed the sale of all of MarkWest Energy’s equity interests in MarkWest Javelina Company L.L.C., MarkWest Javelina Pipeline Company L.L.C., and MarkWest Gas Services L.L.C. (collectively, “Javelina”) pursuant to the terms of an Equity Purchase Agreement entered into with a third party on December 23, 2020. The agreement included adjustments for working capital as well as an earnout provision based on the performance of the assets. No gain or loss was recorded on the sale. The estimated value of the earnout provision was recorded as a contingent asset shown within “Other noncurrent assets” on the Consolidated Balance Sheets as of June 30, 2021. Javelina’s assets and liabilities sold are shown on the Consolidated Balance Sheets as “Assets held for sale” and “Liabilities held for sale” for the year ended December 31, 2020. Prior to the sale, Javelina was reported within the G&P segment. Wholesale Exchange

Investments and Noncontrolling

Investments and Noncontrolling Interest	6 Months Ended
	Jun. 30, 2021
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests [Text Block]	Investments and Noncontrolling Interests The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at June 30, June 30, December 31, (In millions, except ownership percentages) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 460 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 257 254 LOOP LLC 41% 264 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 191 194 Minnesota Pipe Line Company, LLC 17% 185 188 Whistler Pipeline LLC (2) 38% 181 185 Explorer Pipeline Company 25% 65 72 W2W Holdings LLC (2)(3) 50% 70 72 Other (2) 109 103 Total L&S 1,782 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 690 698 Sherwood Midstream LLC (2) 50% 550 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 327 307 MarkWest Torñado GP, L.L.C. (2) 60% 213 188 Rendezvous Gas Services, L.L.C. (2) 78% 153 159 Sherwood Midstream Holdings LLC (2) 51% 141 148 Centrahoma Processing LLC 40% 131 145 Other (2) 46 49 Total G&P 2,251 2,251 Total $ 4,033 $ 4,036 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL. (2) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (3) Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest Wink to Webster Pipeline LLC. For those entities that have been deemed to be VIEs, neither MPLX nor any of its subsidiaries have been deemed to be the primary beneficiary due to voting rights on significant matters. While we have the ability to exercise influence through participation in the management committees which make all significant decisions, we have equal influence over each committee as a joint interest partner and all significant decisions require the consent of the other investors without regard to economic interest and as such we have determined that these entities should not be consolidated and apply the equity method of accounting with respect to our investments in each entity. Sherwood Midstream LLC (“Sherwood Midstream”) has been deemed the primary beneficiary of Sherwood Midstream Holdings LLC (Sherwood Midstream Holdings”) due to its controlling financial interest through its authority to manage the joint venture. As a result, Sherwood Midstream consolidates Sherwood Midstream Holdings. Therefore, MPLX also reports its portion of Sherwood Midstream Holdings’ net assets as a component of its investment in Sherwood Midstream. As of June 30, 2021, MPLX has a 24.55 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream. MPLX’s maximum exposure to loss as a result of its involvement with equity method investments includes its equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of its compensation received for the performance of the operating services. MPLX did not provide any financial support to equity method investments that it was not contractually obligated to provide during the six months ended June 30, 2021. During the first quarter of 2020, we recorded an other than temporary impairment for three joint ventures in which we have an interest as discussed in Note 1. Impairment of these investments was $1,264 million, of which $1,251 million was related to MarkWest Utica EMG, L.L.C. and its investment in Ohio Gathering Company, L.L.C. The impairment was recorded through “Income from equity method investments.” The impairments were largely due to a reduction in forecasted volumes gathered and processed by the systems operated by the joint ventures. Summarized financial information for MPLX’s equity method investments for the six months ended June 30, 2021 and 2020 is as follows: Six Months Ended June 30, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 336 $ 607 $ 943 Costs and expenses 217 284 501 Income from operations 119 323 442 Net income 105 283 388 Income from equity method investments (1) $ 73 $ 63 $ 136 (1) Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million. Six Months Ended June 30, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ (43) $ 640 $ 597 Costs and expenses 202 274 476 (Loss)/income from operations (245) 366 121 Net (loss)/income (283) 332 49 (Loss)/income from equity method investments (1) $ (1,178) $ 83 $ (1,095) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of June 30, 2021 and December 31, 2020 is as follows: June 30, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 355 $ 464 $ 819 Noncurrent assets 7,401 4,909 12,310 Current liabilities 183 281 464 Noncurrent liabilities $ 2,379 $ 862 $ 3,241 December 31, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 530 $ 318 $ 848 Noncurrent assets 6,889 4,997 11,886 Current liabilities 323 187 510 Noncurrent liabilities $ 1,904 $ 830 $ 2,734 As of June 30, 2021 and December 31, 2020, the underlying net assets of MPLX’s investees in the G&P segment exceeded the carrying value of its equity method investments by approximately $56 million and $57 million, respectively. As of June 30, 2021 and December 31, 2020, the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $332 million and $331 million, respectively.

Related Party Agreements and Tr

Related Party Agreements and Transactions	6 Months Ended
	Jun. 30, 2021
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions MPLX engages in transactions with both MPC and certain of its equity method investments as part of its normal business; however, transactions with MPC make up the majority of MPLX’s related party transactions. Transactions with related parties are further described below. MPLX has various long-term, fee-based commercial agreements with MPC. Under these agreements, MPLX provides transportation, terminal, fuels distribution, marketing, storage, management, operational and other services to MPC. MPC has committed to provide MPLX with minimum quarterly throughput volumes on crude oil and refined products and other fees for storage capacity; operating and management fees; as well as reimbursements for certain direct and indirect costs. MPC has also committed to provide a fixed fee for 100 percent of available capacity for boats, barges and third-party chartered equipment under the marine transportation service agreement. MPLX also has a keep-whole commodity agreement with MPC under which MPC pays us a processing fee for NGLs related to keep-whole agreements and delivers shrink gas to the producers on our behalf. We pay MPC a marketing fee in exchange for assuming the commodity risk. Additionally, MPLX has obligations to MPC for services provided to MPLX by MPC under omnibus and employee services-type agreements as well as other various agreements. Related Party Loan MPLX is party to a loan agreement with MPC Investment LLC (“MPC Investment”) (the “MPC Loan Agreement”). Under the terms of the agreement, MPC Investment extends loans to MPLX on a revolving basis as requested by MPLX and as agreed to by MPC Investment. The borrowing capacity of the MPC Loan Agreement is $1.5 billion aggregate principal amount of all loans outstanding at any one time. The loan agreement is scheduled to expire, and borrowings under the loan agreement are scheduled to mature and become due and payable on July 31, 2024, provided that MPC Investment may demand payment of all or any portion of the outstanding principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), at any time prior to maturity. Borrowings under the MPC Loan Agreement bear interest at LIBOR plus 1.25 percent or such lower rate as would be applicable to such loans under the MPLX Credit Agreement as discussed in Note 15. Activity on the MPC Loan Agreement was as follows: (In millions) Six Months Ended June 30, 2021 Year Ended December 31, 2020 Borrowings $ 4,435 $ 6,264 Average interest rate of borrowings 1.354 % 2.278 % Repayments $ 3,942 $ 6,858 Outstanding balance at end of period (1) $ 493 $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets. Related Party Revenue Related party sales to MPC consist of crude oil and refined products pipeline and trucking transportation services based on tariff/contracted rates; storage, terminal and fuels distribution services based on contracted rates; and marine transportation services. Related party sales to MPC also consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Service revenues - related parties MPC $ 907 $ 857 $ 1,778 $ 1,784 Other — — 1 1 Total Service revenue - related parties 907 857 1,779 1,785 Rental income - related parties MPC 168 237 410 471 Product sales - related parties (1) MPC 31 30 73 63 Sales-type lease revenue - related parties MPC 136 38 173 76 Other income - related parties MPC 11 10 23 20 Other 16 15 32 31 Total Other income - related parties $ 27 $ 25 $ 55 $ 51 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and six months ended June 30, 2021, these sales totaled $177 million and $345 million, respectively. For the three and six months ended June 30, 2020, these sales totaled $52 million and $225 million, respectively. Related Party Expenses MPC provides executive management services and certain general and administrative services to MPLX under the terms of our omnibus agreements (“Omnibus charges”). Omnibus charges included in “Rental cost of sales - related parties” primarily relate to services that support MPLX’s rental operations and maintenance of assets available for rent. Omnibus charges included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Omnibus charges included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. MPLX also obtains employee services from MPC under employee services agreements (“ESA charges”). ESA charges for personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties.” ESA charges for personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” ESA charges for personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses.” In addition to these agreements, MPLX purchases products from MPC, makes payments to MPC in its capacity as general contractor to MPLX, and has certain lease agreements with MPC. Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Rental cost of sales - related parties MPC $ 23 $ 41 $ 62 $ 87 Purchases - related parties MPC 293 276 587 547 Other 4 4 8 9 Total Purchases - related parties 297 280 595 556 General and administrative expenses MPC $ 63 $ 68 $ 120 $ 132 Some charges incurred under the omnibus and ESA agreements are related to engineering services and are associated with assets under construction. These charges are added to “Property, plant and equipment, net” on the Consolidated Balance Sheets. For the three and six months ended June 30, 2021, these charges totaled $15 million and $27 million, respectively. For the three and six months ended June 30, 2020, these charges totaled $15 million and $51 million, respectively. MPC has also been advancing certain strategic priorities to lay a foundation for long-term success, including plans to optimize its assets and structurally lower costs in 2021 and beyond. In 2020, MPC approved and executed an involuntary workforce reduction plan, which together with employee reductions resulting from MPC's indefinite idling of its Martinez, California and Gallup, New Mexico refineries, affected approximately 2,050 employees. All of the employees that conduct MPLX’s business are directly employed by affiliates of MPC, and certain of those employees were affected by MPC’s workforce reductions. During the third and fourth quarters of 2020, MPLX reimbursed MPC for $37 million related to severance and employee benefits related expenses that MPC recorded in connection with its workforce reductions. There were no such costs in the first six months of 2021. Related Party Assets and Liabilities Assets and liabilities with related parties appearing on the Consolidated Balance Sheets are detailed in the table below. This table identifies the various components of related party assets and liabilities, including those associated with leases (see Note 20 for additional information) and deferred revenue on minimum volume commitments. If MPC fails to meet its minimum committed volumes, MPC will pay MPLX a deficiency payment based on the terms of the agreement. The deficiency amounts are recorded as “Current liabilities - related parties.” In many cases, MPC may then apply the amount of any such deficiency payments as a credit for volumes in excess of its minimum volume commitment in future periods under the terms of the applicable agreements. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes in excess of minimum quarterly volume commitments, where it is probable the customer will not use the credit in future periods or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Current liabilities - related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized as revenue over the remaining term of the applicable agreements or in some cases as an equity contribution from its sponsor. (In millions) June 30, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 519 $ 615 Receivables - Other 8 27 Prepaid - MPC 12 4 Other - MPC 2 1 Lease Receivables - MPC 73 30 Total 614 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 844 386 Unguaranteed residual asset - MPC 38 23 Total 1,144 672 Current liabilities - related parties Payables - MPC 689 215 Payables - Other 31 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 51 66 Deferred revenue - Project reimbursements - MPC 39 30 Deferred revenue - Project reimbursements - Other 1 1 Total 812 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 229 Long-term deferred revenue - Project reimbursements - MPC 70 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 306 $ 283 Other Related Party Transactions From time to time, MPLX may also sell to or purchase from related parties, assets and inventory at the lesser of average unit cost or net realizable value. Sales to related parties for the six months ended June 30, 2021 and 2020 were $15 million and $4 million, respectively. Purchases from related parties were immaterial for the six months ended June 30, 2021 and 2020.

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
Earnings Per Share [Text Block]	Net Income/(Loss) Per Limited Partner Unit Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding. During the three and six months ended June 30, 2021 and 2020, MPLX had participating securities consisting of common units, certain equity-based compensation awards, Series A preferred units and Series B preferred units and had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three and six months ended June 30, 2021 and 2020 were less than 1 million. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Net income/(loss) attributable to MPLX LP $ 706 $ 648 $ 1,445 $ (2,076) Less: Distributions declared on Series A preferred units (1) 21 21 41 41 Distributions declared on Series B preferred units (1) 10 10 21 21 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 705 715 1,412 1,443 Undistributed net gain/(loss) attributable to MPLX LP $ (30) $ (98) $ (29) $ (3,581) (1) See Note 7 for distribution information. Three Months Ended June 30, 2021 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 705 $ 21 $ 10 $ 736 Undistributed net loss attributable to MPLX LP (30) — — (30) Net income attributable to MPLX LP (1) $ 675 $ 21 $ 10 $ 706 Weighted average units outstanding: Basic 1,029 Diluted 1,029 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.66 Diluted $ 0.66 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended June 30, 2020 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 715 $ 21 $ 10 $ 746 Undistributed net loss attributable to MPLX LP (98) — — (98) Net income attributable to MPLX LP (1) $ 617 $ 21 $ 10 $ 648 Weighted average units outstanding: Basic 1,059 Diluted 1,059 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.58 Diluted $ 0.58 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Six Months Ended June 30, 2021 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,412 $ 41 $ 21 $ 1,474 Undistributed net loss attributable to MPLX LP (29) — — (29) Net income attributable to MPLX LP (1) $ 1,383 $ 41 $ 21 $ 1,445 Weighted average units outstanding: Basic 1,033 Diluted 1,033 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.34 Diluted $ 1.34 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Six Months Ended June 30, 2020 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,443 $ 41 $ 21 $ 1,505 Undistributed net loss attributable to MPLX LP (3,581) — — (3,581) Net (loss)/income attributable to MPLX LP (1) $ (2,138) $ 41 $ 21 $ (2,076) Weighted average units outstanding: Basic 1,059 Diluted 1,059 Net income attributable to MPLX LP per limited partner unit: Basic $ (2.02) Diluted $ (2.02) (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

