MPLX (MPLX) 10-K 28 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Entity Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1,552,000
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Public Float			$ 9.8
Common Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		794,158,848
Class B Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		0
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		0

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 5,007
Rental income		349		$ 277	$ 298
Rental income - related parties		718		279	235
Income/(loss) from equity method investments	[1],[2]	240		78	(74)
Other income		7		6	7
Other income - related parties		99		92	86
Total revenues and other income		6,425		3,867	3,029
Costs and expenses:
Rental cost of sales		135		62	57
Rental cost of sales - related parties		5		2	1
Purchases - related parties		860		455	388
Depreciation and amortization	[3]	766		683	591
Impairment expense		0		0	130
General and administrative expenses		291		241	227
Other taxes		72		54	50
Total costs and expenses		3,922		2,676	2,346
Income from operations		2,503		1,191	683
Related party interest and other financial costs		5		2	1
Interest expense (net of amounts capitalized of $33 million, $32 million, and $28 million, respectively)		534		296	210
Other financial costs		122		56	50
Income before income taxes		1,842		837	422
Provision/(benefit) for income taxes		8		1	(12)
Net income		1,834		836	434
Less: Net income attributable to noncontrolling interests		16		6	2
Less: Net income attributable to Predecessor		0		36	199
Net income attributable to MPLX LP	[4]	1,818		794	233
Less: Preferred unit distributions		75		65	41
Less: General partner’s interest in net income attributable to MPLX LP		0		318	191
Limited partners’ interest in net income attributable to MPLX LP		$ 1,743		$ 411	$ 1
Weighted average limited partner units outstanding:
Common - basic (in units)		761		393	338
Common - diluted (in units)		761		396	345
Limited Partners Common Units
Costs and expenses:
Net income attributable to MPLX LP	[4]	$ 1,743		$ 411	$ 1
Net income attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 2.29		$ 1.07	$ 0
Common - diluted (in USD per unit)		$ 2.29		$ 1.06	$ 0
Weighted average limited partner units outstanding:
Common - basic (in units)		761		385	331
Common - diluted (in units)		761		388	338
Service [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,704		$ 1,156	$ 958
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		2,159		1,082	936
Product [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		902		889	572
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		49		8	11
Oil and Gas, Refining and Marketing [Member]
Costs and expenses:
Cost of Goods and Services Sold		948	[5]	528	454
Natural Gas, Midstream [Member]
Costs and expenses:
Cost of Goods and Services Sold		845		651	448
Service, Other [Member]
Revenues and other income:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 198		$ 0	$ 0
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	Depreciation and amortization attributable to L&S was $240 million, $163 million and $128 million for the years ended 2018, 2017 and 2016, respectively. Depreciation and amortization attributable to G&P was $526 million, $520 million and $463 million for 2018, 2017 and 2016, respectively.
[4]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.
[5]	Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”

Consolidated Statements of In_2

Consolidated Statements of Income Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Interest costs capitalized	$ 33	$ 32	$ 28

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 1,834	$ 836	$ 434
Other comprehensive (loss)/income, net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	(2)	0	0
Comprehensive income	1,832	836	434
Less comprehensive income attributable to:
Noncontrolling interests	16	6	2
Income attributable to Predecessor	0	36	199
Comprehensive income attributable to MPLX LP	$ 1,816	$ 794	$ 233

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 68	$ 5
Receivables, net		417	292
Receivables - related parties		289	160
Inventories		77	65
Other current assets		46	37
Total current assets		897	559
Equity method investments		4,174	4,010
Property, plant and equipment, net		14,639	12,187
Intangibles, net		424	453
Goodwill		2,586	2,245
Long-term receivables - related parties		24	20
Other noncurrent assets		35	26
Total assets		22,779	19,500
Current liabilities:
Accounts payable		162	151
Accrued liabilities		250	231
Payables - related parties		203	516
Deferred revenue - related parties		51	43
Accrued property, plant and equipment		294	194
Accrued interest payable		143	88
Other current liabilities		83	81
Total current liabilities		1,186	1,304
Long-term deferred revenue		80	42
Long-term deferred revenue - related parties		43	43
Long-term debt		13,392	6,945
Deferred income taxes		13	5
Deferred credits and other liabilities		197	188
Total liabilities		14,911	8,527
Commitments and contingencies (see Note 25)
Redeemable preferred units		1,004	1,000
Equity
Total MPLX LP partners’ capital		6,708	9,827
Accumulated other comprehensive loss		(16)	(14)
Noncontrolling interests		156	146
Total equity		6,864	9,973
Total liabilities, preferred units and equity		22,779	19,500
MPC
Current assets:
Receivables - related parties		281	153
Long-term receivables - related parties		24	20
Current liabilities:
Payables - related parties	[1]	131	470
Limited Partners Common Units | Public
Equity
Total MPLX LP partners’ capital		8,336	8,379
Total equity		8,336	8,379
Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital		(1,612)	2,099
Total equity		(1,612)	2,099
Limited Partners Class B Units | Public
Equity
Total equity		0	0
General Partner | MPC
Equity
Total MPLX LP partners’ capital		0	(637)
Total equity		0	(637)
LOOP and Explorer
Equity
Accumulated other comprehensive loss		$ (16)	$ (14)
[1]	Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017. There was no outstanding balance on the MPC Loan Agreement as of December 31, 2018.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Dec. 31, 2018	Dec. 31, 2017
Limited Partners Common Units | Public
Units issued	289	289
Units outstanding	289	289
Limited Partners Common Units | MPC
Units issued	505	118
Units outstanding	505	118
Limited Partners Class B Units
Units issued	0	0
Units outstanding	0	0
Common Unitholder GP | MPC
Units issued	0	0
Units outstanding	0
General Partner | MPC
Units issued	0	8
Units outstanding	0	8

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)		Dec. 31, 2017USD ($)		Dec. 31, 2016USD ($)
Operating activities:
Net income	$ 1,834		$ 836		$ 434
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs	59		53		46
Depreciation and amortization	766	[1]	683	[1]	591	[1]
Impairment expense	0		0		130
Deferred income taxes	8		(1)		(17)
Asset retirement expenditures	(7)		(2)		(6)
Loss/(gain) on disposal of assets	2		0		(1)
Income from equity method investments	(240)	[2],[3]	(78)	[2],[3]	74	[2],[3]
Distributions from unconsolidated affiliates	400		241		148
Changes in:
Current receivables	(122)		8		(52)
Inventories	(5)		(3)		(8)
Fair value of derivatives	(10)		6		43
Current accounts payable and accrued liabilities	100		48		102
Receivables from/liabilities to related parties	(50)		63		(19)
Prepaid other current assets from related parties	7		(8)		0
Deferred revenue	39		33		10
All other, net	45		28		16
Net cash provided by operating activities	2,826		1,907		1,491
Investing activities:
Additions to property, plant and equipment	(1,919)		(1,411)		(1,313)
Acquisitions, net of cash acquired	(451)		(249)		0
Investments - net related party loans	0		80		(17)
Disposal of assets	8		7		1
Investments in unconsolidated affiliates	(341)		(761)		(87)
Distributions from unconsolidated affiliates - return of capital	16		26		0
All other, net	1		0		(1)
Net cash used in investing activities	(2,686)		(2,308)		(1,417)
Financing activities:
Long-term debt - borrowings	13,186		2,911		434
Long-term debt - repayments	(6,780)		(416)		(1,312)
Related party debt - borrowings	3,962		2,369		2,532
Related party debt - repayments	(4,347)		(1,983)		(2,540)
Debt issuance costs	(76)		(29)		0
Net proceeds from equity offerings	0		483		792
Issuance of redeemable preferred units	0		0		984
Distributions to preferred unitholders	(71)		(65)		(25)
Distributions to MPC for acquisitions	(4,111)		(1,951)		0
Distributions to MPC from Predecessor	0		113		104
Distributions to unitholders and general partner	(1,819)		(1,120)		(845)
Distributions to noncontrolling interests	(17)		(7)		(3)
Contributions from MPC	0		0		225
Contributions from noncontrolling interests	11		129		6
Consideration payment to Class B unitholders	0		(25)		(25)
All other, net	(11)		(12)		(6)
Net cash (used in)/provided by financing activities	(73)		171		113
Net increase/(decrease) in cash, cash equivalents and restricted cash	67		(230)		187
Cash, cash equivalents and restricted cash at beginning of period	76	[4]	9	[4]	239
Cash, cash equivalents and restricted cash at end of period	$ 76	[4]	$ 9	[4]	$ 239
[1]	Depreciation and amortization attributable to L&S was $240 million, $163 million and $128 million for the years ended 2018, 2017 and 2016, respectively. Depreciation and amortization attributable to G&P was $526 million, $520 million and $463 million for 2018, 2017 and 2016, respectively.
[2]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[3]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[4]	As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($)	Total	PublicLimited Partners Common Units	PublicLimited Partners Class B Units	MPCLimited Partners Common Units	MPCGeneral Partner	AOCI Attributable to Parent	Noncontrolling Interest	Equity of PredecessorMPC
Beginning balance at Dec. 31, 2015	$ 9,946,000,000	$ 7,691,000,000	$ 266,000,000	$ 465,000,000	$ 819,000,000	$ 0	$ 13,000,000	$ 692,000,000
Distributions to MPC from Predecessor	(104,000,000)	0	0	0	0	0	0	104,000,000
Contribution from MPC	225,000,000	0	0	84,000,000	141,000,000	0	0	0
Unit issuances under ATM Program	792,000,000	776,000,000	0	0	16,000,000	0	0	0
Stockholders' Equity, Other	6,000,000	6,000,000	0	0	0	0	0	0
Net income (loss)	(393,000,000)	(5,000,000)	0	6,000,000	191,000,000	0	(2,000,000)	(199,000,000)
Allocation of MPC's net investment at acquisition	0	0	0	(669,000,000)	337,000,000	0	0	332,000,000
MPC of MarkWest Hydrocarbon	(188,000,000)	0	0	0	(188,000,000)	0	0	0
Unitholders and GP	(845,000,000)	(513,000,000)	0	(142,000,000)	(190,000,000)	0	0	0
Noncontrolling interests	(3,000,000)	0	0	0	0	0	(3,000,000)	0
Noncash Partner Capital Account Distributions	563,000,000	0	0	0	563,000,000	0	0	0
Contributions from noncontrolling interests	6,000,000	0	0	0	0	0	6,000,000	0
Unit conversion	0	(133,000,000)	133,000,000	0	0	0	0	0
Non-cash contribution from MPC	336,000,000	0	0	0	0	0	0	(336,000,000)
Deferred income tax impact from changes in equity	17,000,000	2,000,000	0	13,000,000	2,000,000	0	0	0
Ending balance at Dec. 31, 2016	11,110,000,000	8,086,000,000	133,000,000	1,069,000,000	1,013,000,000	0	18,000,000	791,000,000
Distributions to MPC from Predecessor	(113,000,000)	0	0	0	0	0	0	113,000,000
Distribution declared, general partner	(32,000,000)	0	0	0	(32,000,000)	0	0	0
Contribution from MPC	675,000,000	0	0	0	0	(14,000,000)	0	689,000,000
Unit issuances under ATM Program	483,000,000	473,000,000	0	0	10,000,000	0	0	0
Stockholders' Equity, Other	8,000,000	8,000,000	0	0	0	0	0	0
Net income (loss)	(771,000,000)	301,000,000	0	110,000,000	318,000,000	0	(6,000,000)	(36,000,000)
Allocation of MPC's net investment at acquisition	0	0	0	(1,669,000,000)	266,000,000	0	0	1,403,000,000
Distribution to MPC for acquisitions	(1,931,000,000)	0	0	(537,000,000)	(1,394,000,000)	0	0	0
Unitholders and GP	(1,120,000,000)	(622,000,000)	0	(212,000,000)	(286,000,000)	0	0	0
Noncontrolling interests	(7,000,000)	0	0	0	0	0	(7,000,000)	0
Contributions from noncontrolling interests	129,000,000	0	0	0	0	0	129,000,000	0
Unit conversion	0	(133,000,000)	133,000,000	0	0	0	0	0
Ending balance at Dec. 31, 2017	9,973,000,000	8,379,000,000	0	2,099,000,000	(637,000,000)	(14,000,000)	146,000,000	0
Distributions to MPC from Predecessor	0
Contribution from MPC	1,046,000,000	0	0	0	0	0	0	1,046,000,000
Stockholders' Equity, Other	11,000,000	12,000,000	0	0	1,000,000	(2,000,000)	0	0
Net income (loss)	(1,759,000,000)	667,000,000	0	1,076,000,000	0	0	(16,000,000)	0
Allocation of MPC's net investment at acquisition	0	0	0	(5,172,000,000)	4,126,000,000	0	0	1,046,000,000
Distribution to MPC for acquisitions	(4,100,000,000)	0	0	(936,000,000)	(3,164,000,000)	0	0	0
Unitholders and GP	(1,819,000,000)	(722,000,000)	0	(1,097,000,000)	0	0	0	0
Noncontrolling interests	(17,000,000)	0	0	0	0	0	(17,000,000)	0
Contributions from noncontrolling interests	11,000,000	0	0	0	0	0	11,000,000	0
Unit conversion	0	0	0	(7,926,000,000)	7,926,000,000	0	0	0
Ending balance at Dec. 31, 2018	$ 6,864,000,000	$ 8,336,000,000	$ 0	$ (1,612,000,000)	$ 0	$ (16,000,000)	$ 156,000,000	$ 0

Description of the Business and

Description of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation (“MPC”) that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. MPLX is engaged in the gathering, processing and transportation of natural gas; the gathering, transportation, fractionation, storage and marketing of NGLs; and the transportation, storage and distribution of crude oil and refined petroleum products. MPLX’s principal executive office is located in Findlay, Ohio. MPLX was formed on March 27, 2012 as a Delaware limited partnership and completed its Initial Offering on October 31, 2012 . MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil and refined petroleum products, and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 10 for additional information regarding the operations and results of these segments. Basis of Presentation – The consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly-owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in a VIE in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. Certain prior period financial statement amounts have been reclassified to conform to current period presentation. The accompanying consolidated financial statements of MPLX have been prepared in accordance with GAAP.