Equity

Equity	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Equity	Equity The changes in the number of common units outstanding during the six months ended June 30, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 183,111 Units redeemed in unit repurchase program (11,929,998) Balance at June 30, 2021 1,027,031,091 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 six months ended June 30, 2021, 11,929,998 public common units were repurchased at an average cost per unit of $26.02. Total cash paid for units repurchased during 2021 was $310 million with $657 million remaining available under the authorization for future repurchases as of June 30, 2021. As of June 30, 2021, we had agreements to acquire 126,293 additional common units for $4 million, which settled in July 2021. Wholesale Exchange In connection with the Wholesale Exchange as discussed in Note 3, 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. Series B Preferred Units MPLX has outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of MPLX with a price to the public of $1,000 per unit (the “Series B preferred units”). The Series B preferred units are pari passu with the Series A preferred units with respect to distribution rights and rights upon liquidation. Series B preferred unitholders are entitled to receive a fixed distribution of $68.75 per unit, per annum, payable semi-annually in arrears on the 15th day, or the first business day thereafter, of February and August of each year up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent, in each case assuming a distribution is declared by the Board of Directors. The changes in the Series B preferred unit balance from December 31, 2020 through June 30, 2021 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Series B preferred units”. (In millions) Series B Preferred Units Balance at December 31, 2020 $ 611 Net income allocated 21 Distributions received by Series B preferred unitholders (21) Balance at June 30, 2021 $ 611 Cash distributions – In accordance with the MPLX partnership agreement, on July 27, 2021, MPLX declared a quarterly cash distribution for the second quarter of 2021, totaling $705 million, or $0.6875 per common unit. This rate will also be received by Series A preferred unitholders. These distributions will be paid on August 13, 2021 to common unitholders of record on August 6, 2021. Additionally, in accordance with the distribution rights discussed above, MPLX will make a cash distribution totaling $21 million to Series B unitholders on August 16, 2021. Quarterly distributions for 2021 and 2020 are summarized below: (Per common unit) 2021 2020 March 31, $ 0.6875 $ 0.6875 June 30, $ 0.6875 $ 0.6875 The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three and six months ended June 30, 2021 and 2020. MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 705 $ 715 $ 1,412 $ 1,443 Series A preferred unit distributions 21 21 41 41 Series B preferred unit distributions 10 10 21 21 Total cash distributions declared $ 736 $ 746 $ 1,474 $ 1,505

Series A Preferred Units

Series A Preferred Units	6 Months Ended
	Jun. 30, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units	Series A Preferred Units On May 13, 2016, MPLX LP issued approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The Series A preferred units rank senior to all common units and pari passu with all Series B preferred units with respect to distributions and rights upon liquidation. The holders of the Series A preferred units are entitled to receive, when and if declared by the board, a quarterly distribution equal to the greater of $0.528125 per unit or the amount of distributions they would have received on an as converted basis. On July 27, 2021, MPLX declared a quarterly cash distribution of $0.6875 per common unit for the second quarter of 2021. Holders of the Series A preferred units will receive the common unit rate in lieu of the lower $0.528125 base amount. The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate. Approximately 29.6 million Series A preferred units remaining outstanding as of June 30, 2021. The changes in the redeemable preferred balance from December 31, 2020 through June 30, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated 41 Distributions received by Series A preferred unitholders (41) Balance at June 30, 2021 $ 968 The Series A preferred units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside MPLX’s control. Therefore, they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The Series A preferred units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value and declared distributions decrease the carrying value of the Series A preferred units. As the Series A preferred units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the Series A preferred units would become redeemable.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2021
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Segment InformationMPLX’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews MPLX’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. MPLX has two reportable segments: L&S and G&P. Each of these segments is organized and managed based upon the nature of the products and services it offers. • L&S – transports, stores, distributes and markets crude oil, asphalt, refined petroleum products and water. Also includes an inland marine business, terminals, rail facilities, storage caverns and refining logistics. • G&P – gathers, processes and transports natural gas; and gathers, transports, fractionates, stores and markets NGLs. Our CEO evaluates the performance of our segments using Segment Adjusted EBITDA. Amounts included in net income and excluded from Segment Adjusted EBITDA include: (i) depreciation and amortization; (ii) provision/(benefit) for income taxes; (iii) amortization of deferred financing costs; (iv) extinguishment of debt; (v) non-cash equity-based compensation; (vi) impairment expense; (vii) net interest and other financial costs; (viii) income/(loss) from equity method investments; (ix) distributions and adjustments related to equity method investments; (x) unrealized derivative gains/(losses); (xi) acquisition costs; (xii) noncontrolling interest; and (xiii) other adjustments as deemed necessary. These items are either: (i) believed to be non-recurring in nature; (ii) not believed to be allocable or controlled by the segment; or (iii) are not tied to the operational performance of the segment. The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 L&S Service revenue $ 992 $ 931 $ 1,945 $ 1,935 Rental income 176 246 425 488 Product related revenue 4 21 8 40 Sales-type lease revenue - related parties 136 38 173 76 Income from equity method investments 35 40 71 90 Other income 16 14 31 27 Total segment revenues and other income (1) 1,359 1,290 2,653 2,656 Segment Adjusted EBITDA (2) 947 839 1,843 1,711 Capital expenditures 76 108 135 292 Investments in unconsolidated affiliates 13 74 22 128 G&P Service revenue 493 489 1,001 1,025 Rental income 91 89 183 177 Product related revenue 407 151 804 373 Income/(loss) from equity method investments 31 49 65 (1,185) Other income 14 13 28 27 Total segment revenues and other income (1) 1,036 791 2,081 417 Segment Adjusted EBITDA (2) 427 388 883 810 Capital expenditures 36 110 66 244 Investments in unconsolidated affiliates $ 36 $ 57 $ 62 $ 94 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $267 million for the three and six months ended June 30, 2021, respectively, and $146 million and $304 million for the three and six months ended June 30, 2020, respectively. Third party revenues for the G&P segment were $988 million and $1,977 million for the three and six months ended June 30, 2021, respectively, and $748 million and $323 million for the three and six months ended June 30, 2020, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (In millions) June 30, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 8 $ 15 L&S 20,869 20,938 G&P 14,896 15,461 Total assets $ 35,773 $ 36,414 The table below provides a reconciliation between “Net income/(loss)” and Segment Adjusted EBITDA. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 947 $ 839 $ 1,843 $ 1,711 G&P Segment Adjusted EBITDA 427 388 883 810 Total reportable segments 1,374 1,227 2,726 2,521 Depreciation and amortization (1) (318) (321) (647) (646) Provision for income taxes — — (1) — Amortization of deferred financing costs (18) (15) (35) (29) Non-cash equity-based compensation (2) (3) (5) (8) Impairment expense (42) — (42) (2,165) Net interest and other financial costs (198) (208) (418) (424) Gain on extinguishment of debt — — 12 — Income/(loss) from equity method investments 66 89 136 (1,095) Distributions/adjustments related to equity method investments (121) (115) (242) (239) Unrealized derivative (losses)/gains (2) (36) (6) (39) 9 Other — (1) (2) (2) Adjusted EBITDA attributable to noncontrolling interests 10 8 20 17 Net income/(loss) $ 715 $ 655 $ 1,463 $ (2,061) (1) Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories

Inventories	6 Months Ended
	Jun. 30, 2021
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) June 30, 2021 December 31, 2020 NGLs $ 5 $ 5 Line fill 18 13 Spare parts, materials and supplies 103 100 Total inventories $ 126 $ 118

Property, Plant and Equipment

Property, Plant and Equipment	6 Months Ended
	Jun. 30, 2021
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) June 30, 2021 December 31, 2020 L&S Pipelines $ 6,045 $ 6,026 Refining logistics 1,789 2,333 Terminals 1,652 1,643 Marine 965 965 Land, building and other 1,575 1,584 Construction-in-progress 217 262 Total L&S property, plant and equipment 12,243 12,813 G&P Gathering and transportation 7,574 7,547 Processing and fractionation 5,726 5,721 Land, building and other 511 507 Construction-in-progress 276 287 Total G&P property, plant and equipment 14,087 14,062 Total property, plant and equipment 26,330 26,875 Less accumulated depreciation 5,978 5,657 Property, plant and equipment, net $ 20,352 $ 21,218 Long-lived assets used in operations are assessed for impairment whenever changes in facts and circumstances indicate that the carrying value of the assets may not be recoverable based on the expected undiscounted future cash flow of an asset group. For purposes of impairment evaluation, long-lived assets must be grouped at the lowest level for which independent cash flows can be identified, which is at least at the segment level and in some cases for similar assets in the same geographic region where cash flows can be separately identified. If the sum of the undiscounted cash flows is less than the carrying value of an asset group, fair value is calculated, and the carrying value is written down if greater than the calculated fair value.

Goodwill and Intangibles (Notes

Goodwill and Intangibles (Notes)	6 Months Ended
	Jun. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets Disclosure [Text Block]	Goodwill and Intangibles Goodwill MPLX annually evaluates goodwill for impairment as of November 30, as well as whenever events or changes in circumstances indicate it is more likely than not that the fair value of a reporting unit with goodwill is less than its carrying amount. There were no impairments recorded as a result of our November 30, 2020 annual goodwill impairment evaluation. During the first quarter of 2020, we determined that an interim impairment analysis of the goodwill recorded was necessary based on consideration of a number of first quarter events and circumstances as discussed in Note 1. Our producer customers in our Eastern G&P region reduced production forecasts and drilling activity in response to the global economic downturn. Additionally, a decline in NGL prices impacted our future revenue forecast. After performing our evaluations related to the interim impairment of goodwill during the first quarter of 2020, we recorded an impairment of $1,814 million within the Eastern G&P reporting unit, which was recorded to “Impairment expense” on the Consolidated Statements of Income. The impairment was primarily driven by additional guidance related to the slowing of drilling activity, which reduced production growth forecasts from our producer customers. Changes in the carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2019 $ 7,722 $ 3,141 $ 10,863 Accumulated impairment losses — (1,327) (1,327) Balance as of December 31, 2019 7,722 1,814 9,536 Impairment losses — (1,814) (1,814) Wholesale Exchange (Note 3) (65) — (65) Balance as of December 31, 2020 7,657 — 7,657 Balance as of June 30, 2021 7,657 — 7,657 Gross goodwill as of June 30, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of June 30, 2021 $ 7,657 $ — $ 7,657 Intangible Assets During the first quarter of 2020, we also determined that an impairment analysis of intangibles within our Western G&P reporting unit was necessary. See Note 11 for additional information regarding our assessment around the Western G&P reporting unit, and more specifically our East Texas G&P asset group. The fair value of the intangibles in our East Texas G&P asset group was determined based on applying the multi-period excess earnings method, which is an income approach. Key assumptions included management’s best estimates of the expected future cash flows from existing customers, customer attrition rates and the discount rate. After performing our evaluations related to the impairment of intangible assets associated with our East Texas G&P asset group during the first quarter of 2020, we recorded an impairment of $177 million to “Impairment expense” on the Consolidated Statements of Income related to our customer relationships. No additional impairments have been recorded since that time. MPLX’s remaining intangible assets are comprised of customer contracts and relationships. Gross intangible assets with accumulated amortization as of June 30, 2021 and December 31, 2020 is shown below: June 30, 2021 December 31, 2020 (In millions) Useful Lives Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (2) Net L&S 6 - 8 years $ 283 $ (99) $ 184 $ 283 $ (81) $ 202 G&P 6 - 25 years 1,288 (576) 712 1,288 (531) 757 $ 1,571 $ (675) $ 896 $ 1,571 $ (612) $ 959 (1) Amortization expense attributable to the L&S and G&P segments for the six months ended June 30, 2021 was $18 million and $45 million, respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization. Estimated future amortization expense related to the intangible assets at June 30, 2021 is as follows: (In millions) 2021 $ 64 2022 127 2023 127 2024 127 2025 113 Thereafter 338 Total $ 896

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values – Recurring Fair value measurements and disclosures relate primarily to MPLX’s derivative positions as discussed in Note 14. The following table presents the financial instruments carried at fair value on a recurring basis as of June 30, 2021 and December 31, 2020 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. June 30, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (102) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (102) $ — $ (63) Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.64 to $1.59 per gallon with a weighted average of $0.81 per gallon and (2) the probability of renewal of 100 percent for the first five five Changes in Level 3 Fair Value Measurements The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended June 30, 2021 Three Months Ended June 30, 2020 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (66) $ — $ (45) Total (losses)/gains (realized and unrealized) included in earnings (1) — (39) — (7) Settlements — 3 — 1 Fair value at end of period — (102) — (51) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized (losses)/gains relating to liabilities still held at end of period $ — $ (39) $ — $ (6) Six Months Ended June 30, 2021 Six Months Ended June 30, 2020 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (63) $ — $ (60) Total (losses)/gains (realized and unrealized) included in earnings (1) — (45) — 7 Settlements — 6 — 2 Fair value at end of period — (102) — (51) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized (losses)/gains relating to liabilities still held at end of period $ — $ (41) $ — $ 7 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income. Fair Values – Reported MPLX’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, lease receivables from related parties, accounts payable, payables to related parties and long-term debt. MPLX’s fair value assessment incorporates a variety of considerations, including (1) the duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. MPLX believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 14). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. The long-term debt fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of our debt, excluding finance leases: June 30, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,838 $ 19,335 $ 22,951 $ 20,244

Derivative Financial Instrument

Derivative Financial Instruments	6 Months Ended
	Jun. 30, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of June 30, 2021, MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below. Embedded Derivative - MPLX has a natural gas purchase commitment embedded in a keep-whole processing agreement with a producer customer in the Southern Appalachian region expiring in December 2022. The customer has the unilateral option to extend the agreement for two consecutive five Certain derivative positions are subject to master netting agreements, therefore, MPLX has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of June 30, 2021 and December 31, 2020, there were no derivative assets or liabilities that were offset on the Consolidated Balance Sheets. The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) June 30, 2021 December 31, 2020 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (14) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (88) — (56) Total $ — $ (102) $ — $ (63) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, see Note 13. There were no material changes to MPLX’s policy regarding the accounting for Level 2 and Level 3 instruments as previously disclosed in MPLX’s Annual Report on Form 10-K for the year ended December 31, 2020. The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Purchased product costs Realized (loss)/gain $ (3) $ (1) $ (6) $ (2) Unrealized (loss)/gain (36) (6) (39) 9 Purchased product costs derivative (loss)/gain (39) (7) (45) 7 Total derivative (loss)/gain $ (39) $ (7) $ (45) $ 7

Debt

Debt	6 Months Ended
	Jun. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings consist of the following: (In millions) June 30, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ — $ 175 Floating rate senior notes 1,000 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations (1) 9 11 Total 19,609 20,536 Unamortized debt issuance costs (109) (116) Unamortized discount/premium (265) (281) Amounts due within one year (1) (764) Total long-term debt due after one year $ 19,234 $ 19,375 (1) See Note 20 for lease information. Credit Agreement MPLX’s amended and restated revolving credit facility (as amended, the “MPLX Credit Agreement”), has a borrowing capacity of up to $3.5 billion and a term that extends to July 2024. During the six months ended June 30, 2021, MPLX borrowed $2.8 billion under the MPLX Credit Agreement, at an average interest rate of 1.345 percent, and repaid $2.975 billion. At June 30, 2021, MPLX had no outstanding borrowings and less than $1 million in letters of credit outstanding under the MPLX Credit Agreement, resulting in total availability of $3.50 billion, or 100 percent of the borrowing capacity . Floating Rate Senior Notes MPLX has $1.0 billion of aggregate principal amount of floating rate senior notes due September 2022. The notes were offered at a price to the public of 100 percent of par and are callable, in whole or in part, at par plus accrued and unpaid interest at any time on or after September 10, 2020. Interest is payable quarterly in March, June, September and December. The interest rate applicable to the notes is LIBOR plus 1.1 percent per annum. Fixed Rate Senior Notes MPLX’s senior notes, including those issued by consolidated subsidiaries, consist of various series of senior notes expiring between 2022 and 2058 with interest rates ranging from 1.750 percent to 5.500 percent. Interest on each series of notes is payable semi-annually in arrears on various dates depending on the series of the notes.