Summary of Principal Accounting

Summary of Principal Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Principal Accounting Policies	Summary of Principal Accounting Policies Use of Estimates – The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods. Actual results could differ materially from those estimates. Estimates are subject to uncertainties due to the levels of subjectivity and judgment necessary to account for highly uncertain matters or the susceptibility of such matters to change and affect items such as valuing identified intangible assets; determining the fair value of derivative instruments; evaluating impairments of long-lived assets, goodwill and equity investments; establishing estimated useful lives for long-lived assets; acquisition accounting; recognizing share-based compensation expense; estimating revenues, expense accruals and capital expenditures; valuing AROs; and determining liabilities, if any, for environmental and legal contingencies. Revenue Recognition – As a result of the adoption of the new revenue recognition standard, as described further in Note 3 , MPLX has updated its policies as they relate to revenue recognition. Revenue is measured based on consideration specified in a contract with a customer. MPLX recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. MPLX enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” MPLX provides services under the following types of arrangements: • Fee-based arrangements – Under fee-based arrangements, MPLX receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue MPLX earns from these arrangements is generally directly related to the volume of natural gas, NGLs, refined products or crude oil that is handled by or flows through MPLX’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, MPLX’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed. In certain instances when specifically stated in the contract terms, MPLX purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis, as MPLX takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, MPLX: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, MPLX delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes MPLX retains to third parties. Revenue is recognized on a net basis when MPLX acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, MPLX gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, MPLX must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require MPLX to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” on the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as MPLX is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by MPLX. • Purchase arrangements – Under purchase arrangements, MPLX purchases natural gas at either the wellhead or the tailgate of a plant. MPLX then gathers and delivers the natural gas to pipelines where MPLX may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs.” Often, MPLX earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements is reported as “Product sales” on the Consolidated Statements of Income and is recognized on a gross basis as MPLX purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, MPLX provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, MPLX records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of MPLX’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions, and the services offered and whether they are deemed distinct. MPLX allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. Revenue from MPLX’s service arrangements will generally be recognized over time as the performance obligation is satisfied as services are provided. MPLX has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period. In instances in which tiered pricing structures do not reflect our efforts to perform, MPLX will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue. Based on the terms of certain natural gas gathering, transportation and processing agreements, MPLX is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. Revenue and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as “Rental income” and “Rental cost of sales,” respectively, on the Consolidated Statements of Income. Revenue and Expense Accruals – MPLX routinely makes accruals based on estimates for both revenues and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling MPLX’s records with those of third parties. The delayed information from third parties includes, among other things, actual volumes purchased, transported or sold, adjustments to inventory and invoices for purchases, actual natural gas and NGL deliveries and other operating expenses. MPLX makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and MPLX’s internal records have been reconciled. Cash and Cash Equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with initial maturities of three months or less. Restricted Cash – Restricted cash consists of cash and investments that must be maintained as collateral for letters of credit issued to certain third-party producer customers. The balances will be outstanding until certain capital projects are completed and the third party releases the restriction. Restricted cash also consists of cash advances to be used for the operation and maintenance of an operated pipeline. At December 31, 2018 and 2017 , the amount of restricted cash included in “ Other current assets ” on the Consolidated Balance Sheets was $8 million and $4 million , respectively. Receivables – Receivables primarily consist of customer accounts receivable, which are recorded at the invoiced amount and generally do not bear interest. Management reviews the allowance quarterly. Past-due balances over 90 days and other higher- risk amounts are reviewed individually for collectability. Balances that remain outstanding after reasonable collection efforts have been unsuccessful are written off through a charge to the valuation allowance and a credit to accounts receivable. Inventories – Inventories consist primarily of natural gas, propane, other NGLs and materials and supplies to be used in operations. Natural gas, propane, and other NGLs are valued at the lower of cost or market value. Materials and supplies are stated at the lower of cost or market value. Cost for materials and supplies are determined primarily using the weighted-average cost method. Imbalances – Within our pipelines and storage assets, we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive imbalances are recorded as other current assets and negative imbalances are recorded as accounts payable. Positive and negative product imbalances are settled in cash, settled by physical delivery of gas from a different source, or tracked and settled in the future. Property, Plant and Equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis over the estimated useful lives of the assets. Expenditures that extend the useful lives of assets are capitalized. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment assessment is performed and the excess of the book value over the fair value is recorded as an impairment loss. Interest costs for the construction or development of long-lived assets are capitalized and amortized over the related asset’s estimated useful life. When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported on the Consolidated Statements of Income. Gains on the disposal of property, plant and equipment are recognized when they occur, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale. Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined and compared to the carrying value of the reporting unit. The fair value is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future Net operating margins, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the implied fair value of goodwill is calculated. The excess, if any, of the book value over the implied fair value of goodwill is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. There were no impairments as a result of MPLX’s November 30, 2018 and November 30, 2017 annual goodwill impairment analyses. During 2016, impairment charges of approximately $130 million were recorded. Other Taxes – Other taxes primarily include real estate taxes. Environmental Costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. MPLX recognizes remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure. Asset Retirement Obligations – An ARO is a legal obligation associated with the retirement of tangible long-lived assets that generally result from the acquisition, construction, development or normal operation of the asset. AROs are recorded at fair value in the period in which they are incurred, if a reasonable estimate of fair value can be made, and added to the carrying amount of the associated asset. This additional carrying amount is then depreciated over the life of the asset. The liability is determined using a credit adjusted risk free interest rate and increases due to the passage of time based on the time value of money until the obligation is settled. MPLX recognizes a liability of a conditional ARO as soon as the fair value of the liability can be reasonably estimated. A conditional ARO is defined as an unconditional legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within the control of the entity. AROs have not been recognized for certain assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates. Investment in Unconsolidated Affiliates – Equity investments in which MPLX exercises significant influence, but does not control and is not the primary beneficiary, are accounted for using the equity method and are reported in “ Equity method investments ” on the accompanying Consolidated Balance Sheets. This includes entities in which we hold majority ownership but the minority shareholders have substantive participating rights. Differences in the basis of the investments and the separate net asset values of the investees, if any, are amortized into net income over the remaining useful lives of the underlying assets and liabilities, except for the excess related to goodwill. MPLX believes the equity method is an appropriate means for it to recognize increases or decreases measured by GAAP in the economic resources underlying the investments. Regular evaluation of these investments is appropriate to evaluate any potential need for impairment. MPLX uses evidence of a loss in value to identify if an investment has an other than a temporary decline. Deferred Financing Costs – Deferred financing costs are an asset for credit facility costs and netted against debt for senior notes. These costs are amortized over the contractual term of the related obligations using the effective interest method or, in certain circumstances, accelerated if the obligation is refinanced. Derivative Instruments – MPLX uses commodity derivatives to economically hedge a portion of its exposure to commodity price risk. All derivative instruments (including derivatives embedded in other contracts) are recorded at fair value. Certain commodity derivatives are reflected on the consolidated balance sheets on a net basis by counterparty as they are governed by master netting arrangements. MPLX discloses the fair value of all derivative instruments under the captions “ Other noncurrent assets ,” “ Other current liabilities ” and “ Deferred credits and other liabilities ” on the Consolidated Balance Sheets. Changes in the fair value of derivative instruments are reported on the Consolidated Statements of Income in accounts related to the item whose value or cash flows are being managed. All derivative instruments are marked to market through “ Product sales ,” “ Purchased product costs ,” or “Cost of revenues” on the Consolidated Statements of Income. Revenue gains and losses relate to contracts utilized to manage the cash flow for the sale of a product, typically NGLs. Purchased product costs gains and losses relate to contracts utilized to manage the cost of natural gas purchases, typically related to keep-whole arrangements. Cost of revenues gains and losses relate to a contract utilized to manage electricity costs. Changes in risk management for unrealized activities are reported as an adjustment to net income in computing cash flow from operating activities on the accompanying Consolidated Statements of Cash Flows. During the years ended December 31, 2018 , 2017 and 2016 , MPLX did not elect hedge accounting for any derivatives. MPLX has elected the normal purchases and normal sales designation for certain contracts related to the physical purchase of electric power. Fair Value of Financial Instruments – Management believes the carrying amount of financial instruments, including cash and cash equivalents, receivables, receivables from related parties, other current assets, accounts payable, accounts payable to related parties and accrued liabilities approximate fair value because of the short-term maturity of these instruments. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximate fair value due to the variable interest rate that approximates current market rates (see Note 16 ). Derivative instruments are recorded at fair value, based on available market information (see Note 17 ). Fair Value Measurement – Financial assets and liabilities recorded at fair value in the Consolidated Balance Sheets are categorized based upon the fair value hierarchy established by GAAP, which classifies the inputs used to measure fair value into Level 1, Level 2 or Level 3. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The methods and assumptions utilized may produce a fair value that may not be realized in future periods upon settlement. Furthermore, while MPLX believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. For further discussion see Note 16 . Equity-Based Compensation Arrangements – MPLX issues phantom units under its share-based compensation plan as described further in Note 22 . A phantom unit entitles the grantee a right to receive a common unit upon the issuance of the phantom unit. The fair value of phantom unit awards granted to employees and non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. The fair value of the units awarded is amortized into earnings using a straight-line amortization schedule over the period of service corresponding with the vesting period. For phantom units that vest immediately and are not forfeitable, equity-based compensation expense is recognized at the time of grant. Performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards and use a Monte Carlo valuation model to calculate a grant date fair value. To satisfy common unit awards, MPLX may issue new common units, acquire common units in the open market or use common units already owned by the general partner. Tax Effects of Share-Based Compensation – MPLX elected to adopt the simplified method to establish the beginning balance of the additional paid-in capital pool (“APIC Pool”) related to the tax effects of employee share-based compensation and to determine the subsequent impact on the APIC Pool and Consolidated Statements of Cash Flows of the tax effects of share-based compensation awards that were outstanding upon adoption. Additional paid-in capital is reported as “Common unitholders - public ” on the accompanying Consolidated Balance Sheets. Income Taxes – MPLX is not a taxable entity for federal income tax purposes. As a result of the MarkWest Merger, MarkWest was the surviving entity for tax purposes. MarkWest is not a taxable entity for federal income tax purposes. As such, MPLX does not directly pay federal income tax. Taxes on MPLX’s net income generally are borne by its partners through the allocation of taxable income. MPLX’s taxable income or loss, which may vary substantially from the net income or loss reported on the Consolidated Statements of Income, is includable in the federal income tax returns of each partner. MPLX and certain legal entities are, however, taxable entities under certain state jurisdictions. MPLX accounts for income taxes under the asset and liability method. Deferred income taxes are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, capital loss carryforwards and net operating loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect of any tax rate change on deferred taxes is recognized as tax expense/(benefit) from continuing operations in the period that includes the enactment date of the tax rate change. Realizability of deferred tax assets is assessed and, if not more likely than not, a valuation allowance is recorded to reflect the deferred tax assets at net realizable value as determined by management. All deferred tax balances are classified as long-term in the accompanying Consolidated Balance Sheets. All changes in the tax bases of assets and liabilities are allocated among operations and items charged or credited directly to equity. Distributions – In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 9 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued as a liability until declared. However, when distributions related to the eliminated IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in below. Net Income Per Limited Partner Unit – MPLX uses the two-class method when calculating the net income per unit applicable to limited partners, because there is more than one class of participating security. The classes of participating securities include common units, general partner units, preferred units, certain equity-based compensation awards and eliminated IDRs. Class B units are considered to be a separate class of common units that do not participate in distributions. Net income attributable to MPLX LP is allocated to the unitholders differently for preparation of the Consolidated Statements of Equity and the calculation of net income per limited partner unit. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule and subsequently allocated to remaining unitholders in accordance with their respective ownership percentages. However, prior to 2018 when distributions related to the eliminated IDRs were made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders, except Class B unitholders, based on their respective ownership percentages. Subsequent to the conversion of the general partner to a non-economic interest as described in Note 8 , no earnings will be allocated to the general partner. 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 7 . In preparing net income per limited partner units, during periods in which a net loss attributable to MPLX is reported or periods in which the total distributions exceed the reported net income attributable to MPLX’s unitholders, the amount allocable to certain equity-based compensation awards is based on actual distributions to the equity-based compensation awards. Diluted earnings per unit is calculated by dividing net income attributable to MPLX’s common unitholders, after deducting amounts allocable to other participating securities, by the weighted average number of common units and potential common units outstanding during the period. Potential common units are excluded from the calculation of diluted earnings per unit during periods in which net income attributable to MPLX’s unitholders, after deducting amounts that are allocable to the outstanding equity-based compensation awards, preferred units, and eliminated IDRs, is a loss as the impact would be anti-dilutive. Business Combinations – MPLX recognizes and measures the assets acquired and liabilities assumed in a business combination based on their estimated fair values at the acquisition date, with any remaining difference recorded as goodwill or gain from a bargain purchase. For all material acquisitions, management engages an independent valuation specialist to assist with the determination of fair value of the assets acquired, liabilities assumed, noncontrolling interests, if any, and goodwill, based on recognized business valuation methodologies. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent to the acquisition, and not later than one year from the acquisition date, MPLX will record any material adjustments to the initial estimate based on new information obtained that would have existed as of the acquisition date. An adjustment that arises from information obtained that did not exist as of the date of the acquisition will be recorded in the period of the adjustment. An income, market or cost valuation method may be utilized to estimate the fair value of the assets acquired, liabilities assumed, and noncontrolling interests, if any, in a business combination. The income valuation method represents the present value of future cash flows over the life of the asset using: (i) discrete financial forecasts, which rely on management’s estimates of volumes, NGL prices, revenue and operating expenses; (ii) long-term growth rates; and (iii) appropriate discount rates. The market valuation method uses prices paid for a reasonably similar asset by other purchasers in the market, with adjustments relating to any differences between the assets. The cost valuation method is based on the replacement cost of a comparable asset at prices at the time of the acquisition reduced for depreciation of the asset. Acquisition-related costs are expensed as incurred in connection with each business combination. See Note 4 for more information about the acquisitions. Accounting for Changes in Ownership Interests in Subsidiaries – MPLX’s ownership interest in a consolidated subsidiary may change if it sells a portion of its interest or acquires additional interest or if the subsidiary issues or repurchases its own shares. If the transaction does not result in a change in control over the subsidiary, the transaction is accounted for as an equity transaction. If a sale results in a loss of control, it would result in the deconsolidation of a subsidiary with a gain or loss recognized on the Consolidated Statements of Income unless the subsidiary meets the definition of in-substance real estate. Deconsolidation of in-substance real estate is recorded at cost with no gain or loss recognized. If the purchase of additional interest occurs which changes the acquirer’s ownership interest from noncontrolling to controlling, the acquirer’s preexisting interest in the acquiree is remeasured to its fair value, with a resulting gain or loss recorded in earnings upon consummation of the business combination. Once an entity has control of a subsidiary, its acquisitions of some or all of the noncontrolling interests in that subsidiary are accounted

Accounting Standards

Accounting Standards	12 Months Ended
	Dec. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Standards	Accounting Standards Recently Adopted ASU 2014-09, Revenue - Revenue from Contracts with Customers In May 2014, the FASB issued ASU 2014-09, which created ASC Topic 606 (“ASC 606”), Revenue from Contracts with Customers. The guidance in ASC 606 states that revenue is recognized when a customer obtains control of a good or service. Recognition of revenue involves a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and recognizing revenue as the obligations are satisfied. Additional disclosures are required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. MPLX adopted the standard as of January 1, 2018 using the modified retrospective method by recognizing the cumulative effect of initially applying the new revenue standard as an adjustment to opening equity. The comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. See Note 19 for further details. We also adopted the following standards during 2018 , none of which had a material impact to our financial statements or financial statement disclosures: ASU Effective Date 2017-09 Stock Compensation - Scope of Modification Accounting January 1, 2018 2017-05 Gains and Losses from the Derecognition of Nonfinancial Assets - Clarifying the Scope of Asset Derecognition Guidance January 1, 2018 2017-01 Business Combinations - Clarifying the Definition of a Business January 1, 2018 2016-18 Statement of Cash Flows - Restricted Cash January 1, 2018 2016-15 Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments January 1, 2018 2016-01 Financial Instruments - Recognition and Measurement of Financial Assets and Liabilities January 1, 2018

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions Mt. Airy Terminal On September 26, 2018, MPLX acquired an eastern U.S. Gulf Coast export terminal (the “Mt. Airy Terminal”) from Pin Oak Holdings, LLC for total consideration of $451 million . The terminal includes 4 million barrels of third-party leased storage capacity and a 120 mbpd dock. The Mt. Airy Terminal is located on the Mississippi River between New Orleans and Baton Rouge, is in close proximity to several Gulf Coast refineries including MPC’s Garyville Refinery and is near numerous rail lines and pipelines. The Mt. Airy Terminal is accounted for within the L&S segment. Based on the fair value estimates of assets acquired and liabilities assumed at the acquisition date, the purchase price was allocated as follows: (In millions) Balance as of September 26, 2018 Receivables, net $ 3 Other current assets 1 Property, plant and equipment, net 336 Intangibles, net 9 Goodwill 126 Accounts payable (17 ) Other current liabilities (7 ) Net assets acquired $ 451 Goodwill represents the significant growth potential of the terminal due to the multiple pipelines and rail lines which cross the property, the terminal’s position as an aggregation point for liquids growth in the region for both ocean-going export vessels and inland barges, the proximity of the terminal to MPC’s Garyville refinery and other refineries in the region as well as the opportunity to expand and construct an additional dock at the site. All of the goodwill recognized related to this transaction is tax deductible. The amount of revenue and income from operations associated with the acquisition of the Mt. Airy Terminal included on the Consolidated Statement of Income since the September 26, 2018 acquisition date was not material to the financial statements. Assuming the acquisition had occurred on January 1, 2017, the consolidated pro forma results would not have been materially different from the reported results. Refining Logistics and Fuels Distribution Acquisition On February 1, 2018 , MPC and MPLX LP closed on an agreement for the dropdown of refining logistics assets and fuels distribution services to MPLX LP. MPC contributed these assets and services in exchange for $4.1 billion in cash and a fixed number of MPLX LP common units and general partner units of 111,611,111 and 2,277,778 , respectively. The fair value of the common and general partner units issued as of the acquisition date was $4.3 billion based on the closing common unit price as of February 1, 2018 , as recorded on the Consolidated Statements of Equity, for a total purchase price of $8.4 billion . The equity issued consisted of: (i) 85,610,278 common units to MPLX GP LLC (“MPLX GP”), (ii) 18,176,666 common units to MPLX Logistics Holdings LLC (“MPLX Logistics”) and (iii) 7,824,167 common units to MPLX Holdings Inc. (“MPLX Holdings”). MPLX also issued 2,277,778 general partner units to MPLX GP in order to maintain its two percent general partner interest (“GP Interest”) in MPLX. MPC agreed to waive approximately one-third of the first quarter 2018 distributions on the common units issued in connection with this transaction. As a result of this waiver, MPC did not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. Immediately following this transaction, the GP Interest was converted into a non-economic general partner interest as discussed in Note 8 . MPLX recorded this transaction on a historical basis as required for transactions between entities under common control. No effect was given to the prior periods as these entities were not considered businesses prior to the February 1, 2018 dropdown. In connection with the dropdown, approximately $830 million of net property, plant and equipment was recorded in addition to $85 million and $130 million of goodwill allocated to MPLX Refining Logistics LLC (“Refining Logistics”) and MPLX Fuels Distribution LLC (“Fuels Distribution”), respectively. Both the Refining Logistics assets and the Fuels Distribution services are accounted for within the L&S segment. The Refining Logistics assets include 619 tanks with approximately 56 million barrels of storage capacity (crude, finished products and intermediates), 32 rail and truck racks, 18 docks, and gasoline blenders. These assets generate revenue through storage services agreements with MPC. Refining Logistics provides certain services to MPC related to the receipt, storage, throughput, custody and delivery of petroleum products in and through certain storage and logistical facilities and assets associated with MPC’s refineries. Fuels Distribution, which is a wholly owned subsidiary of MPLXT, generates revenue through a Fuels Distribution Services Agreement with MPC. Fuels Distribution is structured to provide a broad range of scheduling and marketing services as MPC’s agent. The amounts of revenue and income from operations associated with these investments included on the Consolidated Statements of Income, since the February 1, 2018 acquisition date, were as follows: (In millions) Twelve Months Ended Revenues and other income $ 1,359 Income from operations $ 874 Joint-Interest Acquisition On September 1, 2017 , MPLX entered into a Membership Interests and Shares Contributions Agreement with MPLX GP, MPLX Logistics, MPLX Holdings and MPC Investment LLC (“MPC Investment”), each a wholly-owned subsidiary of MPC, whereby MPLX agreed to acquire certain ownership interests in joint venture entities indirectly held by MPC. Pursuant to the agreement, MPC Investment agreed to contribute: all of the membership interests of Lincoln Pipeline LLC, which holds a 35 percent interest in Illinois Extension; all of the membership interests of MPL Louisiana Holdings LLC, which holds a 41 percent interest in LOOP; a 59 percent interest in LOCAP; and a 25 percent interest in Explorer, through a series of intercompany contributions to MPLX for an agreed upon purchase price of approximately $420 million in cash and equity consideration valued at approximately $630 million , for total consideration of $1.05 billion (collectively, the “Joint-Interest Acquisition”). The number of common units representing the equity consideration was then determined by dividing the contribution amount by the simple average of the ten-day trailing volume weighted average NYSE price of a common unit for the ten trading days ending at market close on August 31, 2017. The fair value of the common and general partner units issued was approximately $653 million based on the closing common unit price as of September 1, 2017, as recorded on the Consolidated Statements of Equity, for a total purchase price of $1.07 billion . The equity issued consisted of: (i) 13,719,017 common units to MPLX GP, (ii) 3,350,893 common units to MPLX Logistics and (iii) 1,441,224 common units to MPLX Holdings. MPLX also issued 377,778 general partner units to MPLX GP in order to maintain its two percent GP Interest in MPLX. Illinois Extension operates the 168 -mile, 24 -inch diameter Southern Access Extension crude oil pipeline from Flanagan, Illinois to Patoka, Illinois, as well as additional tankage and two pump stations. LOOP owns and operates midstream crude oil infrastructure, including a deep-water oil port offshore of Louisiana, pipelines and onshore storage facilities. LOOP also manages the operations of LOCAP, an affiliate pipeline system. LOCAP owns and operates a crude oil pipeline and tank facility in St. James, Louisiana, that distributes oil received from LOOP’s storage facilities and other connecting pipelines to nearby refineries and into the Mid-Continent region of the United States. Explorer owns and operates an approximate 1,830 -mile common carrier pipeline that primarily transports gasoline, diesel, diluent and jet fuel from the Gulf Coast region to the Midwest United States. MPLX accounts for the Joint-Interest Acquisition entities as equity method investments within its L&S segment. As a transfer between entities under common control, MPLX recorded the Joint-Interest Acquisition on its Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. MPLX recognizes an “Accumulated other comprehensive loss” on its Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by the LOOP and Explorer joint-interests to their employees. MPLX LP is not a sponsor of these benefit plans. Distributions of cash received from the entities and interests acquired in the Joint-Interest Acquisition related to periods prior to the acquisition were prorated on a daily basis with MPLX LP retaining the portion of distributions beginning on the closing date. All amounts distributed to MPLX LP related to periods before the acquisition have been paid to MPC. Additionally, MPLX LP agreed to pay MPC for any distributions of cash from LOOP related to the sale of LOOP’s excess crude oil inventory. Because the future distributions or payments could not be reasonably quantified, a liability was not recorded in connection with the acquisition. MPLX LP subsequently received distributions related to the time period prior to the acquisition, which it remitted to MPC and recorded a corresponding decrease to the general partner’s equity for $32 million . MPLX accounts for the interests acquired in the Joint-Interest Acquisition one month in arrears, which is the most recently available information. The amount of income associated with these investments included on the Consolidated Statements of Income under the caption “ Income/(loss) from equity method investments ” for the twelve months ended December 31, 2018 and December 31, 2017 totaled $118 million and $21 million , respectively. MPC agreed to waive approximately two-thirds of the third quarter 2017 distributions on the common units issued in connection with the Joint-Interest Acquisition. As a result of this waiver, MPC did not receive approximately two-thirds of the distributions or IDRs that would have otherwise accrued on such common units with respect to the third quarter 2017 distributions. The value of these waived distributions was $10 million . Acquisition of Hardin Street Transportation LLC, Woodhaven Cavern LLC and MPLX Terminals LLC MPC contributed the assets of HST, WHC and MPLXT to newly created and wholly-owned subsidiaries and entered into commercial agreements related to services provided by these new entities to MPC on January 1, 2015 for HST and WHC and April 1, 2016 for MPLXT. Pursuant to a Membership Interests Contributions Agreement entered into on March 1, 2017 by MPLX with MPLX GP, MPLX Logistics, MPLX Holdings and MPC Investment (each a wholly-owned subsidiary of MPC), MPC Investment agreed to contribute the outstanding membership interests in HST, WHC and MPLXT through a series of intercompany contributions to MPLX for approximately $1.5 billion in cash and equity consideration valued at approximately $504 million . The number of common units representing the equity consideration was determined by dividing the contribution amount by the simple average of the ten-day trailing volume weighted average NYSE price of a common unit for the ten trading days ending at market close on February 28, 2017. The fair value of the common and general partner units issued was approximately $503 million , and consisted of (i) 9,197,900 common units to MPLX GP, (ii) 2,630,427 common units to MPLX Logistics and (iii) 1,132,049 common units to MPLX Holdings. MPLX also issued 264,497 general partner units to MPLX GP in order to maintain its two percent GP Interest in MPLX. MPC agreed to waive two-thirds of the first quarter 2017 distributions on the common units issued in connection with the Transaction. As a result of this waiver, MPC did not receive two-thirds of the general partner distributions or IDRs that would have otherwise accrued on such common units with respect to the first quarter 2017 distributions. The value of these waived distributions was $6 million . HST owns and operates various private crude oil and refined product pipeline systems and associated storage tanks. As of the acquisition date, these pipeline systems consisted of 174 miles of crude oil pipelines and 430 miles of refined products pipelines. WHC owns and operates eight butane and propane storage caverns located in Michigan with approximately 1.8 million barrels of NGL storage capacity. As of the acquisition date, MPLXT owned and operated 59 terminals for the receipt, storage, blending, additization, handling and redelivery of refined petroleum products. Additionally, MPLXT operated one leased terminal and had partial ownership interest in two terminals. Collectively, these 62 terminals have a combined shell capacity of approximately 23.6 million barrels. The terminal facilities are located primarily in the Midwest, Gulf Coast and Southeast regions of the United States. MPLX accounts for these businesses within its L&S segment. MPLX retrospectively adjusted the historical financial results for all periods to give effect to the acquisition of HST and WHC effective January 1, 2015, and the acquisition of MPLXT effective April 1, 2016, as required for transactions between entities under common control. Prior to these dates, these entities were not considered businesses and, therefore, there are no financial results from which to recast. Acquisition of Ozark Pipeline On March 1, 2017, MPLX acquired the Ozark pipeline from Enbridge Pipelines (Ozark) LLC for approximately $219 million , including purchase price adjustments made in the second quarter of 2017. Based on the final fair value estimates of assets acquired and liabilities assumed at the acquisition date, the purchase price was primarily allocated to property, plant and equipment. The Ozark pipeline is a 433 -mile, 22 -inch crude oil pipeline originating in Cushing, Oklahoma, and terminating in Wood River, Illinois, capable of transporting approximately 230 mbpd. MPLX accounts for the Ozark pipeline within its L&S segment. The amounts of revenue and income from operations associated with the acquisition included on the Consolidated Statements of Income, since the March 1, 2017 acquisition date are as follows: (In millions) Twelve Months Ended December 31, 2017 Revenues and other income $ 64 Income from operations $ 20 Assuming the acquisition of the Ozark pipeline had occurred on January 1, 2016, the consolidated pro forma results would not have been materially different from reported results. MarEn Bakken On February 15, 2017, MPLX closed on a joint venture, MarEn Bakken Company, LLC (“MarEn Bakken”), with Enbridge Energy Partners L.P. in which MPLX LP acquired a partial, indirect interest in the Dakota Access Pipeline and Energy Transfer Crude Oil Company Pipeline projects, collectively referred to as the Bakken Pipeline system, from Energy Transfer Partners, L.P. and Sunoco Logistics Partners, LP. The Bakken Pipeline system is capable of transporting more than 520 mbpd of crude oil from the Bakken/Three Forks production area in North Dakota to the Midwest through Patoka, Illinois and ultimately to the Gulf Coast. MPLX contributed $500 million of the $2.0 billion purchase price paid by MarEn Bakken to acquire a 37 percent indirect interest in the Bakken Pipeline system. MPLX holds, through a subsidiary, a 25 percent interest in MarEn Bakken, which equates to an approximate 9 percent indirect interest in the Bakken Pipeline system. MPLX accounts for its investment in MarEn Bakken as an equity method investment and bases the equity method accounting for this joint venture one month in arrears which is the most recently available information. The amount of income or loss associated with these investments included on the Consolidated Statements of Income under the caption “ Income/(loss) from equity method investments ” for the twelve months ended December 31, 2018 and December 31, 2017 totaled $48 million and $15 million , respectively. In connection with MPLX’s acquisition of a partial, indirect equity interest in the Bakken Pipeline system, MPC agreed to waive its right to receive incentive distributions of $1.6 million per quarter for twelve consecutive quarters, beginning with distributions declared in the first quarter of 2017 and paid to MPC in the second quarter of 2017, which was prorated to $0.8 million from the acquisition date. This waiver is no longer applicable as a result of the conversion of the GP Interest to a non-economic general partner interest as discussed in Note 8. Acquisition of Hardin Street Marine LLC On March 14, 2016, MPLX entered into a Membership Interests Contribution Agreement with MPLX GP, MPLX Logistics and MPC Investment (each a wholly-owned subsidiary of MPC), related to the acquisition of HSM, MPC’s inland marine business, from MPC. Pursuant to the agreement, the transaction was valued at $600 million , consisting of a fixed number of common units and general partner units of 22,534,002 and 459,878 , respectively. The general partner units maintained MPC’s two percent GP Interest in MPLX. The acquisition closed on March 31, 2016 and the fair value of the common units and general partner units issued was $669 million and $14 million , respectively. MPC agreed to waive distributions in the first quarter of 2016 on common units issued in connection with this transaction. As a result of this waiver, MPC did not receive general partner distributions or IDRs that would have otherwise accrued on such common units with respect to the first quarter 2016 distributions. The value of these waived distributions was $15 million . The inland marine business, comprised of 18 tow boats and 219 owned and leased barges as of the acquisition date, which transport light products, heavy oils, crude oil, renewable fuels, chemicals and feedstocks in the Midwest and Gulf Coast regions of the United States, accounted for nearly 60 percent of the total volumes MPC shipped by inland marine vessels as of March 31, 2016. MPLX accounts for HSM within its L&S segment.

Investments and Noncontrolling

Investments and Noncontrolling Interests	12 Months Ended
	Dec. 31, 2018
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests	Investments and Noncontrolling Interests The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at December 31, December 31, (In millions, except ownership percentages) 2018 2018 2017 Explorer 25% 90 89 Illinois Extension Pipeline 35% 275 284 LOCAP 59% 27 24 LOOP 41% 226 225 MarEn Bakken 25% 498 520 Centrahoma Processing LLC 40% 160 121 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 236 164 MarkWest Utica EMG, L.L.C. 56% 2,039 2,139 Sherwood Midstream LLC 50% 366 236 Sherwood Midstream Holdings LLC 60% 157 165 Other 100 43 Total $ 4,174 $ 4,010 Summarized financial information for MPLX’s equity method investments for the years ended December 31, 2018 , 2017 and 2016 is as follows: December 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 238 $ 234 $ 1,364 $ 1,836 Costs and expenses 184 95 709 988 Income from operations 54 139 655 848 Net income 53 139 584 776 (Loss)/income from equity method investments (1) $ (10 ) $ 74 $ 176 $ 240 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 187 $ 86 $ 954 $ 1,227 Costs and expenses 97 42 520 659 Income from operations 90 44 434 568 Net income 90 43 345 478 Income from equity method investments (1) $ 10 $ 20 $ 48 $ 78 December 31, 2016 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs (2) Non-VIEs Total Revenues and other income $ 216 $ 18 $ 148 $ 382 Costs and expenses 100 111 117 328 Income/(loss) from operations 116 (93 ) 31 54 Net income/(loss) 114 (93 ) 31 52 Income/(loss) from equity method investments (1) $ 8 $ (89 ) $ 7 $ (74 ) (1) “Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion. (2) Includes an impairment charge of $89 million for the year ended December 31, 2016 related to MPLX’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table. Summarized balance sheet information for MPLX’s equity method investments as of December 31, 2018 and 2017 is as follows: December 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 82 $ 153 $ 379 $ 614 Noncurrent assets 1,939 1,596 4,715 8,250 Current liabilities 28 127 246 401 Noncurrent liabilities $ 3 $ 186 $ 841 $ 1,030 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 65 $ 46 $ 399 $ 510 Noncurrent assets 2,077 930 4,624 7,631 Current liabilities 39 44 220 303 Noncurrent liabilities $ 3 $ 11 $ 904 $ 918 (1) MarkWest Utica EMG, L.L.C (“MarkWest Utica EMG”), noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $750 million and $790 million as of December 31, 2018 and 2017 , respectively. As of December 31, 2018 and 2017 , the carrying value of MPLX’s equity method investments exceeded the underlying net assets of its investees by $ 1.0 billion for the G&P segment. As of December 31, 2018 and 2017 , the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $114 million and $118 million , respectively. This basis difference is being amortized into net income over the remaining estimated useful lives of the underlying net assets, except for $ 459 million and $39 million of excess related to goodwill for the G&P and L&S segments, respectively. MarkWest Utica EMG Effective January 1, 2012, MarkWest Utica Operating Company, L.L.C. (“Utica Operating”), a wholly-owned and consolidated subsidiary of MPLX, and EMG Utica, LLC (“EMG Utica” and together with Utica Operating, the “Members”) executed agreements to form a joint venture, MarkWest Utica EMG, to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio. The related limited liability company agreement has been amended from time to time (the limited liability company agreement currently in effect is referred to as the “Amended LLC Agreement”). The aggregate funding commitment of EMG Utica was $950 million . Thereafter, Utica Operating was required to fund, as needed, 100 percent of future capital for MarkWest Utica EMG until the aggregate capital that had been contributed by the Members reached $2.0 billion , which occurred prior to the MarkWest Merger. Until such time as the investment balances of Utica Operating and EMG Utica are in the ratio of 70 percent and 30 percent , respectively (such time being referred to as the “Second Equalization Date”), EMG Utica will have the right, but not the obligation, to fund up to ten percent of each capital call for MarkWest Utica EMG, and Utica Operating will be required to fund all remaining capital not elected to be funded by EMG Utica. After the Second Equalization Date, Utica Operating and EMG Utica will have the right, but not the obligation, to fund their pro rata portion (based on their respective investment balances) of any additional required capital and may also fund additional capital that the other party elects not to fund. As of December 31, 2018 , EMG Utica has contributed approximately $1.2 billion and Utica Operating has contributed approximately $ 1.5 billion to MarkWest Utica EMG. Under the Amended LLC Agreement, prior to December 31, 2016, EMG Utica’s investment balance was increased by a quarterly special non-cash allocation of income (“Preference Amount”) calculated based upon the amount of capital contributed by EMG Utica in excess of $500 million . After December 31, 2016, no Preference Amount will accrue to EMG Utica’s investment balance. EMG Utica received a Preference Amount totaling approximately $16 million for the year ended December 31, 2016 . Under the Amended LLC Agreement, after December 31, 2016, cash generated by MarkWest Utica EMG that is available for distribution will be allocated to the Members in proportion to their respective investment balances. As of December 31, 2018 , Utica Operating’s investment balance in MarkWest Utica EMG was approximately 56 percent . MarkWest Utica EMG is deemed to be a VIE. Utica Operating is not deemed to be the primary beneficiary, due to EMG Utica’s voting rights on significant matters. MPLX’s maximum exposure to loss as a result of its involvement with MarkWest Utica EMG 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 MarkWest Utica EMG that it was not contractually obligated to provide during the years ended December 31, 2018 , 2017 and 2016 . MPLX receives management fee revenue for engineering and construction and administrative services for operating MarkWest Utica EMG, and is also reimbursed for personnel services (“Operational Service revenue”). Operational Service revenue is reported as “Other income - related parties” on the Consolidated Statements of Income. The amount of Operational Service revenue related to MarkWest Utica EMG for the years ended December 31, 2018 , 2017 and 2016 totaled $17 million , $17 million and $16 million , respectively. Ohio Gathering Ohio Gathering is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC. As of December 31, 2018 , MPLX had an approximate 34 percent indirect ownership interest in Ohio Gathering. As Ohio Gathering is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, MPLX reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. MPLX receives Operational Service revenue for operating Ohio Gathering which is reported as “Other income-related parties” on the Consolidated Statements of Income. The amount of Operational Service revenue related to Ohio Gathering for the years ended December 31, 2018 , 2017 and 2016 totaled $16 million , $16 million and $15 million , respectively. Sherwood Midstream Effective January 1, 2017, MarkWest Liberty Midstream & Resources, L.L.C. (“MarkWest Liberty Midstream”), a wholly-owned and consolidated subsidiary of MPLX LP, and Antero Midstream Partners LP (“Antero Midstream”) formed a joint venture, Sherwood Midstream LLC (“Sherwood Midstream”), to support Antero Resources Corporation’s (“Antero Resources”) development in the Marcellus Shale. MarkWest Liberty Midstream has a 50 percent ownership interest in Sherwood Midstream. Pursuant to the terms of the related limited liability company agreement (the “LLC Agreement”), MarkWest Liberty Midstream contributed assets then under construction with a fair value of approximately $134 million and cash of approximately $20 million . Antero Midstream made an initial capital contribution of approximately $154 million . Also effective January 1, 2017, MarkWest Liberty Midstream converted all of its ownership interests in MarkWest Ohio Fractionation Company, L.L.C. (“Ohio Fractionation”), a previously wholly-owned subsidiary, to Class A Interests and amended its LLC Agreement to create Class B-3 Interests, which were sold to Sherwood Midstream for $126 million in cash. The Class B-3 Interests provide Sherwood Midstream with the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 3 fractionator. Sherwood Midstream accounts for its investment in Ohio Fractionation, which is a VIE, as an equity method investment as Sherwood Midstream does not control Ohio Fractionation. MarkWest Liberty Midstream has been deemed to be the primary beneficiary of Ohio Fractionation because it has control over the decisions that could significantly impact its financial performance, and as a result, consolidates Ohio Fractionation. The carrying amounts of assets and liabilities included on MPLX’s Consolidated Balance Sheets pertaining to Ohio Fractionation at December 31, 2018 , were current assets of $132 million , non-current assets of $550 million , current liabilities of $75 million and $1 million of non-current liabilities. The creditors of Ohio Fractionation do not have recourse to MPLX LP’s general credit through guarantees or other financial arrangements. The assets of Ohio Fractionation are the property of Ohio Fractionation and cannot be used to satisfy the obligations of MPLX LP. Sherwood Midstream’s interests are reflected in “Net income attributable to noncontrolling interests” on the Consolidated Statements of Income and “Noncontrolling interests” on the Consolidated Balance Sheets. Under the LLC Agreement, cash generated by Sherwood Midstream that is available for distribution will be allocated to the members in proportion to their respective investment balances. Sherwood Midstream is deemed to be a VIE. MarkWest Liberty Midstream is not deemed to be the primary beneficiary, due to Antero Midstream’s voting rights on significant matters. MPLX’s maximum exposure to loss as a result of its involvement with Sherwood Midstream 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 Sherwood Midstream that it was not contractually obligated to provide during the years ended December 31, 2018 and 2017 . MPLX receives Operational Service revenue for operating Sherwood Midstream. The amount of Operational Service revenue related to Sherwood Midstream for the years ended December 31, 2018 and 2017 totaled approximately $12 million and $8 million , respectively, and is reported as “Other income-related parties” on the Consolidated Statements of Income. Sherwood Midstream Holdings Effective January 1, 2017, MarkWest Liberty Midstream and Sherwood Midstream formed a joint venture, Sherwood Midstream Holdings LLC (“Sherwood Midstream Holdings”), for the purpose of owning, operating and maintaining all of the shared assets that support the operations of the gas plants and other assets owned by Sherwood Midstream and the gas plants and de-ethanization facilities owned by MarkWest Liberty Midstream. MarkWest Liberty Midstream initially contributed certain real property, equipment and facilities with a fair value of approximately $209 million to Sherwood Midstream Holdings in exchange for a 79 percent initial ownership interest. Sherwood Midstream contributed cash of approximately $44 million to Sherwood Midstream Holdings in exchange for a 21 percent ownership interest. During the second quarter ended June 30, 2017, true-ups to the initial contributions were finalized. MarkWest Liberty Midstream contributed certain additional real property, equipment and facilities with a fair value of approximately $10 million to Sherwood Midstream Holdings and Sherwood Midstream contributed cash of approximately $4 million to Sherwood Midstream Holdings. Collectively, the real property, equipment, facilities and cash initially contributed, or that may be subsequently constructed by or contributed, to Sherwood Midstream Holdings are referred to as the “Shared Assets.” The net book value of the contributed assets was approximately $203 million . The contribution was determined to be an in-substance sale of real estate. As such, MPLX only recognized a gain for the portion attributable to Antero Midstream’s indirect interest of approximately $2 million , included in “Other income” on the Consolidated Statements of Income. MarkWest Liberty Midstream’s portion of the gain attributable to its direct and indirect interests of approximately $14 million is included in its investment in Sherwood Midstream Holdings and is reported under the caption “Equity method investments” on the Consolidated Balance Sheets. In connection with the initial contributions, MarkWest Liberty Midstream received a special distribution of approximately $45 million . During the year ended December 31, 2018 , MarkWest Liberty Midstream sold to Sherwood Midstream six percent of its equity ownership in Sherwood Midstream Holdings for $15 million . MarkWest Liberty Midstream’s and Sherwood Midstream’s ownership interests in Sherwood Midstream Holdings will fluctuate over time. As new Shared Assets are constructed, the members will make additional capital contributions to Sherwood Midstream Holdings. The amount that each member must contribute will be based on the expected utilization of the Shared Assets, as defined in the LLC Agreement. Pursuant to the terms of the LLC Agreement, MarkWest Liberty Midstream will serve as the operator for Sherwood Midstream Holdings. MPLX accounts for Sherwood Midstream Holdings, which is a VIE, as an equity method investment as Sherwood Midstream is considered to be the general partner and controls all decisions. MPLX’s maximum exposure to loss as a result of its involvement with Sherwood Midstream Holdings 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 operating services. MPLX did not provide any financial support to Sherwood Midstream Holdings that it was not contractually obligated to provide during the years ended December 31, 2018 and 2017 . Sherwood Midstream has been deemed the primary beneficiary of Sherwood Midstream Holdings due to its controlling financial interest through its authority to manage the joint venture. As a result, Sherwood Midstream consolidates Sherwood Midstream Holdings. Therefore, MPLX also reports its portion of Sherwood Midstream Holdings’ net assets as a component of its investment in Sherwood Midstream. As of December 31, 2018 , MPLX has a 20.2 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream.