Revenue

Revenue	6 Months Ended
	Jun. 30, 2021
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Disaggregation of Revenue The following tables represent a disaggregation of revenue for each reportable segment for the three and six months ended June 30, 2021 and 2020: Three Months Ended June 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 91 $ 487 $ 578 Service revenue - related parties 901 6 907 Service revenue - product related — 76 76 Product sales 1 303 304 Product sales - related parties 3 28 31 Total revenues from contracts with customers $ 996 $ 900 1,896 Non-ASC 606 revenue (1) 499 Total revenues and other income $ 2,395 Three Months Ended June 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 77 $ 486 $ 563 Service revenue - related parties 854 3 857 Service revenue - product related — 22 22 Product sales 17 103 120 Product sales - related parties 4 26 30 Total revenues from contracts with customers $ 952 $ 640 1,592 Non-ASC 606 revenue (1) 489 Total revenues and other income $ 2,081 Six Months Ended June 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 175 $ 992 $ 1,167 Service revenue - related parties 1,770 9 1,779 Service revenue - product related — 153 153 Product sales 2 584 586 Product sales - related parties 6 67 73 Total revenues from contracts with customers $ 1,953 $ 1,805 3,758 Non-ASC 606 revenue (1) 976 Total revenues and other income $ 4,734 Six Months Ended June 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 161 $ 1,014 $ 1,175 Service revenue - related parties 1,774 11 1,785 Service revenue - product related — 61 61 Product sales 32 257 289 Product sales - related parties 8 55 63 Total revenues from contracts with customers $ 1,975 $ 1,398 3,373 Non-ASC 606 loss (1) (300) Total revenues and other income $ 3,073 (1) Non-ASC 606 Revenue includes rental income, sales-type lease revenue, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and MPLX’s rights to consideration for work completed exceeds the amount billed to the customer. Contract assets are included in “Other current assets” and “Other noncurrent assets” on the Consolidated Balance Sheets. Contract liabilities, which we refer to as “Deferred revenue” and “Long-term deferred revenue,” typically relate to advance payments for aid in construction agreements and deferred customer credits associated with makeup rights and minimum volume commitments. Related to minimum volume commitments, breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. We classify contract liabilities as current or long-term based on the timing of when we expect to recognize revenue. “Receivables, net” primarily relate to our commodity sales. Portions of the “Receivables, net” balance are attributed to the sale of commodity product controlled by MPLX prior to sale while a significant portion of the balance relates to the sale of commodity product on behalf of our producer customers. The sales and related “Receivable, net” are commingled and excluded from the table below. MPLX remits the net sales price back to our producer customers upon completion of the sale. Each period end, certain amounts within accounts payable relate to our payments to producer customers. Such amounts are not deemed material at period end as a result of when we settle with each producer. The tables below reflect the changes in our contract balances for the six-month periods ended June 30, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 40 $ (25) $ 1 $ 16 Long-term contract assets 2 — — 2 Deferred revenue 37 24 (17) 44 Deferred revenue - related parties 91 37 (50) 78 Long-term deferred revenue 119 9 — 128 Long-term deferred revenue - related parties 48 (8) — 40 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 39 $ (20) $ (1) $ 18 Deferred revenue 23 8 (5) 26 Deferred revenue - related parties 53 48 (29) 72 Long-term deferred revenue 90 11 — 101 Long-term deferred revenue - related parties $ 55 $ (2) $ — $ 53 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations in the current periods. Remaining Performance Obligations The table below includes estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period. As of June 30, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $289 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 $ 933 2022 1,767 2023 1,615 2024 1,491 2025 and thereafter 4,427 Total revenue on remaining performance obligations (1),(2),(3) $ 10,233 (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	6 Months Ended
	Jun. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information Six Months Ended June 30, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 414 $ 423 Income taxes paid 1 1 Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories $ — $ (1) The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Six Months Ended June 30, (In millions) 2021 2020 Decrease in capital accruals $ (34) $ (172)

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	6 Months Ended
	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX LP records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP LLC (“LOOP”) and Explorer Pipeline Company (“Explorer”) to their employees. MPLX LP is not a sponsor of these benefit plans. The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2020 through June 30, 2021. (In millions) Pension Other Total Balance at December 31, 2020 (1) $ (13) $ (2) $ (15) Other comprehensive loss - remeasurements (2) (2) — (2) Balance at June 30, 2021 (1) $ (15) $ (2) $ (17) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through June 30, 2020. (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14) $ (1) $ (15) Other comprehensive loss - remeasurements (2) — (1) (1) Balance at June 30, 2020 (1) $ (14) $ (2) $ (16) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Plan

Equity-Based Compensation Plan	6 Months Ended
	Jun. 30, 2021
Share-based Payment Arrangement [Abstract]
Share-based Payment Arrangement [Text Block]	Equity-Based Compensation Phantom Units The following is a summary of phantom unit award activity of MPLX LP common units for the six months ended June 30, 2021: Number Weighted Outstanding at December 31, 2020 644,023 $ 28.65 Granted 335,904 24.97 Settled (201,400) 30.30 Forfeited (1,330) 33.19 Outstanding at June 30, 2021 777,197 $ 26.63 Performance Units The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the six months ended June 30, 2021: Number of Outstanding at December 31, 2020 3,092,286 Granted 375,730 Settled (1,347,177) Forfeited (1,761) Outstanding at June 30, 2021 2,119,078

Leases Leases (Notes)

Leases Leases (Notes)	6 Months Ended
	Jun. 30, 2021
Disclosure Text Block [Abstract]
Leases of Lessor Disclosure [Text Block]	LeasesDuring the second quarter of 2021 and during the first quarter of 2020, reimbursements for projects and changes to minimum volume commitments at certain L&S locations were agreed to between MPLX and MPC. These reimbursements and minimum volume commitments 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 ASC 842. As a result of the reassessment, certain leases were reclassified from an operating lease to a sales-type lease. Accordingly, 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 during the respective periods. See Note 5 for the location of lease receivables and unguaranteed residual assets on the Consolidated Balance Sheets. The difference between the net book value of the underlying assets and the net investment in the lease has been recorded as a “Contribution from MPC” in the Consolidated Statements of Equity given that the impacted storage services agreements are related to a common control transaction. During the second quarter of 2021, MPLX derecognized approximately $421 million of property, plant and equipment, recorded a lease receivable of approximately $519 million, recorded an unguaranteed residual asset of approximately $14 million with the difference recorded as a deemed “Contribution from MPC” of $112 million. 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 with the difference recorded as a deemed “Contribution from MPC” of $209 million. Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended June 30, 2021 Three Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 144 $ 68 $ 195 $ 66 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 135 — 38 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A Six Months Ended June 30, 2021 Six Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 346 $ 136 $ 381 $ 129 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 172 — 76 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A (1) These amounts are presented net of executory costs. See Note 5 for additional information on where related party lease assets are recorded in the Consolidated Balance Sheets. Third party lease assets are less than $1 million as of June 30, 2021 and are included within the “Receivables, net” and “Other noncurrent assets” captions within the Consolidated Balance Sheets. The following is a schedule of future payments on the sales-type leases as of June 30, 2021: (In millions) Related Party 2021 $ 271 2022 543 2023 544 2024 537 2025 525 2026 and thereafter 1,017 Total minimum future rentals 3,437 Less: present value discount 2,520 Lease receivable $ 917

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and ContingenciesMPLX 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 June 30, 2021 and December 31, 2020, accrued liabilities for remediation totaled $24 million and $17 million, respectively. It is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, that may be imposed. At June 30, 2021 and December 31, 2020, there were no balances with MPC for indemnification of environmental costs. MPLX is involved in environmental enforcement matters arising in the ordinary course of business. While the outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on its consolidated results of operations, financial position or cash flows. MPLX is also a party to a number of other lawsuits and other proceedings arising in the ordinary course of business. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, management believes the resolution of these other lawsuits and proceedings will not, individually or collectively, have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, MPLX has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require MPLX to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. MPLX is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. In connection with our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL, we have entered into a Contingent Equity Contribution Agreement whereby MPLX LP, along with the other joint venture owners in the Bakken Pipeline system, has agreed to make equity contributions to the joint venture upon certain events occurring to allow the entities that own and operate the Bakken Pipeline system to satisfy their senior note payment obligations. The senior notes were issued to repay amounts owed by the pipeline companies to fund the cost of construction of the Bakken Pipeline system. In March 2020, the U.S. District Court for the District of Columbia (the “D.D.C.”) ordered the U.S. Army Corps of Engineers (“Army Corps”), which granted permits and an easement for the Bakken Pipeline system, to conduct a full environmental impact statement (“EIS”), and further requested briefing on whether an easement necessary for the operation of the Bakken Pipeline system should be vacated while the EIS is being prepared. On July 6, 2020, the D.D.C. ordered vacatur of the easement to cross Lake Oahe during the pendency of an EIS and further ordered a shut down of the pipeline by August 5, 2020. The D.D.C. denied a motion to stay that order. Dakota Access and the Army Corps appealed the D.D.C.’s orders to the U.S. Court of Appeals for the District of Columbia Circuit (the “Court of Appeals”). On July 14, 2020, the Court of Appeals issued an administrative stay while the court considered Dakota Access and the Army Corps’ emergency motion for stay pending appeal. On August 5, 2020, the Court of Appeals stayed the D.D.C.’s injunction that required the pipeline be shutdown and emptied of oil by August 5, 2020. The Court of Appeals denied a stay of the D.D.C.’s March order, which required the EIS, and further denied a stay of the D.D.C.’s July order, which vacated the easement. On January 26, 2021, the Court of Appeals upheld the D.D.C.’s order vacating the easement while the Army Corps prepares the EIS. The Court of Appeals reversed the D.D.C.’s order to the extent it directed that the pipeline be shutdown and emptied of oil. On May 21, 2021, the D.D.C. denied a renewed request for an injunction. On June 22, 2021, the D.D.C. issued an order dismissing without prejudice the tribes’ claims against the Dakota Access Pipeline. The judge noted that the plaintiffs may move to reopen the case in the event of a violation of the court’s prior orders. The pipeline remains operational. If the pipeline is temporarily shut down pending completion of the EIS, MPLX would have to contribute its 9.19 percent pro rata share of funds required to pay interest accruing on the notes and any portion of the principal that matures while the pipeline is shutdown. MPLX also expects to contribute its 9.19 percent pro rata share of any costs to remediate any deficiencies to reinstate the permit and/or return the pipeline into operation. If the vacatur of the easement permit results in a permanent shutdown of the pipeline, MPLX would have to contribute its 9.19 percent pro rata share of the cost to redeem the bonds (including the 1% redemption premium required pursuant to the indenture governing the notes) and any accrued and unpaid interest. As of June 30, 2021, our maximum potential undiscounted payments under the Contingent Equity Contribution Agreement were approximately $230 million. Other Legal Proceedings – In early July 2020, Tesoro High Plains Pipeline Company, LLC (“THPP”), a subsidiary of MPLX, received a Notification of Trespass Determination from the Bureau of Indian Affairs (“BIA”) relating to a portion of the Tesoro High Plains Pipeline that crosses the Fort Berthold Reservation in North Dakota. The notification covered the rights of way for 23 tracts of land and demanded the immediate cessation of pipeline operations. The notification also assessed trespass damages of approximately $187 million. THPP appealed this determination, which triggered an automatic stay of the requested pipeline shutdown and payment. On October 29, 2020, the Assistant Secretary - Indian Affairs issued an order vacating the BIA’s trespass order and requiring the Regional Director for the BIA Great Plains Region to issue a new decision on or before December 15 covering all 34 tracts at issue. On December 15, 2020, the Regional Director of the BIA issued a new trespass notice to THPP consistent with the Assistant Secretary - Indian Affairs order vacating the prior trespass order. The new order found that THPP was in trespass and assessed trespass damages of approximately $4 million (including interest), which has been paid. The order also required THPP to immediately cease and desist use of the portion of the pipeline that crosses the property at issue. THPP has complied with the Regional Director’s December 15, 2020 notice. On February 12, 2021, landowners filed suit in the U.S. District Court for the District of North Dakota (the “District of North Dakota”) against THPP, the Department of the Interior, the Assistant Secretary - Indian Affairs, the Interior Board of Indian Appeals and the BIA, requesting, among other things, that decisions by the Assistant Secretary - Indian Affairs and the Interior Board of Indian Appeals be vacated as to the award of damages to plaintiffs. In March 2021, THPP received a copy of an order purporting to vacate all orders related to THPP’s alleged trespass issued by the BIA between July 2, 2020 and January 14, 2021. The order directs the Regional Director of the BIA to reconsider the issue of THPP’s alleged trespass and issue a new order, if necessary, after all interested parties have had an opportunity to be heard. Subsequently, landowners voluntarily dismissed the suit filed in the District of North Dakota. On April 23, 2021, THPP filed a lawsuit in the District of North Dakota against the United States of America, the U.S. Department of the Interior and the BIA challenging the March order purporting to vacate all previous orders related to THPP’s alleged trespass. We continue to work towards a settlement of this matter with holders of the property rights at issue.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventsOn August 4, 2021, MPLX announced the intent to provide notice of the redemption of all of the $1 billion outstanding aggregate principal amount of MPLX's LIBOR plus 1.1 percent per annum floating rate senior notes due September 9, 2022. The notes are expected to be redeemed on September 3, 2021, at a price equal to 100 percent of the principal amount thereof, plus accrued and unpaid interest to, but not including, the redemption date. MPLX expects to fund the redemption amount with a combination of cash on hand and borrowings under the MPLX Credit Agreement or the MPC Loan Agreement.

Description of the Business a_2

Description of the Business and Basis of Presentation Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. On July 31, 2020, MPLX completed the exchange of Western Refining Wholesale, LLC (“WRW”) to Western Refining Southwest, Inc. (now known as Western Refining Southwest LLC) (“WRSW”), a wholly owned subsidiary of MPC, in exchange for the redemption of 18,582,088 MPLX common units held by WRSW (the “Wholesale Exchange”). See Note 3 for additional information regarding the Wholesale Exchange. These financial statements include the results of WRSW through July 31, 2020. Impairments – During the first half of 2021, we continued to see improvements in the environment in which our business operates as COVID-19 impacts continue to subside. The increased availability of vaccinations and easing of COVID-19 restrictions has been followed by an increase in economic activity, however, we are unable to predict the potential effects a resurgence of COVID-19 may have on our financial position and results. In the second quarter of 2021, we recognized impairment expense of $42 million within our G&P segment related to our continued emphasis on portfolio optimization with the anticipated divestiture of several non-core assets and the closure of other non-core assets. During the first quarter of 2020, the overall deterioration in the economy and the environment in which MPLX and its customers operate, as well as a sustained decrease in unit price, were considered triggering events at that time resulting in impairments of the carrying value of certain assets. We recognized impairments related to goodwill, certain equity method investments and certain long-lived assets (including intangibles), within our G&P segment. Many of our producer customers refined and updated production forecasts in response to the environment at that time, which impacted their expected future demand for our services, including the future utilization of our assets. Additionally, certain of our contracts have commodity price exposure, including NGL prices, which experienced increased volatility as noted above. The table below provides information related to the impairments recognized during the first quarter of 2020 as well as the corresponding footnote where additional information can be found. (In millions) Impairment Footnote Reference Goodwill $ 1,814 12 Equity method investments 1,264 4 Intangibles, net 177 12 Property, plant and equipment, net 174 11 Total impairments $ 3,429 Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results to be expected for the full year. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.
Use of Estimates, Policy [Policy Text Block]	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation.
Consolidation, Subsidiaries or Other Investments, Consolidated Entities, Policy [Policy Text Block]	MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in VIEs in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method.
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.
Nature of Operations	We are engaged in the transportation, storage and distribution of crude oil, asphalt and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio.MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil, asphalt and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments.

Net Income Per Limited Partne_2

Net Income Per Limited Partner Unit Accounting Policy (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to Series A and Series B preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6.Net income/(loss) per unit applicable to common units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding.

Investments and Noncontrollin_2

Investments and Noncontrolling Interests (Tables)	6 Months Ended
	Jun. 30, 2021
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments [Table Text Block]	The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at June 30, June 30, December 31, (In millions, except ownership percentages) 2021 2021 2020 L&S MarEn Bakken Company LLC (1) 25% $ 460 $ 465 Illinois Extension Pipeline Company, L.L.C. 35% 257 254 LOOP LLC 41% 264 252 Andeavor Logistics Rio Pipeline LLC (2) 67% 191 194 Minnesota Pipe Line Company, LLC 17% 185 188 Whistler Pipeline LLC (2) 38% 181 185 Explorer Pipeline Company 25% 65 72 W2W Holdings LLC (2)(3) 50% 70 72 Other (2) 109 103 Total L&S 1,782 1,785 G&P MarkWest Utica EMG, L.L.C. (2) 57% 690 698 Sherwood Midstream LLC (2) 50% 550 557 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (2) 67% 327 307 MarkWest Torñado GP, L.L.C. (2) 60% 213 188 Rendezvous Gas Services, L.L.C. (2) 78% 153 159 Sherwood Midstream Holdings LLC (2) 51% 141 148 Centrahoma Processing LLC 40% 131 145 Other (2) 46 49 Total G&P 2,251 2,251 Total $ 4,033 $ 4,036 (1) The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL. (2) Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs. (3) Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest Wink to Webster Pipeline LLC.
Summarized Financial Information For Equity Method Investees Table [Table Text Block]	Summarized financial information for MPLX’s equity method investments for the six months ended June 30, 2021 and 2020 is as follows: Six Months Ended June 30, 2021 (In millions) VIEs Non-VIEs Total Revenues and other income $ 336 $ 607 $ 943 Costs and expenses 217 284 501 Income from operations 119 323 442 Net income 105 283 388 Income from equity method investments (1) $ 73 $ 63 $ 136 (1) Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million. Six Months Ended June 30, 2020 (In millions) VIEs Non-VIEs Total Revenues and other income $ (43) $ 640 $ 597 Costs and expenses 202 274 476 (Loss)/income from operations (245) 366 121 Net (loss)/income (283) 332 49 (Loss)/income from equity method investments (1) $ (1,178) $ 83 $ (1,095) (1) Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million. Summarized balance sheet information for MPLX’s equity method investments as of June 30, 2021 and December 31, 2020 is as follows: June 30, 2021 (In millions) VIEs Non-VIEs Total Current assets $ 355 $ 464 $ 819 Noncurrent assets 7,401 4,909 12,310 Current liabilities 183 281 464 Noncurrent liabilities $ 2,379 $ 862 $ 3,241 December 31, 2020 (In millions) VIEs Non-VIEs Total Current assets $ 530 $ 318 $ 848 Noncurrent assets 6,889 4,997 11,886 Current liabilities 323 187 510 Noncurrent liabilities $ 1,904 $ 830 $ 2,734

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	6 Months Ended
	Jun. 30, 2021
Related Party Transactions [Abstract]
Schedule of Long-term Debt Instruments [Table Text Block]	Activity on the MPC Loan Agreement was as follows: (In millions) Six Months Ended June 30, 2021 Year Ended December 31, 2020 Borrowings $ 4,435 $ 6,264 Average interest rate of borrowings 1.354 % 2.278 % Repayments $ 3,942 $ 6,858 Outstanding balance at end of period (1) $ 493 $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.
Schedule of Entity Wide Information Sales to Related Parties [Table Text Block]	Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Service revenues - related parties MPC $ 907 $ 857 $ 1,778 $ 1,784 Other — — 1 1 Total Service revenue - related parties 907 857 1,779 1,785 Rental income - related parties MPC 168 237 410 471 Product sales - related parties (1) MPC 31 30 73 63 Sales-type lease revenue - related parties MPC 136 38 173 76 Other income - related parties MPC 11 10 23 20 Other 16 15 32 31 Total Other income - related parties $ 27 $ 25 $ 55 $ 51 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and six months ended June 30, 2021, these sales totaled $177 million and $345 million, respectively. For the three and six months ended June 30, 2020, these sales totaled $52 million and $225 million, respectively.
Schedule Of Entity Wide Information Allocated Related Party Employee Benefit Costs By Income Statement Line [Table Text Block]	Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Rental cost of sales - related parties MPC $ 23 $ 41 $ 62 $ 87 Purchases - related parties MPC 293 276 587 547 Other 4 4 8 9 Total Purchases - related parties 297 280 595 556 General and administrative expenses MPC $ 63 $ 68 $ 120 $ 132
Schedule of Related Party Transactions [Table Text Block]	(In millions) June 30, 2021 December 31, 2020 Current assets - related parties Receivables - MPC $ 519 $ 615 Receivables - Other 8 27 Prepaid - MPC 12 4 Other - MPC 2 1 Lease Receivables - MPC 73 30 Total 614 677 Noncurrent assets - related parties Long-term receivables - MPC 32 32 Right of use assets - MPC 230 231 Long-term lease receivables - MPC 844 386 Unguaranteed residual asset - MPC 38 23 Total 1,144 672 Current liabilities - related parties Payables - MPC 689 215 Payables - Other 31 43 Operating lease liabilities - MPC 1 1 Deferred revenue - Minimum volume deficiencies - MPC 51 66 Deferred revenue - Project reimbursements - MPC 39 30 Deferred revenue - Project reimbursements - Other 1 1 Total 812 356 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 229 229 Long-term deferred revenue - Project reimbursements - MPC 70 47 Long-term deferred revenue - Project reimbursements - Other 7 7 Total $ 306 $ 283

Net Income Per Limited Partne_3

Net Income Per Limited Partner Unit (Tables)	6 Months Ended
	Jun. 30, 2021
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class [Table Text Block]	During the three and six months ended June 30, 2021 and 2020, MPLX had participating securities consisting of common units, certain equity-based compensation awards, Series A preferred units and Series B preferred units and had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three and six months ended June 30, 2021 and 2020 were less than 1 million. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Net income/(loss) attributable to MPLX LP $ 706 $ 648 $ 1,445 $ (2,076) Less: Distributions declared on Series A preferred units (1) 21 21 41 41 Distributions declared on Series B preferred units (1) 10 10 21 21 Limited partners’ distributions declared on MPLX common units (including common units of general partner) (1) 705 715 1,412 1,443 Undistributed net gain/(loss) attributable to MPLX LP $ (30) $ (98) $ (29) $ (3,581) (1) See Note 7 for distribution information.
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	Three Months Ended June 30, 2021 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 705 $ 21 $ 10 $ 736 Undistributed net loss attributable to MPLX LP (30) — — (30) Net income attributable to MPLX LP (1) $ 675 $ 21 $ 10 $ 706 Weighted average units outstanding: Basic 1,029 Diluted 1,029 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.66 Diluted $ 0.66 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Three Months Ended June 30, 2020 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 715 $ 21 $ 10 $ 746 Undistributed net loss attributable to MPLX LP (98) — — (98) Net income attributable to MPLX LP (1) $ 617 $ 21 $ 10 $ 648 Weighted average units outstanding: Basic 1,059 Diluted 1,059 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.58 Diluted $ 0.58 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Six Months Ended June 30, 2021 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,412 $ 41 $ 21 $ 1,474 Undistributed net loss attributable to MPLX LP (29) — — (29) Net income attributable to MPLX LP (1) $ 1,383 $ 41 $ 21 $ 1,445 Weighted average units outstanding: Basic 1,033 Diluted 1,033 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.34 Diluted $ 1.34 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period. Six Months Ended June 30, 2020 (In millions, except per unit data) Limited Partners’ Series A Preferred Units Series B Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 1,443 $ 41 $ 21 $ 1,505 Undistributed net loss attributable to MPLX LP (3,581) — — (3,581) Net (loss)/income attributable to MPLX LP (1) $ (2,138) $ 41 $ 21 $ (2,076) Weighted average units outstanding: Basic 1,059 Diluted 1,059 Net income attributable to MPLX LP per limited partner unit: Basic $ (2.02) Diluted $ (2.02) (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.

Equity (Tables)

Equity (Tables)	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of common units outstanding during the six months ended June 30, 2021 are summarized below: (In units) Common Balance at December 31, 2020 1,038,777,978 Unit-based compensation awards 183,111 Units redeemed in unit repurchase program (11,929,998) Balance at June 30, 2021 1,027,031,091 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 six months ended June 30, 2021, 11,929,998 public common units were repurchased at an average cost per unit of $26.02. Total cash paid for units repurchased during 2021 was $310 million with $657 million remaining available under the authorization for future repurchases as of June 30, 2021. As of June 30, 2021, we had agreements to acquire 126,293 additional common units for $4 million, which settled in July 2021. Wholesale Exchange In connection with the Wholesale Exchange as discussed in Note 3, 18,582,088 units were redeemed by MPC in exchange for all of the outstanding membership interests in WRW. These units were cancelled by MPLX immediately following the transaction. Series B Preferred Units MPLX has outstanding 600,000 units of 6.875 percent Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units representing limited partner interests of MPLX with a price to the public of $1,000 per unit (the “Series B preferred units”). The Series B preferred units are pari passu with the Series A preferred units with respect to distribution rights and rights upon liquidation. Series B preferred unitholders are entitled to receive a fixed distribution of $68.75 per unit, per annum, payable semi-annually in arrears on the 15th day, or the first business day thereafter, of February and August of each year up to and including February 15, 2023. After February 15, 2023, the holders of Series B preferred units are entitled to receive cumulative, quarterly distributions payable in arrears on the 15th day of February, May, August and November of each year, or the first business day thereafter, based on a floating annual rate equal to the three-month LIBOR plus 4.652 percent, in each case assuming a distribution is declared by the Board of Directors. The changes in the Series B preferred unit balance from December 31, 2020 through June 30, 2021 are summarized below. Series B preferred units are included in the Consolidated Balance Sheets and Consolidated Statements of Equity within “Series B preferred units”. (In millions) Series B Preferred Units Balance at December 31, 2020 $ 611 Net income allocated 21 Distributions received by Series B preferred unitholders (21) Balance at June 30, 2021 $ 611
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three and six months ended June 30, 2021 and 2020. MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 705 $ 715 $ 1,412 $ 1,443 Series A preferred unit distributions 21 21 41 41 Series B preferred unit distributions 10 10 21 21 Total cash distributions declared $ 736 $ 746 $ 1,474 $ 1,505