Related Party Agreements and Tr

Related Party Agreements and Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions MPLX’s material related parties include: • MPC, which refines, markets and transports crude oil and petroleum products, primarily in the Midwest, Gulf Coast, East Coast and Southeast regions of the United States. • MarkWest Utica EMG, in which MPLX LP has a 56 percent interest as of December 31, 2018 . MarkWest Utica EMG is engaged in natural gas processing and NGL fractionation, transportation and marketing in Ohio. • Ohio Gathering, in which MPLX LP has a 34 percent indirect interest as of December 31, 2018 . Ohio Gathering is a subsidiary of MarkWest Utica EMG providing natural gas gathering service in the Utica Shale region of eastern Ohio. • Sherwood Midstream, in which MPLX LP has a 50 percent interest as of December 31, 2018 . Sherwood Midstream supports the development of Antero Resources’ Marcellus Shale acreage in the rich-gas corridor of West Virginia. • Sherwood Midstream Holdings, in which MPLX LP has an 80 percent total direct and indirect interest as of December 31, 2018 . Sherwood Midstream Holdings owns certain infrastructure at the Sherwood Complex that is shared by and supports the operation of both the Sherwood Midstream and MPLX gas processing plants and de-ethanization facilities. • MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (“Jefferson Dry Gas”), in which MPLX LP has a 67 percent interest as of December 31, 2018 . Jefferson Dry Gas provides natural dry gas gathering and related services in the Utica Shale region of Ohio. Commercial Agreements MPLX has various long-term, fee-based commercial agreements with MPC. Under these agreements, MPLX provides transportation, terminal, fuels distribution, marketing and storage services to MPC. MPC has committed to provide MPLX with minimum quarterly throughput volumes on crude oil and refined products systems in addition to fees for storage capacity. 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. The commercial agreements with MPC include: • Fuels distribution services agreement – Fuels Distribution is a party to a services agreement with MPC in connection with the dropdown of the fuels distribution services. Under this agreement, Fuels Distribution provides services related to the scheduling and marketing of certain petroleum products to MPC. Fuels Distribution does not provide the same services to third parties without the prior written consent of MPC. This agreement has an initial term of 10 years, subject to a five -year renewal period under terms to be renegotiated at that time. Under the Fuels Distribution Services Agreement, MPC pays MPLX a tiered monthly fee-based on the volume of MPC’s products sold by MPLX each month, subject to a maximum annual volume. MPLX has agreed to use commercially reasonable efforts to sell not less than a minimum quarterly volume of MPC’s products during each calendar quarter. If MPLX sells less than the minimum quarterly volume of MPC’s products during any calendar quarter despite its commercially reasonable efforts, MPC will pay MPLX a deficiency payment equal to the volume deficiency multiplied by the applicable tiered fee. The dollar amount of actual sales volume of MPC’s products that exceeds the minimum quarterly volume (an “Excess Sale”) for a particular quarter will be applied as a credit, on a first-in-first-out basis, against any future deficiency payment owed by MPC to MPLX during the four calendar quarters immediately following the calendar quarter in which the Excess Sale occurs. • Transportation services agreements – MPLX has various separate transportation services agreements with terms ranging from five to 15 years , under which MPC pays MPLX fees for transporting crude oil and refined products on various of MPLX’s crude oil and refined product pipelines. MPLX also has a five -year agreement under which MPC pays MPLX fees for handling crude oil and products at MPLX’s Wood River, Illinois barge dock, and a six -year transportation services agreement under which MPC pays MPLX fees for providing marine transportation of crude oil, feedstocks and refined petroleum products, and related services. All of the transportation services agreements include automatic renewal terms ranging from two to five years, unless terminated by either party. Under the terms of these agreements, with the exception of the marine agreement, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay MPLX a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect (the “Quarterly Deficiency Payment”). The amount of any Quarterly Deficiency Payment paid by MPC may be applied as a credit for any volumes transported on the applicable pipeline in excess of MPC’s minimum volume commitment during any of the succeeding four quarters, or eight quarters in the case of the transportation services agreements covering the Wood River to Patoka crude pipeline and the Wood River barge dock, after which time any unused credits will expire. Upon the expiration or termination of a transportation services agreement, MPC will have the opportunity to apply any such remaining credit amounts until the completion of any such four-quarter or eight-quarter period, as applicable. Any such remaining credits may be used against any volumes shipped by MPC on the applicable pipeline, without regard to any minimum volume commitment that may have been in place during the term of the agreement. • Storage services agreements – MPLX has three storage services agreements, with 10 -year, 10 -year, and 17 -year terms, under which MPC pays MPLX fees for providing storage services at MPLX’s Neal, West Virginia butane cavern; Robinson, Illinois butane cavern; and Woodhaven, Michigan butane and propane caverns, respectively. MPLX has various separate three -year storage services agreements under which MPC pays MPLX fees for providing storage services at MPLX’s tank farms, and various separate three -year storage services agreements under which MPC pays MPLX fees for providing storage services at MPLX’s storage tanks associated with MPLX’s crude oil and refined product pipelines. MPLX also has various separate storage services agreements with each of MPC’s refineries under which MPLX provides certain services exclusively to MPC related to the receipt, storage, throughput, custody and delivery of petroleum products in and through certain storage and logistical facilities and assets associated with MPC’s refineries. These agreements have initial terms of 10 years. MPLX’s cavern storage services agreements with MPC contain various automatic renewal terms ranging from zero to 10 years. MPLX’s tank farm storage services agreements with MPC automatically renew for additional one -year terms unless terminated by either party. Under the terms of these agreements, MPLX is obligated to make available to MPC, on a firm basis, the available storage capacity at MPLX LP’s tank farms and caverns. MPC pays MPLX a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. MPLX’s refinery storage services agreements with MPC are subject to five -year renewal periods under terms to be renegotiated at that time. MPC pays MPLX monthly fees for refinery storage and logistical services calculated as set forth in the agreements. The refinery storage and logistical facilities subject to the agreements are to be allocated exclusively to MPC for the term of the agreements. • Terminal services agreement – MPLX has a 10 -year terminal services agreement under which MPC pays MPLX fees for terminal storage for refined petroleum products. The terminal services agreement with MPC includes automatic renewal terms ranging from two to five years, unless terminated by either party. Under the terms of the agreement, MPC pays MPLX monthly based on contractual fees relating to MPC product deliveries as well as any viscosity surcharges, loading, handling, transfers or other related charges. If MPC fails to meet its quarterly minimum volume throughput commitments, MPC will pay a deficiency payment equal to the volume of the deficiency multiplied by the rate then in effect. If the average daily capacity of a terminal falls below the level of MPC’s commitment during a quarter, depending on the cause of the reduction in capacity, MPC’s throughput commitment will be reduced to equal the average daily capacity available during such quarter. Operating Agreements MPLX operates various pipelines owned by MPC under operating services agreements. Under these operating services agreements, MPLX receives an operating fee for operating the assets and is reimbursed for all direct and indirect costs associated with operating the assets. Most of these agreements are indexed for inflation. These agreements range from one to five years in length and automatically renew unless terminated by either party. Co-location services agreements MPLX is party to co-location services agreements with each of MPC’s refineries in connection with the dropdown of the refining logistics assets. Under these agreements, MPC provides management, operational and other services to the subsidiaries of Refining Logistics. Refining Logistics pays MPC monthly fixed fees and direct reimbursements for such services calculated as set forth in the agreements. These agreements have initial terms of 50 years. Ground lease agreements MPLX is party to ground lease agreements with each of MPC’s refineries in connection with the dropdown of the Refining Logistics assets. Under these agreements, MPLX is the lessor of certain sections of property which contain facilities owned by Refining Logistics and are within the premises of MPC’s refineries. Refining Logistics pays MPC monthly fixed fees under these ground leases. These agreements have initial terms of 50 years. Management Services Agreement MPLX, through its subsidiary, HSM, has a management services agreement with MPC under which it provides management services to assist MPC in the oversight and management of the marine business. HSM receives a fixed annual fee for providing the required management services. This fee is adjusted annually on the anniversary of the contract for inflation and any changes in the scope of the management services provided. This agreement is set to expire on January 1, 2021 and automatically renews for two additional renewal terms of five years each unless terminated by either party. Omnibus Agreement MPLX has an omnibus agreement with MPC that addresses its payment of a fixed annual fee to MPC for the provision of executive management services by certain executive officers of the general partner and MPLX’s reimbursement of MPC for the provision of certain general and administrative services to it. It also provides for MPC’s indemnification of MPLX for certain matters, including environmental, title and tax matters; as well as our indemnification of MPC for certain matters under this agreement. Employee Services Agreements MPLX has various employee services agreements with MPC under which MPLX reimburses MPC for employee benefit expenses, along with the provision of operational and management services in support of both our L&S and G&P segments’ operations. Loan Agreement MPLX is party to a loan agreement with MPC Investment (the “MPC Loan Agreement”). Under the terms of the agreement, MPC Investment makes a loan or loans to MPLX on a revolving basis as requested by MPLX and as agreed to by MPC Investment. On April 27, 2018, MPLX and MPC Investment entered into an amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion in aggregate principal amount of all loans outstanding at any one time. The entire unpaid principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), shall become due and payable on December 4, 2020 . 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 December 4, 2020 . Borrowings under the loan will bear interest at LIBOR plus 1.50 percent . During 2018 , MPLX borrowed $4.0 billion and repaid $4.3 billion , resulting in no outstanding balance at December 31, 2018 . During 2017 , MPLX borrowed $2.4 billion and repaid $2.0 billion , resulting in $386 million outstanding balance at December 31, 2017 , which is included in “Payables - related parties” on the Consolidated Balance Sheets. Borrowings were at an average interest rate of 3.473 percent and 2.777 percent for 2018 and 2017 , respectively. Related Party Transactions Related party sales to MPC consisted of crude oil and refined products pipeline transportation services based on tariff 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. Revenue received from related parties related to service, rental, and product sales were as follows: (In millions) 2018 2017 2016 Service revenue MPC $ 2,159 $ 1,082 $ 936 Rental income MPC 718 279 235 Product sales (1) MPC $ 49 $ 8 $ 11 (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 2018 , 2017 , and 2016 , these sales totaled $440 million , $254 million and $46 million , respectively. MPLX has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets, a fixed annual fee for providing oversight and management services required to run the marine business and is also reimbursed for personnel services. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. The revenue received from these related parties, included in “Other income - related parties” on the Consolidated Statements of Income, was as follows: (In millions) 2018 2017 2016 MPC $ 41 $ 40 $ 45 MarkWest Utica EMG 17 17 16 Ohio Gathering 16 16 15 Jefferson Dry Gas 6 6 3 Sherwood Midstream 12 8 — Other 7 5 7 Total $ 99 $ 92 $ 86 MPC provides executive management services and certain general and administrative services to MPLX under the terms of an omnibus agreement. Expenses incurred under this agreement are shown in the table below by the income statement line where they were recorded. Charges for services included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Charges for services included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. These charges were as follows: (In millions) 2018 2017 2016 Rental cost of sales - related parties $ 2 $ 1 $ 1 Purchases - related parties 164 67 39 General and administrative expenses 68 37 45 Total $ 234 $ 105 $ 85 MPLX obtains employee services from MPC under employee services agreements. Expenses incurred under these agreements are shown in the table below by the income statement line where they were recorded. The costs of personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties.” The costs of personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” The costs of personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses” on the Consolidated Statements of Income. These charges were as follows: (In millions) 2018 2017 2016 Rental cost of sales - related parties $ 3 $ 1 $ — Purchases - related parties 528 385 349 General and administrative expenses 109 101 100 Total $ 640 $ 487 $ 449 Also under terms of the omnibus and employee services agreements, some service costs related to engineering services are associated with assets under construction. These costs added to “Property, plant and equipment, net” were as follows: (In millions) 2018 2017 2016 MPC $ 151 $ 42 $ 47 Purchases of products from MPC are classified as “Purchases - related parties.” These purchases include product purchases, payments made to MPC in its capacity as general contractor to MPLX, and certain rent and lease agreements. These purchases were as follows: (In millions) 2018 2017 2016 MPC $ 168 $ 3 $ — Receivables from related parties were as follows: December 31, (In millions) 2018 2017 MPC $ 281 $ 153 Other 8 7 Total $ 289 $ 160 Long-term receivables with related parties, which includes straight-line rental income, were as follows: December 31, (In millions) 2018 2017 MPC $ 24 $ 20 Payables to related parties were as follows: December 31, (In millions) 2018 2017 MPC (1) $ 131 $ 470 MarkWest Utica EMG 51 29 Ohio Gathering 5 8 Sherwood Midstream 16 8 Other — 1 Total $ 203 $ 516 (1) Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017 . There was no outstanding balance on the MPC Loan Agreement as of December 31, 2018 . “Other current assets” included $1 million and $8 million of related party prepaid insurance as of December 31, 2018 and December 31, 2017 , respectively. 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 during the years ended December 31, 2018 and 2017 were $5 million and $11 million , respectively. Purchases from related parties during the years ended December 31, 2018 and 2017 were approximately $8 million and $44 million , respectively. During 2018 and 2017 , MPC did not ship its minimum committed volumes on certain pipelines. Under MPLX’s pipeline transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay MPLX a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as “Deferred revenue-related parties.” MPC may then apply the amount of any such deficiency payments as a credit for volumes transported on the applicable pipeline in excess of its minimum volume commitment during the following four or eight quarters under the terms of the applicable transportation services agreement. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes transported in excess of minimum quarterly volume commitments, when it becomes impossible to physically transport volumes necessary to utilize the credits or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Deferred revenue-related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements. The “Deferred revenue-related parties” balance associated with the minimum volume deficiencies and project reimbursements were as follows: December 31, (In millions) 2018 2017 Minimum volume deficiencies - MPC $ 44 $ 53 Project reimbursements - MPC 50 33 Total $ 94 $ 86