Series A Preferred Units (Table

Series A Preferred Units (Tables)	6 Months Ended
	Jun. 30, 2021
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2020 through June 30, 2021 are summarized below: (In millions) Redeemable Series A Preferred Units Balance at December 31, 2020 $ 968 Net income allocated 41 Distributions received by Series A preferred unitholders (41) Balance at June 30, 2021 $ 968

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2021
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, capital expenditures and investments in unconsolidated affiliates as well as total assets for our reportable segments: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 L&S Service revenue $ 992 $ 931 $ 1,945 $ 1,935 Rental income 176 246 425 488 Product related revenue 4 21 8 40 Sales-type lease revenue - related parties 136 38 173 76 Income from equity method investments 35 40 71 90 Other income 16 14 31 27 Total segment revenues and other income (1) 1,359 1,290 2,653 2,656 Segment Adjusted EBITDA (2) 947 839 1,843 1,711 Capital expenditures 76 108 135 292 Investments in unconsolidated affiliates 13 74 22 128 G&P Service revenue 493 489 1,001 1,025 Rental income 91 89 183 177 Product related revenue 407 151 804 373 Income/(loss) from equity method investments 31 49 65 (1,185) Other income 14 13 28 27 Total segment revenues and other income (1) 1,036 791 2,081 417 Segment Adjusted EBITDA (2) 427 388 883 810 Capital expenditures 36 110 66 244 Investments in unconsolidated affiliates $ 36 $ 57 $ 62 $ 94 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $267 million for the three and six months ended June 30, 2021, respectively, and $146 million and $304 million for the three and six months ended June 30, 2020, respectively. Third party revenues for the G&P segment were $988 million and $1,977 million for the three and six months ended June 30, 2021, respectively, and $748 million and $323 million for the three and six months ended June 30, 2020, respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	(In millions) June 30, 2021 December 31, 2020 Segment assets Cash and cash equivalents $ 8 $ 15 L&S 20,869 20,938 G&P 14,896 15,461 Total assets $ 35,773 $ 36,414
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The table below provides a reconciliation between “Net income/(loss)” and Segment Adjusted EBITDA. Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Reconciliation to Net income/(loss): L&S Segment Adjusted EBITDA $ 947 $ 839 $ 1,843 $ 1,711 G&P Segment Adjusted EBITDA 427 388 883 810 Total reportable segments 1,374 1,227 2,726 2,521 Depreciation and amortization (1) (318) (321) (647) (646) Provision for income taxes — — (1) — Amortization of deferred financing costs (18) (15) (35) (29) Non-cash equity-based compensation (2) (3) (5) (8) Impairment expense (42) — (42) (2,165) Net interest and other financial costs (198) (208) (418) (424) Gain on extinguishment of debt — — 12 — Income/(loss) from equity method investments 66 89 136 (1,095) Distributions/adjustments related to equity method investments (121) (115) (242) (239) Unrealized derivative (losses)/gains (2) (36) (6) (39) 9 Other — (1) (2) (2) Adjusted EBITDA attributable to noncontrolling interests 10 8 20 17 Net income/(loss) $ 715 $ 655 $ 1,463 $ (2,061) (1) Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories (Tables)

Inventories (Tables)	6 Months Ended
	Jun. 30, 2021
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) June 30, 2021 December 31, 2020 NGLs $ 5 $ 5 Line fill 18 13 Spare parts, materials and supplies 103 100 Total inventories $ 126 $ 118

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	6 Months Ended
	Jun. 30, 2021
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) June 30, 2021 December 31, 2020 L&S Pipelines $ 6,045 $ 6,026 Refining logistics 1,789 2,333 Terminals 1,652 1,643 Marine 965 965 Land, building and other 1,575 1,584 Construction-in-progress 217 262 Total L&S property, plant and equipment 12,243 12,813 G&P Gathering and transportation 7,574 7,547 Processing and fractionation 5,726 5,721 Land, building and other 511 507 Construction-in-progress 276 287 Total G&P property, plant and equipment 14,087 14,062 Total property, plant and equipment 26,330 26,875 Less accumulated depreciation 5,978 5,657 Property, plant and equipment, net $ 20,352 $ 21,218

Goodwill and Intangibles (Table

Goodwill and Intangibles (Tables)	6 Months Ended
	Jun. 30, 2021
Finite-Lived Intangible Assets [Line Items]
Schedule of Finite-Lived Intangible Assets [Table Text Block]	Gross intangible assets with accumulated amortization as of June 30, 2021 and December 31, 2020 is shown below: June 30, 2021 December 31, 2020 (In millions) Useful Lives Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (2) Net L&S 6 - 8 years $ 283 $ (99) $ 184 $ 283 $ (81) $ 202 G&P 6 - 25 years 1,288 (576) 712 1,288 (531) 757 $ 1,571 $ (675) $ 896 $ 1,571 $ (612) $ 959 (1) Amortization expense attributable to the L&S and G&P segments for the six months ended June 30, 2021 was $18 million and $45 million, respectively. (2) Impairment charge of $177 million is included within the G&P accumulated amortization.
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	Estimated future amortization expense related to the intangible assets at June 30, 2021 is as follows: (In millions) 2021 $ 64 2022 127 2023 127 2024 127 2025 113 Thereafter 338 Total $ 896

Goodwill and Intangibles Goodwi

Goodwill and Intangibles Goodwill and Intangibles (Tables)	6 Months Ended
	Jun. 30, 2021
Goodwill [Line Items]
Schedule of Goodwill [Table Text Block]	Changes in the carrying amount of goodwill were as follows: (In millions) L&S G&P Total Gross goodwill as of December 31, 2019 $ 7,722 $ 3,141 $ 10,863 Accumulated impairment losses — (1,327) (1,327) Balance as of December 31, 2019 7,722 1,814 9,536 Impairment losses — (1,814) (1,814) Wholesale Exchange (Note 3) (65) — (65) Balance as of December 31, 2020 7,657 — 7,657 Balance as of June 30, 2021 7,657 — 7,657 Gross goodwill as of June 30, 2021 7,657 3,141 10,798 Accumulated impairment losses — (3,141) (3,141) Balance as of June 30, 2021 $ 7,657 $ — $ 7,657

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2021
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value on a recurring basis as of June 30, 2021 and December 31, 2020 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. June 30, 2021 December 31, 2020 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (102) $ — $ (63) Total carrying value on Consolidated Balance Sheets $ — $ (102) $ — $ (63)
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase commitment embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.64 to $1.59 per gallon with a weighted average of $0.81 per gallon and (2) the probability of renewal of 100 percent for the first five five
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]	The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended June 30, 2021 Three Months Ended June 30, 2020 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (66) $ — $ (45) Total (losses)/gains (realized and unrealized) included in earnings (1) — (39) — (7) Settlements — 3 — 1 Fair value at end of period — (102) — (51) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized (losses)/gains relating to liabilities still held at end of period $ — $ (39) $ — $ (6) Six Months Ended June 30, 2021 Six Months Ended June 30, 2020 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (63) $ — $ (60) Total (losses)/gains (realized and unrealized) included in earnings (1) — (45) — 7 Settlements — 6 — 2 Fair value at end of period — (102) — (51) The amount of total (losses)/gains for the period included in earnings attributable to the change in unrealized (losses)/gains relating to liabilities still held at end of period $ — $ (41) $ — $ 7 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
Fair Value Carrying Value by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of our debt, excluding finance leases: June 30, 2021 December 31, 2020 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt (including amounts due within one year) $ 21,838 $ 19,335 $ 22,951 $ 20,244

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of June 30, 2021, MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below.
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) June 30, 2021 December 31, 2020 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (14) $ — $ (7) Other noncurrent assets / Deferred credits and other liabilities — (88) — (56) Total $ — $ (102) $ — $ (63) (1) Includes embedded derivatives in commodity contracts as discussed above.
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2021 2020 2021 2020 Purchased product costs Realized (loss)/gain $ (3) $ (1) $ (6) $ (2) Unrealized (loss)/gain (36) (6) (39) 9 Purchased product costs derivative (loss)/gain (39) (7) (45) 7 Total derivative (loss)/gain $ (39) $ (7) $ (45) $ 7

Debt (Tables)

Debt (Tables)	6 Months Ended
	Jun. 30, 2021
Debt Disclosure [Abstract]
Schedule of Debt [Table Text Block]	MPLX’s outstanding borrowings consist of the following: (In millions) June 30, 2021 December 31, 2020 MPLX LP: Bank revolving credit facility $ — $ 175 Floating rate senior notes 1,000 1,000 Fixed rate senior notes 18,532 19,240 Consolidated subsidiaries: MarkWest 23 23 ANDX 45 87 Financing lease obligations (1) 9 11 Total 19,609 20,536 Unamortized debt issuance costs (109) (116) Unamortized discount/premium (265) (281) Amounts due within one year (1) (764) Total long-term debt due after one year $ 19,234 $ 19,375 (1) See Note 20 for lease information.

Revenue (Tables)

Revenue (Tables)	6 Months Ended
	Jun. 30, 2021
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	The following tables represent a disaggregation of revenue for each reportable segment for the three and six months ended June 30, 2021 and 2020: Three Months Ended June 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 91 $ 487 $ 578 Service revenue - related parties 901 6 907 Service revenue - product related — 76 76 Product sales 1 303 304 Product sales - related parties 3 28 31 Total revenues from contracts with customers $ 996 $ 900 1,896 Non-ASC 606 revenue (1) 499 Total revenues and other income $ 2,395 Three Months Ended June 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 77 $ 486 $ 563 Service revenue - related parties 854 3 857 Service revenue - product related — 22 22 Product sales 17 103 120 Product sales - related parties 4 26 30 Total revenues from contracts with customers $ 952 $ 640 1,592 Non-ASC 606 revenue (1) 489 Total revenues and other income $ 2,081 Six Months Ended June 30, 2021 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 175 $ 992 $ 1,167 Service revenue - related parties 1,770 9 1,779 Service revenue - product related — 153 153 Product sales 2 584 586 Product sales - related parties 6 67 73 Total revenues from contracts with customers $ 1,953 $ 1,805 3,758 Non-ASC 606 revenue (1) 976 Total revenues and other income $ 4,734 Six Months Ended June 30, 2020 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 161 $ 1,014 $ 1,175 Service revenue - related parties 1,774 11 1,785 Service revenue - product related — 61 61 Product sales 32 257 289 Product sales - related parties 8 55 63 Total revenues from contracts with customers $ 1,975 $ 1,398 3,373 Non-ASC 606 loss (1) (300) Total revenues and other income $ 3,073
Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block]	The tables below reflect the changes in our contract balances for the six-month periods ended June 30, 2021 and 2020: (In millions) Balance at December 31, 2020 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 40 $ (25) $ 1 $ 16 Long-term contract assets 2 — — 2 Deferred revenue 37 24 (17) 44 Deferred revenue - related parties 91 37 (50) 78 Long-term deferred revenue 119 9 — 128 Long-term deferred revenue - related parties 48 (8) — 40 Long-term contract liability $ 6 $ — $ — $ 6 (In millions) Balance at December 31, 2019 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at Contract assets $ 39 $ (20) $ (1) $ 18 Deferred revenue 23 8 (5) 26 Deferred revenue - related parties 53 48 (29) 72 Long-term deferred revenue 90 11 — 101 Long-term deferred revenue - related parties $ 55 $ (2) $ — $ 53 (1) Balance represents ASC 606 portion of each respective line item.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	As of June 30, 2021, the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $289 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 $ 933 2022 1,767 2023 1,615 2024 1,491 2025 and thereafter 4,427 Total revenue on remaining performance obligations (1),(2),(3) $ 10,233 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	6 Months Ended
	Jun. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information [Table Text Block]	Six Months Ended June 30, (In millions) 2021 2020 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 414 $ 423 Income taxes paid 1 1 Non-cash investing and financing activities: Net transfers of property, plant and equipment (to)/from materials and supplies inventories $ — $ (1)
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures [Table Text Block]	The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Six Months Ended June 30, (In millions) 2021 2020 Decrease in capital accruals $ (34) $ (172)

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2020 through June 30, 2021. (In millions) Pension Other Total Balance at December 31, 2020 (1) $ (13) $ (2) $ (15) Other comprehensive loss - remeasurements (2) (2) — (2) Balance at June 30, 2021 (1) $ (15) $ (2) $ (17) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2019 through June 30, 2020. (In millions) Pension Other Total Balance at December 31, 2019 (1) $ (14) $ (1) $ (15) Other comprehensive loss - remeasurements (2) — (1) (1) Balance at June 30, 2020 (1) $ (14) $ (2) $ (16) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	6 Months Ended
	Jun. 30, 2021
Phantom Units
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units for the six months ended June 30, 2021: Number Weighted Outstanding at December 31, 2020 644,023 $ 28.65 Granted 335,904 24.97 Settled (201,400) 30.30 Forfeited (1,330) 33.19 Outstanding at June 30, 2021 777,197 $ 26.63
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	Performance Units The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the six months ended June 30, 2021: Number of Outstanding at December 31, 2020 3,092,286 Granted 375,730 Settled (1,347,177) Forfeited (1,761) Outstanding at June 30, 2021 2,119,078

Leases Lessor Disclosure (Table

Leases Lessor Disclosure (Tables)	6 Months Ended
	Jun. 30, 2021
Lessor Disclosure [Abstract]
Operating Lease, Lease Income [Table Text Block]	Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended June 30, 2021 Three Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 144 $ 68 $ 195 $ 66 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 135 — 38 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A Six Months Ended June 30, 2021 Six Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 346 $ 136 $ 381 $ 129 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 172 — 76 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A (1) These amounts are presented net of executory costs. See Note 5 for additional information on where related party lease assets are recorded in the Consolidated Balance Sheets. Third party lease assets are less than $1 million as of June 30, 2021 and are included within the “Receivables, net” and “Other noncurrent assets” captions within the Consolidated Balance Sheets. The following is a schedule of future payments on the sales-type leases as of June 30, 2021: (In millions) Related Party 2021 $ 271 2022 543 2023 544 2024 537 2025 525 2026 and thereafter 1,017 Total minimum future rentals 3,437 Less: present value discount 2,520 Lease receivable $ 917
Sales-type Lease, Lease Income [Table Text Block]	Lease revenues included on the Consolidated Statements of Income were as follows: Three Months Ended June 30, 2021 Three Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 144 $ 68 $ 195 $ 66 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 135 — 38 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A Six Months Ended June 30, 2021 Six Months Ended June 30, 2020 (In millions) Related Party Third Party Related Party Third Party Operating leases: Operating lease revenue (1) $ 346 $ 136 $ 381 $ 129 Sales-type leases: Profit/(loss) recognized at the commencement date — — — N/A Interest income (Sales-type lease revenue- fixed minimum) 172 — 76 N/A Interest income (Revenue from variable lease payments) $ 1 $ — $ — N/A (1) These amounts are presented net of executory costs. See Note 5 for additional information on where related party lease assets are recorded in the Consolidated Balance Sheets. Third party lease assets are less than $1 million as of June 30, 2021 and are included within the “Receivables, net” and “Other noncurrent assets” captions within the Consolidated Balance Sheets. The following is a schedule of future payments on the sales-type leases as of June 30, 2021: (In millions) Related Party 2021 $ 271 2022 543 2023 544 2024 537 2025 525 2026 and thereafter 1,017 Total minimum future rentals 3,437 Less: present value discount 2,520 Lease receivable $ 917

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions	Jul. 31, 2020shares	Jun. 30, 2021USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Jun. 30, 2021USD ($)	Jun. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss							$ 1,814
Equity Method Investment, Other than Temporary Impairment		$ 6	$ 1,264	$ 1,264	$ 6
Impairment of Intangible Assets, Finite-lived				177
Number of reportable segments					2
Impairment of Long-Lived Assets Held-for-use				174
Impairment Charges				3,429
Goodwill and Intangible Asset Impairment		$ 42	$ 0		$ 42	$ 2,165
G&P
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss				$ 1,814			$ 1,814
Western Refining Wholesale & Western Refining Product Transport [Member]
Description Of Business And Basis Of Presentation [Line Items]
Partners' Capital Account, Units, Redeemed | shares	18,582,088

Javelina Sale (Details)

Javelina Sale (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021USD ($)
Sale of Javelina Assets and Liabilities
Noncash or Part Noncash Divestitures [Line Items]
Gain (Loss) on Disposition of Assets	$ 0

Wholesale Exchange (Details)

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

Investments and Noncontrollin_3

Investments and Noncontrolling Interests (Summary of Equity Method Investment Financial Information) (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Revenues		$ 2,395	$ 2,081		$ 4,734		$ 3,073
Equity method investments		4,033			4,033				$ 4,036
Costs and Expenses		1,464	1,203		2,829		4,681
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		715	655		1,463		(2,061)
Income/(loss) from equity method investments(1)	[1]	66	89		136	[2]	(1,095)	[3]
Assets, Current		1,332			1,332				1,515
Liabilities, Current		1,727			1,727				2,086
Equity Method Investment, Other than Temporary Impairment		$ 6	1,264	$ 1,264	$ 6
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%			25.00%
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%			35.00%
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%			41.00%
Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%			67.00%
Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		17.00%			17.00%
Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		38.00%			38.00%
W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%			50.00%
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%			25.00%
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		57.00%			57.00%
Equity Method Investment, Other than Temporary Impairment				$ 1,251
Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%			50.00%
MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%			67.00%
Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		78.00%			78.00%
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%			40.00%
MarkWest Tornado GP, L.L.C.
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		60.00%			60.00%
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		9.19%			9.19%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		24.55%			24.55%
Indirect Ownership Interest [Member] | Wink to Webster Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		15.00%			15.00%
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		51.00%			51.00%
G&P
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		$ (56)			$ (56)				(57)
Difference between carrying amount and underlying equity portion related to goodwill		31			31				31
G&P | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	690			690				698
G&P | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	550			550				557
G&P | MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	327			327				307
G&P | Rendezvous Gas Services, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	153			153				159
G&P | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	141			141				148
G&P | Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		131			131				145
G&P | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	46			46				49
G&P | MarkWest Tornado GP, L.L.C.
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	213			213				188
L&S
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		332			332				331
Difference between carrying amount and underlying equity portion related to goodwill		167			167				167
L&S | MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[5]	460			460				465
L&S | Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investments		257			257				254
L&S | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments		264			264				252
L&S | Andeavor Logistics Rio Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	191			191				194
L&S | Minnesota Pipe Line Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		185			185				188
L&S | Whistler Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	181			181				185
L&S | W2W Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4],[6]	70			70				72
L&S | Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investments		65			65				72
L&S | Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	[4]	109			109				103
Operating Segments | G&P
Schedule of Equity Method Investments [Line Items]
Revenues	[7]	1,036	791		2,081		417
Equity method investments		2,251			2,251				2,251
Income/(loss) from equity method investments(1)		31	49		65		(1,185)
Operating Segments | L&S
Schedule of Equity Method Investments [Line Items]
Revenues	[7]	1,359	1,290		2,653		2,656
Equity method investments		1,782			1,782				1,785
Income/(loss) from equity method investments(1)		35	$ 40		71		90
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					943		597
Costs and Expenses					501		476
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					442		121
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					388		49
Assets, Current		819			819				848
Assets, Noncurrent		12,310			12,310				11,886
Liabilities, Current		464			464				510
Liabilities, Noncurrent		3,241			3,241				2,734
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					607		640
Costs and Expenses					284		274
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					323		366
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					283		332
Income/(loss) from equity method investments(1)					63	[2]	83	[3]
Assets, Current		464			464				318
Assets, Noncurrent		4,909			4,909				4,997
Liabilities, Current		281			281				187
Liabilities, Noncurrent		862			862				830
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues					336		(43)
Costs and Expenses					217		202
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					119		(245)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					105		(283)
Income/(loss) from equity method investments(1)					73	[2]	$ (1,178)	[3]
Assets, Current		355			355				530
Assets, Noncurrent		7,401			7,401				6,889
Liabilities, Current		183			183				323
Liabilities, Noncurrent		$ 2,379			$ 2,379				$ 1,904
[1]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[2]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	Investments deemed to be VIEs. Some investments included within “Other” have also been deemed to be VIEs.
[5]	The investment in MarEn Bakken Company LLC includes our 9.19 percent indirect interest in a joint venture (“Dakota Access”) that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system or DAPL.
[6]	Through our ownership interest in W2W Holdings LLC, we have a 15 percent equity interest Wink to Webster Pipeline LLC.
[7]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $267 million for the three and six months ended June 30, 2021, respectively, and $146 million and $304 million for the three and six months ended June 30, 2020, respectively. Third party revenues for the G&P segment were $988 million and $1,977 million for the three and six months ended June 30, 2021, respectively, and $748 million and $323 million for the three and six months ended June 30, 2020, respectively.

Related Party Agreements and _3

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

Related Party Agreements and _4

Related Party Agreements and Transactions MPC Loan Agreement (Details) - Related Party Revolving Credit Agreement [Member] - MPC Investment [Member] - USD ($) $ in Millions		Jul. 31, 2019	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 1,500
Debt Instrument, Description of Variable Rate Basis		LIBOR plus 1.25 percent
Related party debt - borrowings			$ 4,435	$ 2,708	$ 6,264
Line of Credit Facility, Interest Rate During Period			1.354%		2.278%
Repayments of Lines of Credit			$ 3,942	$ 3,302	$ 6,858
Long-term Line of Credit	[1]		$ 493		$ 0
[1]	Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.

Related Party Disclosures (Deta

Related Party Disclosures (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Related Party Transaction [Line Items]
Operating Lease, Lease Income		$ 99	$ 98	$ 198	$ 194
Related Party Transaction, Other Revenues from Transactions with Related Party		27	25	55	51
Revenue from Related Parties				15	4
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Operating Lease, Lease Income		168	237	410	471
Related Party Transaction, Other Revenues from Transactions with Related Party		11	10	23	20
Sales Revenue, Goods, Related Party, Net Zero		177	52	345	225
Sales-type Lease, Revenue		136	38	173	76
Marathon Petroleum Corporation [Member] | Construction-in-progress
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions		15	15	27	51
Other Affiliates [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Other Revenues from Transactions with Related Party		16	15	32	31
Product [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		31	30	73	63
Product [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	[1]	31	30	73	63
Service [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		907	857	1,779	1,785
Service [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		907	857	1,778	1,784
Service [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		$ 0	$ 0	$ 1	$ 1
[1]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and six months ended June 30, 2021, these sales totaled $177 million and $345 million, respectively. For the three and six months ended June 30, 2020, these sales totaled $52 million and $225 million, respectively.

Summary of Charges for Employee

Summary of Charges for Employee Services and Omnibus Agreements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 297	$ 280	$ 595	$ 556
General and Administrative Expense	87	96	173	193
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	293	276	587	547
General and Administrative Expense	63	68	120	132
Marathon Petroleum Corporation [Member] | Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	23	41	62	87
Other Affiliates [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	4	4	8	9
Construction-in-progress | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 15	$ 15	$ 27	$ 51

Other Assets and Liabilities fr

Other Assets and Liabilities from Related Parties (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Other current assets	$ 46	$ 65
Current Assets, Related Parties	614	677
Due from Related Parties, Noncurrent	1,144	672
Operating Lease, Right-of-Use Asset	297	309
Due to Related Parties, Current	812	356
Operating Lease, Liability, Current	63	63
Operating Lease, Liability, Noncurrent	232	244
Liabilities, Related Parties, Noncurrent	306	283
Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	519	615
Prepaid Insurance	12	4
Other current assets	2	1
Sales-type Lease, Interest Income	73	30
Due from Related Parties, Noncurrent	32	32
Operating Lease, Right-of-Use Asset	230	231
Net Investment in Lease, Noncurrent	844	386
Sales-type Lease, Unguaranteed Residual Asset	38	23
Due to Related Parties, Current	689	215
Operating Lease, Liability, Current	1	1
Operating Lease, Liability, Noncurrent	229	229
Other Affiliates [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	8	27
Due to Related Parties, Current	31	43
Minimum Committed Volume Contracts [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	51	66
Reimbursable Projects [Member] | Marathon Petroleum Corporation [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	39	30
Deferred Revenue, Noncurrent, Related Parties	70	47
Reimbursable Projects [Member] | Other Affiliates [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	1	1
Deferred Revenue, Noncurrent, Related Parties	$ 7	$ 7

Net Income Per Limited Partne_4

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020
Net Income Per Share [Line Items]
Potentially dilutive units		1		1		1		1
Net Income (Loss) Attributable to Parent		$ 706	[1]	$ 648	[1]	$ 1,445	[1]	$ (2,076)
Distribution Made to Limited Partner, Cash Distributions Declared		736		746		1,474		1,505
Undistributed net income (loss) attributable to MPLX LP		(30)		(98)		(29)		(3,581)
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent		675	[1]	617	[1]	1,383	[1]	(2,138)
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	705		715		1,412		1,443
Undistributed net income (loss) attributable to MPLX LP		(30)		(98)		(29)		(3,581)
Series A Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent		21	[1]	21	[1]	41	[1]	41
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	21		21		41		41
Undistributed net income (loss) attributable to MPLX LP		0		0		0		0
Series B Preferred Stock [Member] | Series A Preferred Stock [Member]
Net Income Per Share [Line Items]
Net Income (Loss) Attributable to Parent		10	[1]	10	[1]	21	[1]	21
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	10		10		21		21
Undistributed net income (loss) attributable to MPLX LP		$ 0		$ 0		$ 0		$ 0
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the distribution priorities applicable to the period.
[2]	See Note 7 for distribution information.

Net Income Per Limited Partne_5

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Details) - Limited Partners Common Units [Member] - $ / shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Weighted average units outstanding:
Common - basic (in shares)	1,029	1,059	1,033	1,059
Common - diluted (in shares)	1,029	1,059	1,033	1,059
Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax	$ 0.66	$ 0.58	$ 1.34	$ (2.02)
Common - diluted (in USD per unit)	$ 0.66	$ 0.58	$ 1.34	$ (2.02)

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ / shares in Units, $ in Millions		Aug. 16, 2021	Jul. 27, 2021	Jul. 02, 2021	Apr. 27, 2021	Jul. 31, 2020	Jul. 28, 2020	Apr. 28, 2020	Jul. 29, 2019	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity										$ (693)	$ (729)	$ (634)	$ 2,736
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders										(707)	(735)	(728)	(738)
Distribution Made to Limited Partner, Cash Distributions Declared										736		746		$ 1,474	$ 1,505
Stock Repurchase Program, Authorized Amount										1,000				$ 1,000
Stock Repurchased and Retired During Period, Shares														11,929,998
Treasury Stock Acquired, Average Cost Per Share														$ 26.02
Stock Repurchased and Retired During Period, Value										155	$ 155			$ 310
Stock Repurchase Program, Remaining Authorized Repurchase Amount										657				657
Total MPLX LP partners’ capital										$ 12,552				$ 12,552		$ 12,772
Cash distributions declared per limited partner common unit					$ 0.6875		$ 0.6875	$ 0.6875
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2020				1,027,031,091							1,038,777,978			1,038,777,978
Unit-based compensation awards														183,111
Balance at June 30, 2021										1,027,031,091				1,027,031,091
Distribution Made to Limited Partner, Cash Distributions Declared	[1]									$ 705		715		$ 1,412	1,443
Series B Preferred Stock [Member]
Stockholders Equity [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Excluding Portion Attributable to Temporary Equity										(10)	$ (11)	(10)	(11)	(21)
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders										0	(21)	0	(21)	(21)
Stock Repurchased and Retired During Period, Value										0	0
Total MPLX LP partners’ capital										611	$ 601	611	$ 601	611	611	$ 611	$ 611
Subsequent Event
Stockholders Equity [Line Items]
Stock Repurchased and Retired During Period, Shares				126,293
Stock Repurchased and Retired During Period, Value				$ 4
Cash distributions declared per limited partner common unit			$ 0.6875
Subsequent Event | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared			$ 705
Series B Preferred Stock [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Distribution Made to Limited Partner, Cash Distributions Declared	[1]									$ 10		$ 10		$ 21	$ 21
Series B Preferred Stock [Member] | ANDX LP [Member]
Stockholders Equity [Line Items]
Preferred Units, Outstanding									600,000
Dividend rate, percentage									6.875%					4.652%
Price per share									$ 1,000
Preferred Stock, Dividend Payment Rate, Variable														LIBOR plus
Series B Preferred Stock [Member] | ANDX LP [Member] | Preferred Partner [Member]
Stockholders Equity [Line Items]
Cash distributions declared per limited partner common unit														$ 68.75
Series B Preferred Stock [Member] | Subsequent Event
Stockholders Equity [Line Items]
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		$ (21)
Western Refining Wholesale & Western Refining Product Transport [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Contributed						(18,582,088)
[1]	See Note 7 for distribution information.