Net Income (Loss) Per Limited P

Net Income (Loss) Per Limited Partner Unit	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Income (Loss) Per Limited Partner Unit	Net Income/(Loss) Per Limited Partner Unit Net income/(loss) per unit applicable to common limited partner units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding. The classes of participating securities include common units, certain equity-based compensation awards, Series A Convertible preferred units; and prior to 2018, general partner units and IDRs. The HSM, HST, WHC and MPLXT acquisitions were transfers between entities under common control as discussed in Note 4 . As entities under common control with MPC, prior periods were retrospectively adjusted to furnish comparative information. Accordingly, the prior period earnings have been allocated to the general partner and do not affect the net income/(loss) per unit calculation. The earnings for the entities acquired under common control will be included in the net income/(loss) per unit calculation prospectively as described above. In 2018 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards. In 2017 and 2016 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Potential common units omitted from the diluted earnings per unit calculation for the years ended December 31, 2018 , 2017 and 2016 were less than 1 million . (In millions) 2018 2017 2016 Net income attributable to MPLX LP $ 1,818 $ 794 $ 233 Less: Limited partners’ distributions declared on preferred units (1) 75 65 41 General partner’s distributions declared (includes IDRs) (1)(2) — 328 205 Limited partners’ distributions declared on common units (including common units of general partner) (1) 1,985 895 692 Undistributed net loss attributable to MPLX LP $ (242 ) $ (494 ) $ (705 ) (1) See Note 8 for distribution information. (2) Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared. 2018 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared $ 1,985 $ 75 $ 2,060 Undistributed net loss attributable to MPLX LP (242 ) — (242 ) Net income attributable to MPLX LP (1) $ 1,743 $ 75 $ 1,818 Weighted average units outstanding: Basic 761 761 Diluted 761 761 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.29 Diluted $ 2.29 2017 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 328 $ 895 $ 65 $ 1,288 Undistributed net loss attributable to MPLX LP (10 ) (484 ) — (494 ) Net income attributable to MPLX LP (1) $ 318 $ 411 $ 65 $ 794 Weighted average units outstanding: Basic 8 385 393 Diluted 8 388 396 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.07 Diluted $ 1.06 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distribution declared $ 205 $ 692 $ 41 $ 938 Undistributed net loss attributable to MPLX LP (14 ) (691 ) — (705 ) Net income attributable to MPLX LP (1) $ 191 $ 1 $ 41 $ 233 Weighted average units outstanding: Basic 7 331 338 Diluted 7 338 345 Net income attributable to MPLX LP per limited partner unit: Basic $ — Diluted $ — (1) Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.

Equity

Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Equity	Equity Units Outstanding – MPLX had 794,089,518 common units outstanding as of December 31, 2018 . Of that number, 504,701,934 were owned by MPC, which also owns the non-economic GP interest as described below. GP/IDR Exchange – On February 1, 2018, MPC cancelled its IDRs and converted its economic GP Interest in MPLX LP to a non-economic general partner interest in exchange for 275 million newly issued MPLX LP common units. These units had a fair value of $10.4 billion as of the transaction date as recorded on the Consolidated Statements of Equity. As a result of this transaction, the general partner units and IDRs were eliminated, are no longer outstanding, and no longer participate in distributions of cash from MPLX. MPC continues to own the non-economic GP Interest in MPLX LP. See Note 7 for more information on the net income per unit calculation. Reorganization Transactions – On September 1, 2016, MPLX and various affiliates initiated a series of reorganization transactions in order to simplify MPLX’s ownership structure and its financial and tax reporting requirements (the “Class A Reorganization”). In connection with these transactions, all of the issued and outstanding MPLX LP Class A units, all of which were held by MarkWest Hydrocarbon, L.L.C. (“MarkWest Hydrocarbon”), were either distributed to, or purchased by, MPC in exchange for $84 million in cash, 21,401,137 MPLX LP common units and 436,758 MPLX LP general partner units. Following these initial transactions, the MPLX LP Class A units were exchanged on a one-for-one basis for newly issued common units representing limited partner interests in MPLX LP. MPC also contributed $141 million to facilitate the repayment of intercompany debt between MarkWest Hydrocarbon and MarkWest. As a result of these transactions, the MPLX LP Class A units were eliminated, are no longer outstanding and no longer participate in distributions of cash from MPLX. Cash that is derived from or attributable to MarkWest Hydrocarbon’s operations is now treated in the same manner as cash derived from or attributable to other operations of MPLX and its subsidiaries. Class B Conversions - On July 1, 2016 and July 1, 2017, each Class B unit of MPLX LP was converted, in two equal installments, into 1.09 MPLX LP common units and the right to receive $6.20 in cash. Upon the conversion of each tranche of the Class B units, the right of the unitholder, M&R MWE Liberty LLC and certain of its affiliates (“M&R”), to vote as a common unitholder of MPLX was limited to a maximum of five percent of MPLX’s outstanding common units. Additionally, M&R was given the right with respect to such converted units to participate in MPLX’s underwritten offerings of our common units including continuous equity or similar programs in an amount up to 20 percent of the total number of common units offered by MPLX. M&R may freely transfer such converted units, and M&R has the right to demand that MPLX LP conduct up to three underwritten offerings beginning in 2017, but restricted to no more than one offering in any twelve-month period. Following the July 1, 2017 conversion, all MPLX LP Class B units were eliminated, are no longer outstanding and no longer participate in distributions of cash from MPLX. ATM Program – On March 13, 2018, MPLX entered into a Third Amended and Restated Distribution Agreement, providing for the at-the-market issuances of common units having an aggregate offering price of up to approximately $ 1.7 billion , in amounts, at prices and on terms determined by market conditions and other factors at the time of the offerings (such continuous offering program, or at-the-market program is referred to as the “ATM Program”). During the year ended December 31, 2018 , MPLX issued no common units under the ATM Program. During the years ended December 31, 2017 and 2016 , MPLX issued an aggregate of 13,846,998 and 26,347,887 common units, respectively, under our ATM Program, generating net proceeds of approximately $473 million and $776 million , respectively. MPLX used the net proceeds from sales under the ATM Program for general business purposes, including repayment or refinancing of debt, and funding for acquisitions, working capital requirements and capital expenditures. The table below summarizes the changes in the number of units outstanding for the years ended December 31, 2016 , 2017 , and 2018 : (In units) Common Class B General Partner (1) Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards 120,989 — 2,470 123,459 Issuance of units under the ATM Program 26,347,887 — 537,710 26,885,597 Contribution of HSM (See Note 4) 22,534,002 — 459,878 22,993,880 Class B Conversion 4,350,057 (3,990,878 ) 7,330 366,509 Class A Reorganization 7,153,177 — (436,758 ) 6,716,419 Balance at December 31, 2016 357,193,288 3,990,878 7,371,105 368,555,271 Unit-based compensation awards 268,167 — 5,472 273,639 Issuance of units under the ATM Program 13,846,998 — 282,591 14,129,589 Contribution of HST/WHC/MPLXT (See Note 4) 12,960,376 — 264,497 13,224,873 Contribution of the Joint-Interest Acquisition (See Note 4) 18,511,134 — 377,778 18,888,912 Class B conversion 4,350,057 (3,990,878 ) 7,330 366,509 Balance at December 31, 2017 407,130,020 — 8,308,773 415,438,793 Unit-based compensation awards 348,387 — 140 348,527 Contribution of Refining Logistics and Fuels Distribution (See Note 4) 111,611,111 — 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 — (10,586,691 ) 264,413,309 Balance at December 31, 2018 794,089,518 — — 794,089,518 (1) Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest. Issuance of Additional Securities – The Partnership Agreement authorizes MPLX to issue an unlimited number of additional securities for the consideration and on the terms and conditions determined by the general partner without the approval of the unitholders. Net Income Allocation – In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders first and subsequently allocated to the limited partner unitholders in accordance with their respective ownership percentages. Prior to 2018, when distributions related to the IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings are allocated to the unitholders, based on their respective ownership percentages. The following table presents the allocation of the general partner’s GP Interest in net income attributable to MPLX LP, for income statement periods occurring prior to the exchange of the GP economic interests: (In millions) 2017 2016 Net income attributable to MPLX LP $ 794 $ 233 Less: Preferred unit distributions 65 41 General partner's IDRs and other 310 191 Net income attributable to MPLX LP available to general and limited partners 419 1 General partner's two percent GP Interest in net income attributable to MPLX LP 8 — General partner's IDRs and other 310 191 General partner's GP Interest in net income attributable to MPLX LP $ 318 $ 191 Cash Distributions – The Partnership Agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common unitholders and preferred unitholders will receive. In accordance with the Partnership Agreement, on January 25, 2019 , MPLX declared a quarterly cash distribution, based on the results of the fourth quarter of 2018 , totaling $514 million , or $0.6475 per common unit; this rate was also received by preferred unitholders. These distributions were paid on February 14, 2019 to unitholders of record on February 5, 2019 . Distributions for the fourth quarter of 2017 were $0.6075 per common unit while distributions for the twelve months ended December 31, 2018 and 2017 were $2.5300 and $2.2975 per common unit, respectively. The allocation of total quarterly cash distributions to general, limited, and preferred unitholders is as follows for the years ended December 31, 2018 , 2017 and 2016 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. (In millions) 2018 2017 2016 General partner's distributions: General partner's distributions on general partner units $ — $ 25 $ 18 General partner's distributions on IDRs (1) — 303 187 Total distribution on general partner units and IDRs — 328 205 Limited partners' distributions: Common unitholders, includes common units of general partner 1,985 895 692 Preferred unit distributions 75 65 41 Total cash distributions declared $ 2,060 $ 1,288 $ 938 (1) Includes distributions of fourth quarter 2017 income declared on general partner common units issued February 1, 2018 in exchange for the economic general partner interest.

Redeemable Preferred Units

Redeemable Preferred Units	12 Months Ended
	Dec. 31, 2018
Redeemable Preferred Units Disclosure [Abstract]
Redeemable Preferred Units	Redeemable Preferred Units Private Placement of Preferred Units – On May 13, 2016, MPLX LP completed the private placement of approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The aggregate net proceeds of approximately $984 million from the sale of the Preferred units were used for capital expenditures, repayment of debt and general business purposes. Preferred Unit Distribution Rights - The preferred units rank senior to all common units with respect to distributions and rights upon liquidation. The holders of the preferred units received cumulative quarterly distributions equal to $0.528125 per unit for each quarter prior to the second quarter of 2018. Beginning with the second quarter of 2018, the holders of the preferred units are entitled to receive 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. For the income earned in the second through fourth quarters of 2018, the distribution rate declared to common unitholders was greater than $0.528125 per unit; accordingly, the preferred unitholders received the common unit rates in lieu of the lower $0.528125 base amount. The changes in the redeemable preferred balance for 2018 and 2017 are summarized below: (In millions) 2018 2017 Balance at beginning of period $ 1,000 $ 1,000 Net income allocated 75 65 Distributions received by preferred unitholders (71 ) (65 ) Balance at end of period $ 1,004 $ 1,000 The holders may convert their preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the 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 LP common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the 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 preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the 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 preferred units to common units at the then applicable change of control conversion rate. The 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 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 preferred units. As the 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 preferred units would become redeemable.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information MPLX’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews MPLX’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. MPLX has two reportable segments: L&S and G&P. Each of these segments is organized and managed based upon the nature of the products and services it offers. • L&S – transports, stores, distributes and markets crude oil and refined petroleum products. • G&P – gathers, processes and transports natural gas; gathers, transports, fractionates, stores and markets NGLs. During the second quarter of 2018, our CEO began to evaluate the performance of our segments using Segment Adjusted EBITDA. We have modified our presentation of segment performance metrics to be consistent with this change, including prior periods presented for consistent and comparable presentation. Amounts included in net income and excluded from Segment Adjusted EBITDA include: (i) depreciation and amortization; (ii) provision/(benefit) for income taxes; (iii) amortization of deferred financing costs; (iv) extinguishment of debt; (v) non-cash equity-based compensation; (vi) impairment expense; (vii) net interest and other financial costs; (viii) income/(loss) from equity method investments; (ix) distributions and adjustments related to equity method investments; (x) unrealized derivative gains/(losses); (xi) acquisition costs; (xii) noncontrolling interests; and (xiii) other adjustments as deemed necessary. These items are either: (i) believed to be non-recurring in nature; (ii) not believed to be allocable or controlled by the segment; or (iii) are not tied to the operational performance of the segment. The tables below present information about revenues and other income, capital expenditures and total assets for our reportable segments: (In millions) 2018 2017 2016 L&S Service revenue $ 2,289 $ 1,200 $ 1,006 Rental income 725 279 235 Product related revenue 14 — — Income from equity method investments 166 36 — Other income 46 47 53 Total segment revenues and other income (1) 3,240 1,562 1,294 Segment Adjusted EBITDA (2) 2,057 775 395 Maintenance capital expenditures 104 79 58 Growth capital expenditures 452 433 493 G&P Service revenue 1,574 1,038 888 Rental income 342 277 298 Product related revenue 1,135 897 583 Income/(loss) from equity method investments (3) 74 42 (74 ) Other income 60 51 40 Total segment revenues and other income (1) 3,185 2,305 1,735 Segment Adjusted EBITDA (2) 1,418 1,229 1,024 Maintenance capital expenditures 42 24 26 Growth capital expenditures $ 1,432 $ 948 $ 720 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $313 million , $160 million and $77 million for 2018 , 2017 and 2016 , respectively. Third party revenues for the G&P segment were $3,087 million , $2,246 million and $1,684 million for 2018 , 2017 and 2016 , respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (3) Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016. December 31, (In millions) 2018 2017 Segment Assets Cash and cash equivalents $ 68 $ 5 L&S (1) 6,566 4,611 G&P (1) 16,145 14,884 Total assets $ 22,779 $ 19,500 (1) Equity method investments included in L&S assets were $1.12 billion at December 31, 2018 and $1.15 billion at December 31, 2017 . Equity method investments included in G&P assets were $3.05 billion at December 31, 2018 and $2.86 billion at December 31, 2017 . The table below provides a reconciliation between “Net income” and Segment Adjusted EBITDA. (In millions) 2018 2017 2016 Reconciliation to Net income: L&S Segment Adjusted EBITDA $ 2,057 $ 775 $ 395 G&P Segment Adjusted EBITDA 1,418 1,229 1,024 Total reportable segments 3,475 2,004 1,419 Depreciation and amortization (1) (766 ) (683 ) (591 ) (Provision)/benefit for income taxes (8 ) (1 ) 12 Amortization of deferred financing costs (59 ) (53 ) (46 ) Loss on extinguishment of debt (46 ) — — Non-cash equity-based compensation (19 ) (15 ) (10 ) Impairment expense — — (130 ) Net interest and other financial costs (556 ) (301 ) (215 ) Income/(loss) from equity method investments (2) 240 78 (74 ) Distributions/adjustments related to equity method investments (447 ) (231 ) (150 ) Unrealized derivative gains/(losses) (3) 5 (6 ) (36 ) Acquisition costs (3 ) (11 ) 1 Adjusted EBITDA attributable to noncontrolling interests 18 8 3 Adjusted EBITDA attributable to Predecessor (4) — 47 251 Net income $ 1,834 $ 836 $ 434 (1) Depreciation and amortization attributable to L&S was $240 million , $163 million and $128 million for the years ended 2018 , 2017 and 2016 , respectively. Depreciation and amortization attributable to G&P was $526 million , $520 million and $463 million for 2018 , 2017 and 2016 , respectively. (2) Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016. (3) MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (4) The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.

Major Customers and Concentrati

Major Customers and Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2018
Risks and Uncertainties [Abstract]
Major Customers and Concentration of Credit Risk	Major Customers and Concentration of Credit Risk MPC accounted for 48 percent , 37 percent and 39 percent of MPLX’s operating revenues for 2018 , 2017 and 2016, respectively. Operating revenues consist of service revenue, rental income and product sales. MPC accounted for 46 percent , 36 percent and 41 percent of total revenues and other income for 2018 , 2017 and 2016, respectively. The revenues are accounted for primarily within the L&S segment. The percent calculations exclude revenues attributable to volumes shipped by MPC under joint tariffs with third parties, which are treated as third-party revenue for accounting purposes. MPLX has a concentration of trade receivables due from customers in the same industry: MPC, integrated oil companies, independent refining companies and other pipeline companies. These concentrations of customers may impact MPLX’s overall exposure to credit risk as they may be similarly affected by changes in economic, regulatory and other factors. MPLX manages its exposure to credit risk through credit analysis, credit limit approvals and monitoring procedures; and for certain transactions, it may request letters of credit, prepayments or guarantees.