Equity - Cash Distributions (De

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

Aug. 16, 2021

Aug. 13, 2021

Aug. 06, 2021

Jul. 27, 2021

Apr. 27, 2021

Jul. 28, 2020

Apr. 28, 2020

Jul. 29, 2019

Jun. 30, 2021

Mar. 31, 2021

Jun. 30, 2020

Mar. 31, 2020

Jun. 30, 2021

Jun. 30, 2020

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

Distribution Made to Limited Partner, Cash Distributions Declared

$ 736

$ 746

$ 1,474

$ 1,505

Cash distributions declared per limited partner common unit

$ 0.6875

$ 0.6875

$ 0.6875

Partners' Capital Account, Distributions

707

$ 735

728

$ 738

Subsequent Event

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

Distribution Made to Limited Partner, Declaration Date

Jul. 27, 2021

Cash distributions declared per limited partner common unit

$ 0.6875

Date of record

Aug. 6, 2021

Subsequent Event | Series B Preferred Stock [Member]

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

Distribution date

Aug. 16, 2021

Partners' Capital Account, Distributions

$ 21

Subsequent Event | Limited Partners Common Units [Member]

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

Distribution date

Aug. 13, 2021

Limited Partners Common Units [Member]

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

Distribution Made to Limited Partner, Cash Distributions Declared

[1]

705

715

1,412

1,443

Limited Partners Common Units [Member] | Subsequent Event

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

Distribution Made to Limited Partner, Cash Distributions Declared

$ 705

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

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

Distribution Made to Limited Partner, Cash Distributions Declared

[1]

10

10

21

21

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

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

Distribution Made to Limited Partner, Cash Distributions Declared

[1]

$ 21

$ 21

$ 41

$ 41

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

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

Preferred Stock, Dividend Payment Rate, Variable

LIBOR plus

Preferred Units, Outstanding

600,000

Preferred Stock, Dividend Rate, Percentage

6.875%

4.652%

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

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

Cash distributions declared per limited partner common unit

$ 68.75

[1]

See Note 7 for distribution information.

Series A Preferred Units (Narra

Series A Preferred Units (Narrative) (Details) - $ / shares shares in Millions	Jul. 27, 2021	Apr. 27, 2021	Jul. 28, 2020	Apr. 28, 2020	May 13, 2016	Jun. 30, 2021
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Distributions Declared, Per Unit		$ 0.6875	$ 0.6875	$ 0.6875
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units					30.8
Dividend rate, percentage					6.50%
Price per share					$ 32.50
Dividend rate, per-dollar-amount					$ 0.528125
Description					The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then change of control conversion rate.
Partners' Capital Account, Units						29.6
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Declaration Date	Jul. 27, 2021
Distribution Made to Limited Partner, Distributions Declared, Per Unit	$ 0.6875

Series A Preferred Units (Rollf

Series A Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Redeemable Noncontrolling Interest [Line Items]
Distributions to unitholders and general partner	$ (1,421)	$ (1,445)
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2020	968
Net income allocated	41
Distributions to unitholders and general partner	(41)
Balance at June 30, 2021	$ 968

Segment Information (Details)

Segment Information (Details)	6 Months Ended
	Jun. 30, 2021
Segment Reporting [Abstract]
Number of reportable segments	2

Segment Information - Segment A

Segment Information - Segment Adjusted EBITDA (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended				12 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2020
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	$ 318	$ 321	$ 647		$ 646
Total revenues from contracts with customers		1,896	1,592	3,758		3,373
Income/(loss) from equity method investments(1)	[2]	66	89	136	[3]	(1,095)	[4]
Other income		3	2	4		3
Total segment revenues and other income		(2,395)	(2,081)	(4,734)		(3,073)
Investments in unconsolidated affiliates				(84)		(222)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[5]	(36)	(6)	(39)		9
Marathon Petroleum Corporation [Member]
Segment Reporting Information [Line Items]
Restructuring Charges								$ 37
Sales-type Lease, Revenue		136	38	173		76
L&S
Segment Reporting Information [Line Items]
Depreciation and amortization		136	138	283		276
Total revenues from contracts with customers		996	952	1,953		1,975
Investments in unconsolidated affiliates		(13)	(74)	(22)		(128)
L&S | Marathon Petroleum Corporation [Member]
Segment Reporting Information [Line Items]
Sales-type Lease, Revenue		136	38	173		76
G&P
Segment Reporting Information [Line Items]
Depreciation and amortization		182	183	364		370
Total revenues from contracts with customers		900	640	1,805		1,398
Investments in unconsolidated affiliates		(36)	(57)	(62)		(94)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA		1,374	1,227	2,726		2,521
Operating Segments | L&S
Segment Reporting Information [Line Items]
Rental income		176	246	425		488
Income/(loss) from equity method investments(1)		35	40	71		90
Other income		16	14	31		27
Total segment revenues and other income	[6]	(1,359)	(1,290)	(2,653)		(2,656)
Adjusted EBITDA	[7]	947	839	1,843		1,711
Capital Expenditure		76	108	135		292
Operating Segments | G&P
Segment Reporting Information [Line Items]
Rental income		91	89	183		177
Income/(loss) from equity method investments(1)		31	49	65		(1,185)
Other income		14	13	28		27
Total segment revenues and other income	[6]	(1,036)	(791)	(2,081)		(417)
Adjusted EBITDA	[7]	427	388	883		810
Capital Expenditure		36	110	66		244
Service [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		578	563	1,167		1,175
Service [Member] | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		91	77	175		161
Service [Member] | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		487	486	992		1,014
Service [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		992	931	1,945		1,935
Service [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		493	489	1,001		1,025
Product [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		304	120	586		289
Product [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		4	21	8		40
Product [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		407	151	804		373
Third Party [Member] | L&S
Segment Reporting Information [Line Items]
Total segment revenues and other income		(138)	(146)	(267)		(304)
Third Party [Member] | G&P
Segment Reporting Information [Line Items]
Total segment revenues and other income		$ (988)	$ (748)	$ (1,977)		$ (323)
[1]	Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively.
[2]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[3]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[5]	MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.
[6]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $138 million and $267 million for the three and six months ended June 30, 2021, respectively, and $146 million and $304 million for the three and six months ended June 30, 2020, respectively. Third party revenues for the G&P segment were $988 million and $1,977 million for the three and six months ended June 30, 2021, respectively, and $748 million and $323 million for the three and six months ended June 30, 2020, respectively.
[7]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 8	$ 15
Assets	35,773	36,414
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	20,869	20,938
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,896	$ 15,461

Segment Information - Reconcili

Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended				12 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2020
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization	[1]	$ 318	$ 321	$ 647		$ 646
Income Tax Expense (Benefit)		0	0	(1)		0
Amortization of Debt Issuance Costs				(35)		(29)
Share-based Payment Arrangement, Expense		(2)	(3)	(5)		(8)
Goodwill and Intangible Asset Impairment		(42)	0	(42)		(2,165)
Interest and Other Financial Costs		(198)	(208)	(418)		(424)
Extinguishment of Debt, Gain (Loss), Net of Tax		0	0	12		0
Income/(loss) from equity method investments(1)	[2]	66	89	136	[3]	(1,095)	[4]
Proceeds from Equity Method Investment, Distribution, Return of Capital				(239)		(226)
Other Cost and Expense, Operating		0	1	2		2
Net income		715	655	1,463		(2,061)
Marathon Petroleum Corporation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges								$ (37)
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		1,374	1,227	2,726		2,521
Amortization of Debt Issuance Costs		(18)	(15)	(35)		(29)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(121)	(115)	(242)		(239)
Adjusted EBITDA attributable to noncontrolling interests		10	8	20		17
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		136	138	283		276
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[5]	947	839	1,843		1,711
Income/(loss) from equity method investments(1)		35	40	71		90
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		182	183	364		370
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[5]	427	388	883		810
Income/(loss) from equity method investments(1)		31	49	65		(1,185)
Not Designated as Hedging Instrument [Member] | Purchased product costs
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[6]	$ (36)	$ (6)	$ (39)		$ 9
[1]	Depreciation and amortization attributable to L&S was $136 million and $283 million for the three and six months ended June 30, 2021, respectively, and $138 million and $276 million for the three and six months ended June 30, 2020, respectively. Depreciation and amortization attributable to G&P was $182 million and $364 million for the three and six months ended June 30, 2021, respectively, and $183 million and $370 million for the three and six months ended June 30, 2020, respectively.
[2]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[3]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[4]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[5]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
[6]	MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
NGLs	$ 5	$ 5
Line fill	18	13
Spare parts, materials and supplies	103	100
Total inventories	$ 126	$ 118

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Jun. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 174
Property, plant and equipment, gross		$ 26,330	$ 26,875
Less accumulated depreciation		5,978	5,657
Property, plant and equipment, net		20,352	21,218
L&S
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		12,243	12,813
L&S | Terminals
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		6,045	6,026
L&S | Marine
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		965	965
L&S | Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,652	1,643
L&S | Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,789	2,333
L&S | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,575	1,584
L&S | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		217	262
G&P
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		14,087	14,062
G&P | Pipelines
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		7,574	7,547
G&P | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		5,726	5,721
G&P | Land, building and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		511	507
G&P | Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 276	$ 287

Goodwill and Intangibles Good_2

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

Goodwill and Intangibles Intang

Goodwill and Intangibles Intangibles (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Jun. 30, 2021		Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived	$ 177
Finite-Lived Intangible Assets, Gross		$ 1,571		$ 1,571
Finite-Lived Intangible Asset, Expected Amortization, Remainder of Fiscal Year		64
Finite-Lived Intangible Asset, Expected Amortization, Year One		127
Finite-Lived Intangible Asset, Expected Amortization, Year Two		127
Finite-Lived Intangible Asset, Expected Amortization, Year Four		113
Finite-Lived Intangible Asset, Expected Amortization, after Year Five		338
Finite-Lived Intangible Assets, Accumulated Amortization		(675)	[1]	(612)	[2]
Intangibles, net		896		959
Finite-Lived Intangible Asset, Expected Amortization, Year Three		127
L&S
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		18
Finite-Lived Intangible Assets, Gross		283		283
Finite-Lived Intangible Assets, Accumulated Amortization		(99)	[1]	(81)
Intangibles, net		$ 184		202
L&S | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		6 years
L&S | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		8 years
G&P
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		$ 45
Finite-Lived Intangible Assets, Gross		1,288		1,288
Finite-Lived Intangible Assets, Accumulated Amortization		(576)	[1]	(531)	[2]
Intangibles, net		$ 712		$ 757
G&P | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		6 years
G&P | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		25 years
[1]	Amortization expense attributable to the L&S and G&P segments for the six months ended June 30, 2021 was $18 million and $45 million, respectively.
[2]	Impairment charge of $177 million is included within the G&P accumulated amortization.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Commodity contracts | Not Designated as Hedging Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	(102)	(63)
Embedded derivatives in commodity contracts | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset		0	0
Derivative liability		$ 102	$ 63
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details)		6 Months Ended
		Jun. 30, 2021USD ($)$ / gal	Dec. 31, 2020USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative, Average Forward Price | $ / gal		0.81
Commodity contracts | Not Designated as Hedging Instrument [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	$ (102,000,000)	(63,000,000)
Embedded derivatives in commodity contracts | Natural Gas [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Embedded Derivative Renewal Term		5 years
Embedded Derivative Second Renewal Term		5 years
Fair Value, Inputs, Level 3 [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Probability of Renewal Second Term		100.00%
Fair Value Inputs Probability of Renewal		100.00%
Fair Value, Inputs, Level 3 [Member] | Embedded derivatives in commodity contracts | Fair Value, Recurring [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative Asset		$ 0	$ 0
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price		0.64
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price		$ 1.59
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Commodity Derivative Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		$ 0	$ 0	$ 0	$ 0
Total gains (losses) (realized and unrealized) included in earnings	[1]	0	0	0	0
Settlements		0	0	0	0
Fair value at end of period		0	0	0	0
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		0	0	0	0
Commodity Derivative Contracts (net) | Not Designated as Hedging Instrument [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[2]	0		0		$ 0
Derivative Liability, Fair Value, Gross Liability	[2]	(102)		(102)		$ (63)
Embedded Derivatives in Commodity Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		(66)	(45)	(63)	(60)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(39)	(7)	(45)	7
Settlements		3	1	6	2
Fair value at end of period		(102)	(51)	(102)	(51)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		$ (39)	$ (6)	$ (41)	$ 7
[1]	Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
[2]	Includes embedded derivatives in commodity contracts as discussed above.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including amounts due within one year)	$ 19,335	$ 20,244
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including amounts due within one year)	$ 21,838	$ 22,951

Derivative Financial Instrume_3

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	6 Months Ended
	Jun. 30, 2021USD ($)	Dec. 31, 2020USD ($)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Derivative [Line Items]
Derivative Liability	$ (102)	$ (63)
Natural Gas [Member]
Derivative [Line Items]
Number of Renewals	2
Embedded Derivative Renewal Term	5 years
Embedded Derivative Second Renewal Term	5 years