Income Tax

Income Tax	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax	Income Tax MPLX is not a taxable entity for United States federal income tax purposes or for the majority of states that impose an income tax. Taxes on MPLX’s net income generally are borne by its partners through the allocation of taxable income. On December 22, 2017, the Tax Cuts and Jobs Act was signed into law. While the new law included several key changes to tax law for United States tax payers, as MPLX is not a taxable entity, the new legislation has no impact on MPLX for federal tax purposes. MPLX’s income tax provision/(benefit) primarily results from state and local activity in the states of Texas, Ohio, Kentucky and Tennessee. As a result of the Class A Reorganization discussed in Note 8 , MarkWest Hydrocarbon and MarkWest Hydrocarbon, Inc. (prior to the Class A Reorganization) is no longer a tax paying entity for federal income tax purposes or for the majority of states that impose an income tax effective September 1, 2016. Prior to the Class A Reorganization, in addition to paying tax on its own earnings, MarkWest Hydrocarbon recognized a tax expense or a tax benefit on its proportionate share of Partnership income or loss resulting from MarkWest Hydrocarbon’s ownership of Class A units of MPLX, even though for financial reporting purposes such income or loss was eliminated in consolidation. The deferred income tax component prior to the reorganization related to the change in the temporary book to tax basis difference in the carrying amount of the investment in MPLX, which resulted primarily from timing differences in MarkWest Hydrocarbon’s proportionate share of the book income or loss as compared with the MarkWest Hydrocarbon’s proportionate share of the taxable income or loss of MPLX. MPLX recorded a residual tax provision during the year ended December 31, 2017 related to MarkWest Hydrocarbon’s 2016 income taxes. In connection with the Class A Reorganization, MPC assumed $377 million of MPLX LP’s deferred tax liabilities. MPLX and MarkWest Hydrocarbon recorded income tax expense of $8 million , $1 million and a benefit of $12 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The effective tax rate was less than one percent for 2018 and 2017 and five percent for 2016 . The components of the “Provision/(benefit) for income taxes” are as follows: December 31, (In millions) 2018 2017 2016 Current income tax expense: Federal $ — $ — $ 4 State — 2 1 Total current — 2 5 Deferred income tax expense/(benefit): Federal — — (16 ) State 8 (1 ) (1 ) Total deferred 8 (1 ) (17 ) Provision/(benefit) for income taxes $ 8 $ 1 $ (12 ) A reconciliation of the “Provision/(benefit) for income taxes” and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the year ended December 31, 2016 is as follows: December 31, 2016 (In millions) MarkWest Hydrocarbon (1) Partnership Eliminations Consolidated (Loss)/income before (benefit)/provision for income tax $ (41 ) $ 461 $ 2 $ 422 Federal statutory rate 35 % — % — % Federal income tax at statutory rate (14 ) — — (14 ) State income taxes net of federal benefit (2 ) 1 — (1 ) Provision on income from MPLX LP Class A units 3 — — 3 Change in state statutory rate (1 ) — — (1 ) Other 1 — — 1 (Benefit)/provision for income taxes $ (13 ) $ 1 $ — $ (12 ) (1) MarkWest Hydrocarbon paid tax on its share of MPLX’s income or loss as a result of its ownership of MPLX LP Class A units through September 1, 2016. In taxable jurisdictions, MPLX recorded deferred income taxes on all temporary differences between the book and tax basis of assets and liabilities. MPLX has a net deferred tax liability of $13 million and $5 million for the years ended December 31, 2018 and 2017 , respectively. The net deferred tax liability is principally derived from the difference in the book and tax basis of property, plant and equipment. Significant judgment is required in evaluating tax positions and determining MPLX and MarkWest Hydrocarbon’s provision for income taxes. During the ordinary course of business, there may be transactions and calculations for which the ultimate tax determination is uncertain. However, MPLX and MarkWest Hydrocarbon did not have any material uncertain tax positions for the years ended December 31, 2018 , 2017 or 2016 . Any interest and penalties related to income taxes were recorded as a part of the provision for income taxes. Such interest and penalties were a net expense of less than $1 million in 2018 , and a net benefit of less than $1 million in 2017 and 2016 . As of December 31, 2018 , 2017 and 2016 , no interest and penalties were accrued related to income taxes. In addition, MPLX and MarkWest Hydrocarbon’s former corporate entity have federal tax years 2015 through 2016 and state tax years 2013 through 2017 open to examination.

Inventories

Inventories	12 Months Ended
	Dec. 31, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: December 31, (In millions) 2018 2017 NGLs $ 9 $ 4 Line fill 9 8 Spare parts, materials and supplies 59 53 Total inventories $ 77 $ 65

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: Estimated Useful Lives December 31, (In millions) 2018 2017 Natural gas gathering and NGL transportation pipelines and facilities 5 - 30 years $ 5,926 $ 5,178 Processing, fractionation and storage facilities 10 - 40 years 5,336 3,893 Pipelines and related assets 15 - 51 years 2,560 2,253 Barges and towing vessels 20 years 620 490 Terminals and related assets 4 - 30 years 1,178 821 Refinery related assets 5 - 30 years 938 — Land, building, office equipment and other 3 - 35 years 957 770 Construction-in-progress 801 1,057 Total 18,316 14,462 Less accumulated depreciation 3,677 2,275 Property, plant and equipment, net $ 14,639 $ 12,187 Property, plant and equipment includes gross assets acquired under capital leases of approximately $25 million at December 31, 2018 and 2017 , with related amounts in accumulated depreciation of approximately $9 million at December 31, 2018 and 2017 .

Goodwill and Intangibles

Goodwill and Intangibles	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangibles	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. MPLX has 12 reporting units, eight of which had goodwill totaling approximately $2.6 billion as of November 30, 2018 . MPLX performed its annual impairment tests, and no impairments in the carrying value of goodwill were identified. Significant assumptions used to estimate the reporting units’ fair value include the discount rate as well as estimates of future cash flows, which are impacted primarily by commodity prices and producer customers’ development plans (which impact volumes and capital requirements). During the first quarter of 2016, MPLX determined that an interim impairment analysis of the goodwill recorded in connection with the MarkWest Merger was necessary based on consideration of a number of first quarter events and circumstances, including (i) continued deterioration of near term commodity prices as well as longer term pricing trends, (ii) recent guidance on reductions to forecasted capital spending, the slowing of drilling activity and the resulting reduced production growth forecasts released or communicated by MPLX’s producer customers and (iii) increases in cost of capital. The combination of these factors was considered to be a triggering event requiring an interim impairment test. Based on the first step of the interim goodwill impairment analysis, the fair value for the three reporting units to which goodwill was assigned in connection with the MarkWest Merger was less than the respective carrying value. In step two of the impairment analysis, the implied fair values of the goodwill were compared to the carrying values within those reporting units. Based on this assessment, it was determined that goodwill was impaired in two of the three reporting units. Accordingly, MPLX recorded an impairment charge of approximately $129 million in the first quarter of 2016. In the second quarter of 2016, MPLX completed its purchase price allocation, which resulted in an additional $1 million of impairment expense that would have been recorded in the first quarter of 2016 had the purchase price allocation been completed as of that date. This adjustment to the impairment expense was the result of completing an evaluation of the deferred tax liabilities associated with the MarkWest Merger and their impact on the resulting goodwill that was recognized. The fair value of the reporting units for the interim goodwill impairment analysis described above was determined based on applying the discounted cash flow method, which is an income approach, and the guideline public company method, which is a market approach. The discounted cash flow fair value estimate is based on known or knowable information at the interim measurement date. The significant assumptions that were used to develop the estimates of the fair values under the discounted cash flow method included management’s best estimates of the expected future results and discount rates, which range from 10.5 percent to 11.5 percent . The fair value of the intangibles was determined based on applying the multi-period excess earnings method, which is an income approach. Key assumptions included attrition rates by reporting unit ranging from 5.0 percent to 10.0 percent and discount rates by reporting unit ranging from 11.5 percent to 12.8 percent . Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As a result, there can be no assurance that the estimates and assumptions made for purposes of the interim goodwill impairment test will prove to be an accurate prediction of the future. The fair value measurements for the individual reporting units’ overall fair values, and the fair values of the goodwill assigned thereto, represent Level 3 measurements. The changes in carrying amount of goodwill were as follows for the periods presented: (In millions) L&S G&P Total Gross goodwill as of December 31, 2016 $ 162 $ 2,213 $ 2,375 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2016 162 2,083 2,245 Impairment losses — — — Acquisitions — — — Balance as of December 31, 2017 162 2,083 2,245 Impairment losses — — — Acquisitions 341 — 341 Balance as of December 31, 2018 $ 503 $ 2,083 $ 2,586 Gross goodwill as of December 31, 2018 $ 503 $ 2,213 $ 2,716 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2018 $ 503 $ 2,083 $ 2,586 Intangible Assets MPLX’s intangible assets are comprised of customer contracts and relationships, gross intangible assets with accumulated amortization as of December 31, 2018 and 2017 is shown below: December 31, 2018 December 31, 2017 (In millions) Useful Life Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (1) Net L&S 4-6 years $ 9 $ — $ 9 $ — $ — $ — G&P 11-25 years 533 (118 ) 415 533 (80 ) 453 $ 542 $ (118 ) $ 424 $ 533 $ (80 ) $ 453 (1) Amortization expense attributable to the G&P segment for the years ended December 31, 2018 and 2017 was $38 million in both years. Estimated future amortization expense related to the intangible assets at December 31, 2018 is as follows: (In millions) 2019 $ 40 2020 40 2021 40 2022 39 2023 39 Thereafter 226 Total $ 424

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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 17. The following table presents the financial instruments carried at fair value on a recurring basis as of December 31, 2018 and 2017 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. December 31, 2018 December 31, 2017 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Commodity contracts $ — $ — $ — $ (2 ) Embedded derivatives in commodity contracts — (61 ) — (64 ) Total carrying value in Consolidated Balance Sheets $ — $ (61 ) $ — $ (66 ) Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase agreement embedded in a keep-whole processing agreement. The fair value calculation for Level 3 instruments at December 31, 2018 used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.58 to $1.01 and (2) the probability of renewal of 90 percent for the first five -year term and 80 percent for the second five -year term of the gas purchase agreement and related keep-whole processing agreement. For commodity contracts, increases in forward NGL prices result in a decrease in the fair value of the derivative assets and an increase in the fair value of derivative liabilities. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability. 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. 2018 2017 (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 $ (2 ) $ (64 ) $ (6 ) $ (54 ) Total gains/(losses) (realized and unrealized) included in earnings (1) 6 (9 ) (5 ) (19 ) Settlements (4 ) 12 9 9 Fair value at end of period — (61 ) (2 ) (64 ) The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (8 ) $ (2 ) $ (6 ) (1) Gains and losses on commodity derivatives classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income. Fair Values – Reported MPLX’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, 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 17 ). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. The fair value of the SMR liability is estimated using a discounted cash flow approach based on the contractual cash flows and MPLX’s unsecured borrowing rate. The long-term debt and SMR liability fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of the long-term debt, excluding capital leases, and SMR liability. December 31, 2018 2017 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 13,169 $ 13,484 $ 7,718 $ 6,966 SMR liability $ 92 $ 86 $ 104 $ 91

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments As of December 31, 2018 , MPLX had no outstanding commodity contracts. Embedded Derivative - MPLX has a natural gas purchase commitment embedded in a keep-whole processing agreement with a producer customer in the Southern Appalachian region expiring in December 2022. The customer has the unilateral option to extend the agreement for two consecutive five -year terms through December 2032. For accounting purposes, these natural gas purchase commitment and term extending options have been aggregated into a single compound embedded derivative. The probability of the customer exercising its options is determined based on assumptions about the customer’s potential business strategy decision points that may exist at the time they would elect whether to renew the contract. The changes in fair value of this compound embedded derivative are based on the difference between the contractual and index pricing, the probability of the producer customer exercising its option to extend and the estimated favorability of these contracts compared to current market conditions. The changes in fair value are recorded in earnings through “Purchased product costs” on the Consolidated Statements of Income. As of December 31, 2018 and 2017 , the estimated fair value of this contract was a liability of $61 million and $64 million , respectively. Certain derivative positions are subject to master netting agreements; therefore, MPLX has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of December 31, 2018 and 2017 , 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) December 31, 2018 December 31, 2017 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets /Other current liabilities $ — $ (7 ) $ — $ (14 ) Other noncurrent assets /Deferred credits and other liabilities — (54 ) — (52 ) Total $ — $ (61 ) $ — $ (66 ) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, including the effect of master netting arrangements or collateral, see Note 16 . See Note 2 for a discussion of derivatives MPLX uses and the reasons for them. MPLX does not designate any of its commodity derivative positions as hedges for accounting purposes. The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: December 31, (In millions) 2018 2017 2016 Product sales Realized gains/(losses) $ 4 $ (9 ) $ 2 Unrealized gains/(losses) 2 4 (15 ) Total derivative gains/(losses) related to product sales 6 (5 ) (13 ) Purchased product costs Realized losses (12 ) (9 ) (5 ) Unrealized gains/(losses) 3 (10 ) (22 ) Total derivative loss related to purchased product costs (9 ) (19 ) (27 ) Cost of revenues Realized losses — — (3 ) Unrealized gains — — 1 Total derivative losses related to cost of revenues — — (2 ) Total derivative losses $ (3 ) $ (24 ) $ (42 )

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings at December 31, 2018 and 2017 consisted of the following: December 31, (In millions) 2018 2017 MPLX LP: Bank revolving credit facility due 2022 $ — $ 505 5.500% senior notes due February 2023 — 710 3.375% senior notes due March 2023 500 — 4.500% senior notes due July 2023 989 989 4.875% senior notes due December 2024 1,149 1,149 4.000% senior notes due February 2025 500 500 4.875% senior notes due June 2025 1,189 1,189 4.125% senior notes due March 2027 1,250 1,250 4.000% senior notes due March 2028 1,250 — 4.800% senior notes due February 2029 750 — 4.500% senior notes due April 2038 1,750 — 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 — 5.500% senior notes due February 2049 1,500 — 4.900% senior notes due April 2058 500 — Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 63 Capital lease obligations due 2020 6 7 Total 13,856 7,362 Unamortized debt issuance costs (97 ) (27 ) Unamortized discount (366 ) (389 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 13,392 $ 6,945 The following table shows five years of scheduled debt payments. (In millions) 2019 $ 1 2020 5 2021 — 2022 — 2023 $ 1,500 Credit Agreements On July 21, 2017 , MPLX entered into a syndicated credit agreement to replace its previously outstanding $2 billion five -year bank revolving credit facility and $250 million term loan with a $2.25 billion five -year bank revolving credit facility that expires in July 2022 (the “MPLX Credit Agreement”). The financial covenants and the interest rate terms contained in the new credit agreement are substantially the same as those contained in the previous bank revolving credit facility. On July 19, 2017, MPLX prepaid the previously outstanding principal of the term loan with cash on hand. The borrowings under the term loan facility bore interest between January 1, 2017 and July 19, 2017 at an average interest rate of 2.407 percent . The MPLX Credit Agreement includes letter of credit issuing capacity of up to $222 million and swingline capacity of up to $100 million . The borrowing capacity under the MPLX Credit Agreement may be increased by up to an additional $500 million , subject to certain conditions, including the consent of lenders whose commitments would increase. In addition, the maturity date may be extended, for up to two additional one -year periods, subject to, among other conditions, the approval of lenders holding the majority of the commitments then outstanding, provided that the commitments of any non-consenting lenders will terminate on the then-effective maturity date. Borrowings under the MPLX Credit Agreement bear interest at either the Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin . MPLX is charged various fees and expenses in connection with the agreement, including administrative agent fees, commitment fees on the unused portion of the facility and fees with respect to issued and outstanding letters of credit. The applicable margins to the benchmark interest rates and certain fees fluctuate based on the credit ratings in effect from time to time on MPLX’s long-term debt. The MPLX Credit Agreement contains certain representations and warranties, affirmative and restrictive covenants and events of default that MPLX considers to be usual and customary for an agreement of this type, including a financial covenant that requires MPLX to maintain a ratio of Consolidated Total Debt as of the end of each fiscal quarter to Consolidated EBITDA (both as defined in the MPLX Credit Agreement) for the prior four fiscal quarters of no greater than 5.0 to 1.0 (or 5.5 to 1.0 for up to two fiscal quarters following certain acquisitions). Consolidated EBITDA is subject to adjustments for certain acquisitions and dispositions completed and capital projects undertaken during the relevant period. Other covenants restrict MPLX and/or certain of its subsidiaries from incurring debt, creating liens on our assets and entering into transactions with affiliates. As of December 31, 2018 , MPLX was in compliance with the covenants contained in the MPLX Credit Agreement. During the year ended December 31, 2018 , MPLX borrowed $1,410 million under the MPLX Credit Agreement, at a weighted average interest rate of 3.464 percent , and repaid $1,915 million of these borrowings. At December 31, 2018 , MPLX had no outstanding borrowings and $3 million letters of credit outstanding under the new facility, resulting in total availability of $2.2 billion , or 99.9 percent of the borrowing capacity . During 2017 , MPLX had no borrowings under the previous bank revolving credit facility. During the year ended December 31, 2017 , MPLX borrowed $670 million under the MPLX Credit Agreement, at a weighted average interest rate of 2.748 percent , and repaid $165 million of these borrowings. At December 31, 2017 , MPLX had $505 million outstanding borrowings and $3 million letters of credit outstanding under this facility, resulting in total unused loan availability of $1.7 billion , or 77.4 percent of the borrowing capacity. Senior Notes Interest on each series of MPLX LP and MarkWest senior notes is payable semi-annually in arrears, according to the table below. Senior Notes Interest payable semi-annually in arrears 3.375% senior notes due 2023 March 15 th and September 15 th 4.500% senior notes due 2023 January 15 th and July 15 th 4.875% senior notes due 2024 June 1 st and December 1 st 4.000% senior notes due 2025 February 15 th and August 15 th 4.875% senior notes due 2025 June 1 st and December 1 st 4.125% senior notes due 2027 March 1 st and September 1 st 4.000% senior notes due 2028 March 15 th and September 15 th 4.800% senior notes due 2029 February 15 th and August 15 th 4.500% senior notes due 2038 April 15 th and October 15 th 5.200% senior notes due 2047 March 1 st and September 1 st 4.700% senior notes due 2048 April 15 th and October 15 th 5.500% senior notes due 2049 February 15 th and August 15 th 4.900% senior notes due 2058 April 15 th and October 15 th On December 10, 2018, MPLX redeemed all of the $750 million 5.5 percent senior notes due February 15, 2023, $40 million of which was issued by the MarkWest subsidiary. These notes were redeemed at 101.833 percent of the principal amount, which resulted in a payment of $14 million related to the note premium and the immediate recognition of $46 million of unamortized debt issuance costs. On November 15, 2018, MPLX issued $2.25 billion aggregate principal amount of senior notes in a public offering, consisting of $750 million aggregate principal amount of 4.8 percent unsecured senior notes due February 2029 and $1.5 billion aggregate principal amount of 5.5 percent unsecured senior notes due February 2049 (collectively, the “November 2018 New Senior Notes”). The November 2018 New Senior Notes were offered at a price to the public of 99.432 percent and 98.031 percent of par, respectively. The proceeds were used to repay outstanding borrowings under the MPLX Credit Agreement and the MPC Loan Agreement and to redeem the $750 million 5.5 percent senior notes due February 2023, as well as for general business purposes. Interest on each series of notes in the November 2018 New Senior Notes is payable semi-annually in arrears, commencing on February 15, 2019. On February 8, 2018 , MPLX issued $5.5 billion aggregate principal amount of senior notes in a public offering, consisting of $500 million aggregate principal amount of 3.375 percent unsecured senior notes due March 2023 , $1.25 billion aggregate principal amount of 4.0 percent unsecured senior notes due March 2028 , $1.75 billion aggregate principal amount of 4.5 percent unsecured senior notes due April 2038 , $1.5 billion aggregate principal amount of 4.7 percent unsecured senior notes due April 2048 , and $500 million aggregate principal amount of 4.9 percent unsecured senior notes due April 2058 (collectively, the “February 2018 New Senior Notes”). The February 2018 New Senior Notes were offered at a price to the public of 99.931 percent , 99.551 percent , 98.811 percent , 99.348 percent , and 99.289 percent of par, respectively. Also on February 8, 2018 , $4.1 billion of the net proceeds from the offering were used to repay the 364-day term loan facility, which was drawn on February 1, 2018 to fund the cash portion of the dropdown consideration for Refining Logistics and Fuels Distribution. The remaining proceeds were used to repay outstanding borrowings under the MPLX Credit Agreement and the MPC Loan Agreement, as well as for general business purposes. Interest on each series of notes due in 2023 and 2028 is payable semi-annually in arrears, commencing on September 15, 2018. Interest on each series of notes due in 2038, 2048 and 2058 is payable semi-annually in arrears, commencing on October 15, 2018. On February 10, 2017, MPLX completed a public offering of $1.25 billion aggregate principal amount of 4.125 percent unsecured senior notes due March 2027 (the “2027 Senior Notes”) and $1.0 billion aggregate principal amount of 5.200 percent unsecured senior notes due March 2047 (the “2047 Senior Notes”). The 2027 Senior Notes and the 2047 Senior Notes were offered at a price to the public of 99.834 percent and 99.304 percent of par, respectively. The net proceeds were used to fund the $1.5 billion cash portion of the consideration paid to MPC for the dropdown of assets on March 1, 2017, as well as for general business purposes. SMR Transaction On September 1, 2009, MarkWest completed the sale of the SMR (the “SMR Transaction”). At that time, MarkWest had begun constructing the SMR at its Javelina gas processing and fractionation complex in Corpus Christi, Texas. Under the terms of the agreement, MarkWest received proceeds of $73 million and the purchaser completed the construction of the SMR. MarkWest and the purchaser also executed a related product supply agreement under which MPLX will receive the entire product produced by the SMR through 2030 in exchange for processing fees and the reimbursement of certain other expenses. The processing fee payments began when the SMR commenced operations in March 2010. MarkWest was deemed to have continuing involvement with the SMR as a result of certain provisions in the related agreements. Therefore, the transaction is treated as a financing arrangement under GAAP. MPLX imputes interest on the SMR liability at 6.39 percent annually, its incremental borrowing rate at the time of the purchase accounting valuation. Each processing fee payment has multiple elements: reduction of principal of the SMR liability, interest expense associated with the SMR liability and facility expense related to the operation of the SMR. As part of purchase accounting, the SMR Transaction has been recorded at fair value. As of December 31, 2018 and 2017 , the following amounts related to the SMR are included in the accompanying Consolidated Balance Sheets: (In millions) December 31, 2018 December 31, 2017 Assets Property, plant and equipment, net $ 51 $ 56 Liabilities Accrued liabilities 5 5 Deferred credits and other liabilities $ 81 $ 86

Revenue Revenue

Revenue Revenue	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Effect of ASC 606 Adoption MPLX adopted ASC 606 on January 1, 2018 for all contracts that were not yet completed as of the date of adoption. The details of significant changes and quantitative impact of the new revenue standard are disclosed below. • Third-party reimbursements – Third-party reimbursements, such as electricity costs, are presented gross on the income statement rather than net within cost of revenues. The gross-up for third-party reimbursements (e.g., increase in “Service revenue”; increase in “Cost of revenues”) was $369 million for the year ended December 31, 2018 . MPLX updated the allocation between lease and non-lease components for implicit leases as a result of this ASC 606 gross up. As a result, “Rental income” and “Rental cost of sales” increased by $65 million for the year ended December 31, 2018 . • Noncash consideration – Under certain processing agreements, MPLX is entitled to retain NGLs or other liquids from the customer. We obtain control of these NGLs and are able to direct the use of the goods. Service revenues are recorded based on the value of the NGLs received on the date the services are performed. Historically, revenue was not recorded on these arrangements until the product was sold. The impact to this change was an increase of $52 million to “Service revenue - product related” for the year ended December 31, 2018 . NGL inventory related to keep-whole volumes was also revalued as a result of this change, with a cumulative effect adjustment of $1 million and an increase to inventory of $2 million as of December 31, 2018 . The increase in the inventory basis increased “Purchased product costs” by $50 million for the year ended December 31, 2018 . • Percent-of-proceeds revenues – MPLX’s percentage of proceeds revenue received was historically recorded in product revenues. Upon adoption of ASC 606, these revenues have been classified in service revenue, as the performance obligation related to these contracts is to provide gathering and processing services. Revenues will continue to be recorded net under these arrangements as MPLX does not control the product prior to sale. For the year ended December 31, 2018 , $146 million was recorded in “Service revenue - product related” as opposed to “Product sales.” • Imbalances – Historically, all imbalances were recorded net. In certain instances, MPLX’s arrangements are structured such that imbalances are cashed-out each period end which results in the transfer of control of a commodity and creates a purchase and/or sale of a commodity under ASC 606. Thus, certain imbalances will be grossed up as a result of adoption. The impact of this change was an increase of $55 million to “Product sales” and “Purchased product costs” for the year ended December 31, 2018 . • Aid in construction – Historically, all aid in construction amounts received were deferred and recognized into revenue. Payments received from non-customers will no longer be deferred as the accounting will not be subject to ASC 606. Such payments will be recorded as a reduction to “Property, plant and equipment, net.” The cumulative adjustment wrote down $3 million of “Property, plant and equipment, net.” • Oil Allowances – Historically, oil allowances were recorded when received as consideration for services performed. Under ASC 606, MPLX does not believe such amounts represent consideration from a customer. Any excess product obtained and sold as a result of these allowances is recorded as product sales. This change decreased “Service revenues” and “Service revenues - related party” by $7 million , and increased “Product sales” and “Product sales related party” by $7 million for the year ended December 31, 2018 . The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of ASC 606 was as follows: (In millions) Balance at December 31, 2017 ASC 606 Adjustment Balance at January 1, 2018 Assets Inventories $ 65 $ 1 $ 66 Property, plant and equipment, net 12,187 (3 ) 12,184 Liabilities Long-term deferred revenue 42 (3 ) 39 Equity Common unitholders - public $ 8,379 $ 1 $ 8,380 Aside from the adjustments to the opening balances noted above, the impact of adoption on the Consolidated Balance Sheets for the year ended December 31, 2018 was approximately a $2 million adjustment to “Inventories.” The disclosure of the impact of adoption on the Consolidated Statements of Income for the year ended December 31, 2018 was as follows: December 31, 2018 (In millions) ASC 606 Balance ASC 605 Balance Effect of Change Higher/ (Lower) Revenues and other income: Service revenue $ 1,704 $ 1,342 $ 362 Service revenue - related parties 2,159 2,166 (7 ) Service revenue - product related 198 — 198 Rental income 349 284 65 Product sales (1) 897 982 (85 ) Product sales - related parties 49 42 7 Costs and expenses: Cost of revenues (2) 948 579 369 Purchased product costs 845 740 105 Rental cost of sales 135 70 65 Depreciation and amortization 766 767 (1 ) Net income $ 1,834 $ 1,832 $ 2 (1) G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments. (2) Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.” Disaggregation of Revenue The following table represents a disaggregation of revenue for each reportable segment for the year ended December 31, 2018 : December 31, 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 130 $ 1,574 $ 1,704 Service revenue - related parties 2,159 — 2,159 Service revenue - product related — 198 198 Product sales (1) 7 890 897 Product sales - related parties 7 42 49 Total revenues from contracts with customers $ 2,303 $ 2,704 5,007 Non-ASC 606 revenue (2) 1,418 Total revenues and other income $ 6,425 (1) G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and MPLX’s rights to consideration for work completed exceeds the amount billed to the customer. Contract assets are generally classified as current and included in “Other current assets” on the Consolidated Balance Sheets. Contract liabilities, which we refer to as “Deferred revenue” and “Long-term deferred revenue,” typically relate to advance payments for aid in construction agreements and deferred customer credits associated with makeup rights and minimum volume commitments. Related to minimum volume commitments, breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. We classify contract liabilities as current or long-term based on the timing of when we expect to recognize revenue. “Receivables, net” primarily relate to our commodity sales. Portions of the “Receivables, net” balance are attributed to the sale of commodity product controlled by MPLX prior to sale while a significant portion of the balance relates to the sale of commodity product on behalf of our producer customers. The sales and related “Receivables, net” are commingled and excluded from the table below. MPLX remits the net sales price back to our producer customers upon completion of the sale. Each period end, certain amounts within accounts payable relate to our payments to producer customers. Such amounts are not deemed material at period end as a result of when we settle with each producer. The table below reflects the changes in our contract balances for the year ended December 31, 2018 : (In millions) Balance at January 1, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2018 Contract assets $ 4 $ — $ — $ 4 Deferred revenue 5 8 (9 ) 4 Deferred revenue - related parties 42 40 (32 ) 50 Long-term deferred revenue 5 5 — 10 Long-term deferred revenue - related parties $ 43 $ (1 ) $ — $ 42 (1) Balance represents ASC 606 portion of each respective line item. (2) $1 million revenue was recognized related to past performance obligations in the current year. Remaining Performance Obligations The table below includes estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period. As of December 31, 2018 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $105 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 25 years . Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2019 $ 1,146 2020 1,152 2021 1,166 2022 1,151 2023 and thereafter 5,524 Total revenue on remaining performance obligations (1)(2)(3) $ 10,139 (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. Practical Expedients We do not disclose information on the future performance obligations for any contract with an original expected duration of one year or less.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	12 Months Ended
	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information (In millions) December 31, 2018 December 31, 2017 Cash and cash equivalents $ 68 $ 5 Restricted cash (1) 8 4 Cash, cash equivalents and restricted cash (2) $ 76 $ 9 (1) The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets. (2) As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.” (In millions) 2018 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 484 $ 263 $ 213 Income taxes paid 1 3 4 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 2 6 (3 ) Contribution - fixed assets to joint venture (1) — 337 — Contribution - common units issued (2) $ 4,236 $ 1,133 $ 669 (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5 . (2) For 2016, includes limited partner units issued to MPC as consideration in the acquisition of HSM. For 2017, includes limited and general partner units issued to MPC as consideration in the acquisitions of the joint-interests, HST, WHC and MPLXT. For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. See Note 4 . At December 31, 2017, “Payables - related parties” per the Consolidated Balance Sheets included an $11 million payable to MPC for distributions of cash received from Joint-Interest Acquisition entities that did not affect cash. The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: (In millions) 2018 2017 2016 Increase/(decrease) in capital accruals $ 104 $ 71 $ (22 )

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP and Explorer to their employees. MPLX is not a sponsor of these benefit plans. As a transfer between entities under common control, MPLX recorded the Joint-Interest Acquisition from MPC on the Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. MPLX’s assumption of the accumulated other comprehensive loss balance had no effect on MPLX’s comprehensive income during the period as the balance was accumulated while under the ownership of MPC. The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2016 through December 31, 2018 : (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2016 $ — $ — $ — Joint-Interest Acquisition (13 ) (1 ) (14 ) Balance at December 31, 2017 (1) (13 ) (1 ) (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance as of December 31, 2018 (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.” (2) Components of other comprehensive loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity-Based Compensation	Equity-Based Compensation Description of the Plan Effective March 15, 2018 , the MPLX LP 2012 Incentive Compensation Plan (“MPLX 2012 Plan”) was replaced by the MPLX LP 2018 Incentive Compensation Plan (“MPLX 2018 Plan”). The MPLX 2018 Plan will continue in effect until February 28, 2028 , unless terminated earlier. Subject to customary anti-dilution adjustments, the MPLX 2018 Plan allows for no more than 16 million common units representing limited partnership interests in MPLX to be delivered under the plan. The MPLX LP 2012 Plan allowed for no more than 2.75 million MPLX LP common limited partner units to be delivered. Consistent with the MPLX 2012 Plan, the MPLX 2018 Plan authorizes the MPLX GP board of directors (the “Board”) to grant unit options, unit appreciation rights, restricted units and phantom units, distribution equivalent rights, unit awards, profits interest units, performance units and other unit-based awards to the employees, officers and directors of the General Partner, MPLX, or any of their affiliates, including MPC. Common units delivered pursuant to an award granted under the MPLX 2018 Plan may be newly issued common units or acquired in the open market or from any other person, including an affiliate of MPLX, as determined by the Board. Unit-based Awards under the Plan MPLX expenses all unit-based payments to employees and non-employee directors based on the grant date fair value of the awards over the requisite service period, adjusted for estimated forfeitures. Phantom Units – MPLX has granted phantom units under the MPLX 2018 Plan and the MPLX 2012 Plan to non-employee directors of MPLX LP’s general partner and of MPC. Awards to non-employee directors are accounted for as non-employee awards. Phantom units granted to non-employee directors vest immediately at the time of the grant, as they are non-forfeitable, but are not issued until the director’s departure from the board of directors. Prior to issuance, non-employee directors do not have the right to vote such units and cash distribution equivalents accrue in the form of additional phantom units and will be issued when the director departs from the board of directors. MPLX has granted phantom units under the MPLX 2018 Plan and the MPLX 2012 Plan to certain officers and non-officers of MPLX LP, MPLX LP’s general partner and MPC who make significant contributions to our business. These grants are accounted for as employee awards. In general, these phantom units will vest over a requisite service period of up to three years . Prior to vesting, these phantom unit recipients will not have the right to vote such units and cash distributions declared will be accrued and paid upon vesting. The accrued distributions at December 31, 2018 and 2017 were $4 million and $4 million , respectively. The fair values of phantom units are based on the fair value of MPLX LP common units on the grant date. Performance Units – MPLX has granted performance units under the MPLX 2018 Plan and the MPLX 2012 Plan to certain officers of the general partner and certain eligible MPC officers who make significant contributions to our business. Performance units are designed to pay out 75 percent in cash and 25 percent in MPLX LP common units. The performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards. The performance units granted in 2016 have a three-year performance period of January 1, 2016 through December 31, 2018. The payout of the award is dependent on the total unitholder return of MPLX LP common units as compared to the total unitholder return of a selected group of peer partnerships. The final per unit payout will be based on the average of the results of four measurement periods during the period. The performance units granted in 2017 are hybrid awards having a three -year performance period of January 1, 2017 through December 31, 2019. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s DCF during the last twelve months of the performance period, and a market condition based on MPLX LP’s total unitholder return over the entire three-year performance period. During the first quarter of 2018, a performance award was granted; however, a grant date could not be established based on the nature of the award terms. Given that a grant date cannot be established, no expense or units have been recorded. When a grant date is established, the fair value of the award will be recognized over the remaining service period. Outstanding Phantom Unit Awards The following is a summary of phantom unit award activity of MPLX LP common units in 2018 : Phantom Units Number of Units Weighted Average Fair Value Aggregate Intrinsic Value (In millions) Outstanding at December 31, 2017 1,351,523 $ 34.53 Granted 437,092 33.84 Settled (509,570 ) 34.38 Forfeited (124,710 ) 34.50 Outstanding at December 31, 2018 1,154,335 34.34 Vested and expected to vest at December 31, 2018 1,139,877 34.34 $ 35 Non-forfeitable at December 31, 2018 (1) 321,638 $ 34.59 $ 10 (1) Represents a subset of phantom units held by our non-employee directors and certain of our officers and non-officer employees that are generally non-forfeitable and that would be paid out as common units upon the holder’s separation from service. The following is a summary of the values related to phantom units: Phantom Units Intrinsic Value of Units Issued During the Period (in millions) Weighted Average Grant Date Fair Value of Units Granted During the Period 2018 $ 18 $ 33.84 2017 15 36.26 2016 $ 5 $ 29.42 As of December 31, 2018 , unrecognized compensation cost related to phantom unit awards was $17 million , which is expected to be recognized over a weighted average period of 1.8 years . Outstanding Performance Unit Awards The following table presents a summary of the 2018 activity for performance unit awards to be settled in MPLX LP common units: Performance Units Number of Units Weighted Outstanding at December 31, 2017 2,536,594 $ 0.85 Granted — — Settled (538,594 ) 1.04 Forfeited (56,250 ) 0.90 Outstanding at December 31, 2018 1,941,750 0.80 The number of common units that would be issued upon target vesting, using the closing price of our common units on December 31, 2018 would be 64,084 common units. As of December 31, 2018 , unrecognized compensation cost related to equity-classified performance unit awards was $1 million , which is expected to be recognized over a weighted average period of 1.0 year . Performance units paying out in MPLX LP common units have a grant date fair value calculated using a Monte Carlo valuation model, which requires the input of subjective assumptions. The following table provides a summary of the weighted average inputs used for these assumptions: 2018 2017 2016 Risk-free interest rate N/A 1.52% 0.96% Look-back period N/A 2.83 years 2.83 years Expected volatility N/A 49.34% 47.59% Grant date fair value of performance units granted N/A $0.90 $0.63 The assumption for expected volatility of our unit price reflects the historical volatility of MPLX LP common units. The look-back period reflects the remaining performance period at the grant date. The risk-free interest rate for the remaining performance period as of the grant date is based on the U.S. Treasury yield curve in effect at the time of the grant. No grant date fair value has been calculated for performance units granted in 2018, since due to the award terms, a grant date has not yet been established. Total Unit-Based Compensation Expense Total unit-based compensation expense for awards settling in MPLX LP common units was $22 million in 2018 , $18 million in 2017 and $10 million in 2016 . MPC’s Stock-based Compensation Stock-based compensation expenses charged to MPLX LP under our employee services agreement with MPC were $6 million , $2 million and $5 million for 2018 , 2017 and 2016 , respectively.