Derivative Financial Instrume_4

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	102	63
Other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	14	7
Other noncurrent assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 88	$ 56
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative [Line Items]
Total gain (loss)		$ (39)	$ (7)	$ (45)	$ 7
Purchased product costs
Derivative [Line Items]
Realized gain (loss)		(3)	(1)	(6)	(2)
Total gain (loss)		(39)	(7)	(45)	7
Not Designated as Hedging Instrument [Member] | Purchased product costs
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[1]	$ (36)	$ (6)	$ (39)	$ 9
[1]	MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total		$ 19,609	$ 20,536
Unamortized debt issuance costs		(109)	(116)
Unamortized discount/premium		(265)	(281)
Amounts due within one year		(1)	(764)
Total long-term debt due after one year		19,234	19,375
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,000	1,000
Fixed Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		18,532	19,240
Fixed Rate Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		23	23
Fixed Rate Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		45	87
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Financing lease obligations(1)	[1]	9	11
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		$ 0	$ 175
Minimum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		1.75%
Maximum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.50%
[1]	See Note 20 for lease information.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions		Sep. 09, 2019	Jun. 30, 2021	Jan. 15, 2021	Dec. 31, 2020	Jul. 30, 2019
Debt Instrument [Line Items]
Debt and Lease Obligation			$ 19,609		$ 20,536
Unamortized Debt Issuance Expense			109		116
Debt Instrument, Unamortized Discount			265		281
Debt, Current			1		764
Long-term debt			19,234		19,375
MPLX Revolving Credit Facility due July 2024 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity						$ 3,500
Long-term Line of Credit			0		175
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit			$ 2,800
Debt Instrument, Interest Rate, Effective Percentage			1.345%
Repayments of Long-term Lines of Credit			$ 2,975
Letters of Credit Outstanding			1
Line of Credit Facility, Remaining Borrowing Capacity			$ 3,500
Line of Credit Facility, Remaining Borrowing Capacity, Percentage			100.00%
Fixed Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 18,532		19,240
Fixed Rate Senior Notes [Member] | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			45		87
Fixed Rate Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			23		23
Fixed Rate Senior Notes [Member] | Senior Notes Due January 2025
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt			$ (8)
Long-term Debt, Gross				$ 750
Debt Instrument, Interest Rate, Stated Percentage			525.00%
Percent of Par				102.625%
Payment for Debt Extinguishment or Debt Prepayment Cost			$ 20
Write off of Deferred Debt Issuance Cost			12
Fixed Rate Senior Notes [Member] | Senior Notes Due January 2025 | ANDX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 42
Variable Rate Senior Notes [Member]
Debt Instrument [Line Items]
Percent of Par		100.00%
Variable Rate Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			1,000		1,000
Variable Rate Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR plus 1.1 percent per annum
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Finance Lease, Liability	[1]		$ 9		$ 11
Minimum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			1.75%
Maximum [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			5.50%
[1]	See Note 20 for lease information.

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		$ 1,896	$ 1,592	$ 3,758		$ 3,373
Income/(loss) from equity method investments(1)	[1]	66	89	136	[2]	(1,095)	[3]
Other Operating Income		3	2	4		3
Related Party Transaction, Other Revenues from Transactions with Related Party		27	25	55		51
Operating Income (Loss)	[4]	499	489	976		(300)
Total segment revenues and other income		2,395	2,081	4,734		3,073
L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		996	952	1,953		1,975
G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		900	640	1,805		1,398
Service [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		578	563	1,167		1,175
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		907	857	1,779		1,785
Service [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		91	77	175		161
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		901	854	1,770		1,774
Service [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		487	486	992		1,014
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		6	3	9		11
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		76	22	153		61
Service, Other [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		0	0	0		0
Service, Other [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		76	22	153		61
Product [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers		304	120	586		289
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		31	30	73		63
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		304	120	586		289
Product [Member] | L&S
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		3	4	6		8
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		1	17	2		32
Product [Member] | G&P
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		28	26	67		55
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue		$ 303	$ 103	$ 584		$ 257
[1]	The three and six months ended June 30, 2021 includes $6 million of impairment expense. The six months ended June 30, 2020 includes $1,264 million of impairment expense.
[2]	Includes the impact of any basis differential amortization or accretion in addition to an impairment of $6 million.
[3]	Includes the impact of any basis differential amortization or accretion in addition to the impairment of $1,264 million.
[4]	Non-ASC 606 Revenue includes rental income, sales-type lease revenue, income/(loss) from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.

Revenue Contract Balances (Deta

Revenue Contract Balances (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	[1]
Long-term deferred revenue, beginning balance		$ 314
Long-term deferred revenue, ending balance		349
Liability, change in timeframe, performance obligation satisfied, revenue recognized		0	$ 0
Contract Liability, Noncurrent, Period Increase (Decrease)		0
ASC 606
Contract assets, beginning balance	[1]	40	39
Contract assets, additions/(deletions)		(25)	(20)
Contract assets, revenue recognized	[2]	1	(1)
Contract assets, ending balance		16	18
Contract With Customer Non Current Asset Reclassified To Receivable [Line Items]		0
Contract with Customer, Asset, before Allowance for Credit Loss, Noncurrent		2		$ 2
Deferred revenue, beginning balance	[1]	37	23
Deferred revenue, additions/(deletions)		24	8
Deferred revenue, revenue recognized	[2]	(17)	(5)
Deferred revenue, ending balance		44	26
Deferred revenue - related parties, beginning balance	[1]	91	53
Deferred revenue - related party, additions/(deletions)		37	48
Deferred revenue - related parties, revenue recognized	[2]	(50)	(29)
Deferred revenue - related parties, ending balance		78	72
Long-term deferred revenue, beginning balance	[1]	119	90
Long-term deferred revenue, additions/(deletions)		9	11
Long-term deferred revenue, revenue recognized	[2]	0	0
Long-term deferred revenue, ending balance		128	101
Long-term deferred revenue - related parties, beginning balance	[1]	48	55
Long-term deferred revenue - related party, additions/(deletions)		(8)	(2)
Long-term deferred revenue - related parties, revenue recognized	[2]	0	0
Long-term deferred revenue - related parties, ending balance		40	$ 53
Contract Liability, Noncurrent, Revenue Recognized		0
Contract with Customer, Liability, Noncurrent		6		$ 6
Contract with Customer, Asset Increase (Decrease), Noncurrent		$ 0
[1]	Balance represents ASC 606 portion of each respective line item.
[2]	No significant revenue was recognized related to past performance obligations in the current periods.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	Jun. 30, 2021USD ($)
Revenue from Contract with Customer [Abstract]
Contract with customer, liability	$ 289
Revenue, Remaining Performance Obligation, Amount	10,233	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 933
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,767
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,615
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 1,491
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue from Contract with Customer [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 4,427
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2043-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction, years	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	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 414	$ 423
Income Taxes Paid, Net	1	1
Non-cash investing and financing activities
Net transfers of property, plant and equipment (to)/from materials and supplies inventories	$ 0	$ (1)

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	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Supplemental Cash Flow Elements [Abstract]
Decrease in capital accruals	$ (34)	$ (172)

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions

3 Months Ended

6 Months Ended

Jun. 30, 2021

Jun. 30, 2020

Jun. 30, 2021

Jun. 30, 2020

Mar. 31, 2021

Dec. 31, 2020

[2]

Mar. 31, 2020

Dec. 31, 2019

Accumulated Other Comprehensive Income (Loss) [Line Items]

Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax

[1]

$ 0

$ 0

$ (2)

$ (1)

LOOP and Explorer

Accumulated Other Comprehensive Income (Loss) [Line Items]

Accumulated other comprehensive loss

(17)

[2]

(16)

[2]

(17)

[2]

(16)

[2]

$ (17)

$ (15)

$ (16)

$ (15)

Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax

(2)

[1]

(1)

Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | LOOP and Explorer

Accumulated Other Comprehensive Income (Loss) [Line Items]

Accumulated other comprehensive loss

(15)

[2]

(14)

[2]

(15)

[2]

(14)

[2]

(13)

(14)

Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax

(2)

[1]

0

Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | LOOP and Explorer

Accumulated Other Comprehensive Income (Loss) [Line Items]

Accumulated other comprehensive loss

$ (2)

[2]

$ (2)

[2]

(2)

[2]

(2)

[2]

$ (2)

$ (1)

Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Adjustment, Net of Tax

$ 0

[1]

$ (1)

[1]

Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

[2]

These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Equity-Based Compensation Pla_2

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units	6 Months Ended
	Jun. 30, 2021$ / sharesshares
Number of Units
Outstanding at December 31, 2020 | shares	644,023
Granted | shares	335,904
Settled | shares	201,400
Forfeited | shares	1,330
Outstanding at June 30, 2021 | shares	777,197
Weighted Average Fair Value
Outstanding at December 31, 2020 | $ / shares	$ 28.65
Granted | $ / shares	24.97
Settled | $ / shares	30.30
Forfeited | $ / shares	33.19
Outstanding at June 30, 2021 | $ / shares	$ 26.63

Equity-Based Compensation Pla_3

Equity-Based Compensation Plan - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2021$ / shares
Maximum [Member] | Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [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	$ 2

Equity-Based Compensation Pla_4

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail)	6 Months Ended
	Jun. 30, 2021$ / sharesshares
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2020	3,092,286
Granted	375,730	[1]
Settled	1,347,177
Forfeited	(1,761)
Outstanding at June 30, 2021	2,119,078
Maximum [Member] | Performance Unit Performance Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares	$ 2
[1]	No new performance unit awards were granted during the six months ended June 30, 2021. The performance units shown above as granted represents subsequent tranches of the 2020 and 2019 performance unit awards that were subject to a performance condition based on MPLX LP’s 2021 distributable cash flow, which, due to the nature of the award terms, did not meet the criteria for a grant date fair value until the first quarter of 2021, at which time we began recognizing units and expense related to these tranches. Grant date fair value of the performance condition is based on potential payouts of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes.

Leases Leases Narrative (Detail

Leases Leases Narrative (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Reclassification, Other
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount		$ 421			$ 421
Marathon Petroleum Corporation [Member]
Operating Lease, Lease Income	[1]	144	$ 195		346	$ 381
Sales-type Lease, Unguaranteed Residual Asset		38			38		$ 23
Sales-type Lease, Selling Profit (Loss)		0	0		0	0
Sales-type Lease, Lease Receivable		917			917
Marathon Petroleum Corporation [Member] | Reclassification, Other
Sales-type Lease, Unguaranteed Residual Asset		14			14
Sales-type Lease, Selling Profit (Loss)		112
Sales-type Lease, Lease Receivable		519			519
Refining Logistics [Member] | Reclassification, Other
Lessor, Sales-type Lease, Assumptions and Judgments, Value of Underlying Asset, Amount				$ 171
Refining Logistics [Member] | Marathon Petroleum Corporation [Member] | Reclassification, Other
Sales-type Lease, Unguaranteed Residual Asset				10
Sales-type Lease, Selling Profit (Loss)				209
Sales-type Lease, Lease Receivable				$ 370
Third Party [Member]
Operating Lease, Lease Income	[1]	68	$ 66		136	$ 129
Sales-type Lease, Selling Profit (Loss)		0			0
Sales-type Lease, Lease Receivable		$ 1			$ 1
[1]	These amounts are presented net of executory costs

Leases Lessor Maturity Table (D

Leases Lessor Maturity Table (Details) $ in Millions	Jun. 30, 2021USD ($)
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable	$ 917
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Current Year	271
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year One	543
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received, Three Years	544
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Three	537
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, Year Four	525
Sales-Type and Direct Financing Leases, Lease Receivable, to be Received, after Year Five	1,017
Sales-type and Direct Financing Leases, Lease Receivable, Payments to be Received	3,437
Sales-type and Direct Financing Leases, Lease Receivable, Undiscounted Excess Amount	2,520
Marathon Petroleum Corporation [Member] | Reclassification, Other
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable	519
Third Party [Member]
Lessor, Lease, Description [Line Items]
Sales-type Lease, Lease Receivable	$ 1

Leases Lessor Lease Revenues (D

Leases Lessor Lease Revenues (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Marathon Petroleum Corporation [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	$ 144	$ 195	$ 346	$ 381
Sales-type Lease, Selling Profit (Loss)		0	0	0	0
Sales-type Lease, Interest Income		135	38	172	76
Sales-type Lease, Variable Lease Income		1	0	1	0
Third Party [Member]
Lessor, Lease, Description [Line Items]
Operating Lease, Lease Income	[1]	68	$ 66	136	$ 129
Sales-type Lease, Selling Profit (Loss)		0		0
Sales-type Lease, Interest Income		0		0
Sales-type Lease, Variable Lease Income		$ 0		$ 0
[1]	These amounts are presented net of executory costs

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2021
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation		$ 17	$ 24
Due to Related Parties, Current		356	812
Loss Contingency, Damages Sought, Value		187
Contractual commitments to acquire property, plant and equipment			83
Loss Contingency, Damages Paid, Value	$ 4
Marathon Petroleum Corporation [Member]
Commitments And Contingencies [Line Items]
Due to Related Parties, Current		215	689
Environmental Loss Contingency [Member] | Marathon Petroleum Corporation [Member]
Commitments And Contingencies [Line Items]
Due to Related Parties, Current		$ 0	$ 0
Whistler Pipeline LLC [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage			38.00%
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage			9.19%
Guarantee Type, Other [Member]
Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted			$ 230

Subsequent Events (Details)

Subsequent Events (Details) - Variable Rate Senior Notes [Member] - USD ($) $ in Millions	Aug. 04, 2021	Sep. 09, 2019	Jun. 30, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Percent of Par		100.00%
Floating Rate Senior Notes Due September 2022 [Member]
Subsequent Event [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR plus 1.1 percent per annum
Subsequent Event | Floating Rate Senior Notes Due September 2022 [Member]
Subsequent Event [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 1.1 percent per annum
MPLX LP [Member]
Subsequent Event [Line Items]
Long-term Debt, Gross			$ 1,000	$ 1,000
MPLX LP [Member] | Subsequent Event
Subsequent Event [Line Items]
Long-term Debt, Gross	$ 1,000
Percent of Par	100.00%