Lease Operations

Lease Operations	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Lease Operations	Lease Operations Based on the terms of fee-based transportation and storage services agreements with MPC as well as certain natural gas gathering, transportation and processing agreements, MPLX is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. MPLX’s primary natural gas implicit lease operations relate to a natural gas gathering agreement in the Marcellus Shale for which it earns a fixed-fee for providing gathering services to a single producer using a dedicated gathering system. As the gathering system is expanded, the fixed-fee charged to the producer is adjusted to include the additional gathering assets in the lease. The primary term of the natural gas gathering arrangement expires in 2038 and will continue thereafter on a year-to-year basis until terminated by either party. Other significant natural gas implicit leases relate to a natural gas processing agreement in the Marcellus Shale and a natural gas processing agreement in the Southern Appalachia region for which MPLX earns minimum monthly fees for providing processing services to a single producer using a dedicated processing plant. The primary term of these natural gas processing agreements expires during 2023 and 2033. The transportation and storage services agreements with MPC are described further in Note 6 . MPLX’s revenue from its implicit lease arrangements, excluding executory costs, totaled approximately $928 million in 2018 , $601 million in 2017 and $586 million in 2016 . MPLX’s implicit lease arrangements related to the processing facilities contain contingent rental provisions whereby MPLX receives additional fees if the producer customer exceeds the monthly minimum processed volumes. During the years ended December 31, 2018 and 2017 , MPLX received contingent lease payments of $9 million . During the year ended December 31, 2016 , MPLX received $7 million of contingent lease payments. The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2018 : (In millions) Related Party Third Party Total 2019 $ 748 $ 160 $ 908 2020 750 159 909 2021 627 150 777 2022 627 148 775 2023 616 142 758 2024 and thereafter 2,321 1,111 3,432 Total minimum future rentals $ 5,689 $ 1,870 $ 7,559 The following schedule summarizes MPLX’s investment in assets held for operating lease by major classes as of December 31, 2018 and 2017 : December 31, (In millions) 2018 2017 Natural gas gathering and NGL transportation pipelines and facilities $ 964 $ 851 Processing, fractionation and storage facilities 1,398 573 Pipelines and related assets 266 253 Barges and towing vessels 619 491 Terminals and related assets 1,178 822 Refinery related assets 938 — Land, building, office equipment and other 162 44 Construction-in-progress 189 85 Total 5,714 3,119 Less accumulated depreciation 2,038 1,056 Property, plant and equipment, net $ 3,676 $ 2,063

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	Asset Retirement Obligations MPLX’s assets subject to AROs are primarily certain gas-gathering pipelines and processing facilities, a crude oil pipeline and other related pipeline assets. MPLX also has land leases that require MPLX to return the land to its original condition upon termination of the lease. MPLX reviews current laws and regulations governing obligations for asset retirements and leases, as well as MPLX’s leases and other agreements. The following is a reconciliation of the changes in the ARO from January 1, 2017 to December 31, 2018 : (In millions) 2018 2017 AROs at beginning of period $ 28 $ 25 Liabilities incurred 1 2 Accretion expense 1 1 AROs at end of period $ 30 $ 28 At December 31, 2018 and 2017 , there were no assets legally restricted for purposes of settling AROs. The AROs have been recorded as part of “Deferred credits and other liabilities” on the accompanying Consolidated Balance Sheets. In addition to recorded AROs, MPLX has other AROs related to certain gathering, processing and other assets as a result of environmental and other legal requirements. MPLX is not required to perform such work until it permanently ceases operations of the respective assets. Because MPLX considers the operational life of these assets to be indeterminable, an associated ARO cannot be estimated and is not recorded.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies MPLX is the subject of, or a party to, a number of pending or threatened legal actions, contingencies and commitments involving a variety of matters, including laws and regulations relating to the environment. Some of these matters are discussed below. For matters for which MPLX has not recorded an accrued liability, MPLX is unable to estimate a range of possible losses for the reasons discussed in more detail below. However, the ultimate resolution of some of these contingencies could, individually or in the aggregate, be material. Environmental Matters – MPLX is subject to federal, state and local laws and regulations relating to the environment. These laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites. Penalties may be imposed for non-compliance. At December 31, 2018 and 2017 , accrued liabilities for remediation totaled $14 million and $13 million , respectively. However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, which may be imposed. At December 31, 2017 , there was less than $1 million in payables to MPC for indemnification of environmental costs related to incidents occurring prior to the asset drops. At December 31, 2018 , there was no balance with MPC for these costs. MarkWest Liberty Midstream and its affiliates agreed to pay a cash penalty of approximately $0.6 million and to undertake certain supplemental environmental projects with an estimated cost of approximately $2.4 million , related to civil enforcement allegations associated with permitting and other regulatory obligations for launcher/receiver and compressor station facilities in southeastern Ohio and western Pennsylvania. On April 24, 2018, MarkWest Liberty Midstream and its affiliates entered into a Consent Decree with the EPA and the Pennsylvania Department of Environmental Protection resolving these issues. The Consent Decree was approved by the court on July 9, 2018 and the penalty has been paid. MarkWest Liberty Midstream, Ohio Fractionation and MarkWest Utica EMG, together with other MarkWest affiliates, agreed to pay a penalty of approximately $0.9 million , undertake certain monitoring and emission reduction projects at certain facilities with an estimated cost of approximately $3.3 million , and implement certain process enhancements for its and its affiliates’ leak detection and repair programs at its gas processing and fractionation sites. On November 1, 2018, MPLX and 11 of its subsidiaries entered into a Consent Decree with the EPA, the State of Oklahoma, the Pennsylvania Department of Environmental Protection and the State of West Virginia resolving these issues. The Consent Decree was approved by the court on January 8, 2019 and the penalty has been paid. MPLX is involved in a number of other environmental enforcement matters arising in the ordinary course of business. While the outcome and impact on MPLX LP 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. Other Lawsuits – MPLX, MarkWest, MarkWest Liberty Midstream, MarkWest Liberty Bluestone, L.L.C., Ohio Fractionation and MarkWest Utica EMG (collectively, the “MPLX Parties”) are parties to various lawsuits with Bilfinger Westcon, Inc. (“Westcon”) that were instituted in 2016 and 2017 in the Court of Common Pleas in Butler County, Pennsylvania, the Circuit Court in Wetzel County, West Virginia, and the Court of Common Pleas in Harrison County, Ohio. The lawsuits relate to disputes regarding construction work performed by Westcon at the Bluestone, Mobley and Cadiz processing complexes in Pennsylvania, West Virginia and Ohio, respectively, and the Hopedale fractionation complex in Ohio. With respect to work performed by Westcon at the Mobley and Bluestone processing complexes, one or more of the MPLX Parties have asserted breach of contract, fraud, and with respect to work performed at the Mobley processing complex, MarkWest Liberty Midstream has also asserted negligent misrepresentation claims against Westcon. Westcon has also asserted claims against one or more of the MPLX Parties regarding these construction projects for breach of contract, unjust enrichment, promissory estoppel, fraud and constructive fraud, tortious interference with contractual relations, and civil conspiracy. Collectively, in the several cases, the MPLX Parties seek in excess of $10 million , plus an unspecified amount of punitive damages. Collectively, in the several cases, Westcon seeks in excess of $40 million , plus an unspecified amount of punitive damages. It is possible that, in connection with these lawsuits, the MPLX Parties will incur material amounts of damages. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, MPLX does not consider it reasonably possible that a loss resulting from such lawsuits or other proceedings in excess of any amounts accrued has been incurred that is expected to have a material adverse effect on its consolidated financial position, results of operations, or cash flows. In 2003, the State of Illinois brought an action against the Premcor Refining Group, Inc. (“Premcor”) and Apex Refining Company (“Apex”) asserting claims for environmental cleanup related to the refinery owned by these entities in the Hartford/Wood River, Illinois area. In 2006, Premcor and Apex filed third-party complaints against numerous owners and operators of petroleum products facilities in the Hartford/Wood River, Illinois area, including Marathon Pipe Line LLC (“MPL”). These complaints, which have been amended since filing, assert claims of common law nuisance and contribution under the Illinois Contribution Act and other laws for environmental cleanup costs that may be imposed on Premcor and Apex by the State of Illinois. On September 6, 2016, the trial court approved a settlement between Apex and the State of Illinois whereby Apex agreed to settle all claims against it for a $10 million payment. Premcor filed a motion for permissive appeal and requested a stay to the proceeding until the motion is ruled upon. Premcor reached a settlement with the State of Illinois in the second quarter of 2018, which has been objected to by certain third-party defendants, including MPL, and is subject to court approval. Several third-party defendants in the litigation including MPL have asserted cross-claims in contribution against the various third-party defendants. This litigation is currently pending in the Third Judicial Circuit Court, Madison County, Illinois. The trial concerning Premcor’s claims against third-party defendants, including MPL, previously scheduled to commence September 10, 2018, has been postponed and a new trial date has not been set. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, MPLX does not consider it reasonably possible that a loss resulting from such lawsuits or other proceedings in excess of any amounts accrued has been incurred that is expected to have a material adverse effect on its consolidated financial position, results of operations, or cash flows. Under the omnibus agreement, MPC will indemnify MPLX for the full cost of any losses should MPL be deemed responsible for any damages in this lawsuit. 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, MPLX believes the resolution of these other lawsuits and proceedings will not 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. Contractual Commitments and Contingencies – At December 31, 2018 , MPLX’s contractual commitments to acquire property, plant and equipment totaled $746 million . These commitments were primarily related to plant expansion projects for the Marcellus and Southwest Operations. In addition, from time to time and in the ordinary course of business, MPLX and its affiliates provide guarantees of MPLX’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require MPLX to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of December 31, 2018 , management does not believe there are any indications that MPLX will not be able to meet the construction milestones, that force majeure does not apply or that such fees and charges will otherwise be triggered. Lease and Other Contractual Obligations – MPLX executed transportation and terminalling agreements that obligate us to minimum volume, throughput or payment commitments over the terms of the agreements, which range from 9 to 11 years . After the minimum volume commitments are met in the transportation and terminalling agreements, MPLX pays additional amounts based on throughput. There are escalation clauses in the transportation and terminalling agreements, which are based on Consumer Price Index adjustments. The minimum future payments under these agreements as of December 31, 2018 are as follows: (In millions) 2019 $ 52 2020 52 2021 48 2022 46 2023 46 2024 and thereafter 180 Total $ 424 MPLX has various non-cancellable operating lease agreements, most of these leases include renewal options. MPLX also leases certain pipelines under a capital lease that has a fixed price purchase option in 2020. Future minimum commitments as of December 31, 2018 , for capital lease obligations and for operating lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Capital Obligations Operating Obligations 2019 $ 2 $ 73 2020 5 70 2021 — 67 2022 — 64 2023 — 58 2024 and thereafter — 719 Total minimum lease payments 7 $ 1,051 Less: imputed interest costs 1 Present value of net minimum lease payments $ 6 Operating lease rental expense was: (In millions) 2018 2017 2016 Minimum rental expense $ 85 $ 64 $ 57 SMR Transaction – On September 1, 2009, MarkWest entered into a product supply agreement creating a long-term contractual obligation for the payment of processing fees in exchange for the entire product processed by the SMR. See Note 18 for additional discussion. The product received under this agreement is sold to a refinery customer pursuant to a corresponding long-term agreement. The minimum amounts payable annually under the product supply agreement, excluding the potential impact of inflation adjustments per the agreement, are as follows: (In millions) 2019 $ 17 2020 17 2021 17 2022 17 2023 17 2024 and thereafter 110 Total minimum payments 195 Less: Services element 75 Less: Interest 34 Total SMR liability 86 Less: Current portion of SMR liability 5 Long-term portion of SMR liability $ 81

Select Quarterly Financial Data

Select Quarterly Financial Data	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Select Quarterly Financial Data	Select Quarterly Financial Data (Unaudited) 2018 2017 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 1,420 $ 1,578 $ 1,712 $ 1,715 $ 886 $ 916 $ 980 $ 1,085 Income from operations 557 608 672 666 265 280 311 335 Net income 423 456 516 439 187 191 217 241 Net income attributable to MPLX LP 421 453 510 434 150 190 216 238 Net income attributable to MPLX LP per limited partner unit: Common - basic 0.61 0.55 0.62 0.52 0.20 0.26 0.29 0.31 Common - diluted 0.61 0.55 0.62 0.52 0.19 0.26 0.29 0.31 Subordinated - basic and diluted — — — — — — — — Cash distributions declared per limited partner common unit 0.6175 0.6275 0.6375 0.6475 0.5400 0.5625 0.5875 0.6075 Distributions declared: Limited partner units - Public 179 181 185 187 149 162 170 175 Limited partner units - MPC 288 316 322 327 49 56 62 171 General partner units - MPC — — — — 5 6 7 — IDRs - MPC — — — — 60 70 81 — Redeemable preferred units 16 20 19 20 16 17 16 16 Total distributions declared $ 483 $ 517 $ 526 $ 534 $ 279 $ 311 $ 336 $ 362

Description of the Business a_2

Description of the Business and Basis of Presentation (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of presentation	Basis of Presentation – The consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly-owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in a VIE in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. Certain prior period financial statement amounts have been reclassified to conform to current period presentation. The accompanying consolidated financial statements of MPLX have been prepared in accordance with GAAP.

Summary of Principal Accounti_2

Summary of Principal Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of estimates	Use of Estimates – The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods. Actual results could differ materially from those estimates. Estimates are subject to uncertainties due to the levels of subjectivity and judgment necessary to account for highly uncertain matters or the susceptibility of such matters to change and affect items such as valuing identified intangible assets; determining the fair value of derivative instruments; evaluating impairments of long-lived assets, goodwill and equity investments; establishing estimated useful lives for long-lived assets; acquisition accounting; recognizing share-based compensation expense; estimating revenues, expense accruals and capital expenditures; valuing AROs; and determining liabilities, if any, for environmental and legal contingencies.
Revenue recognition	Revenue Recognition – As a result of the adoption of the new revenue recognition standard, as described further in Note 3 , MPLX has updated its policies as they relate to revenue recognition. Revenue is measured based on consideration specified in a contract with a customer. MPLX recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. MPLX enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” MPLX provides services under the following types of arrangements: • Fee-based arrangements – Under fee-based arrangements, MPLX receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue MPLX earns from these arrangements is generally directly related to the volume of natural gas, NGLs, refined products or crude oil that is handled by or flows through MPLX’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, MPLX’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed. In certain instances when specifically stated in the contract terms, MPLX purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis, as MPLX takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, MPLX: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, MPLX delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes MPLX retains to third parties. Revenue is recognized on a net basis when MPLX acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, MPLX gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, MPLX must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require MPLX to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” on the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as MPLX is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by MPLX. • Purchase arrangements – Under purchase arrangements, MPLX purchases natural gas at either the wellhead or the tailgate of a plant. MPLX then gathers and delivers the natural gas to pipelines where MPLX may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs.” Often, MPLX earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements is reported as “Product sales” on the Consolidated Statements of Income and is recognized on a gross basis as MPLX purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, MPLX provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, MPLX records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of MPLX’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions, and the services offered and whether they are deemed distinct. MPLX allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. Revenue from MPLX’s service arrangements will generally be recognized over time as the performance obligation is satisfied as services are provided. MPLX has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period. In instances in which tiered pricing structures do not reflect our efforts to perform, MPLX will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue. Based on the terms of certain natural gas gathering, transportation and processing agreements, MPLX is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. Revenue and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as “Rental income” and “Rental cost of sales,” respectively, on the Consolidated Statements of Income.
Revenue and expense accruals	Revenue and Expense Accruals – MPLX routinely makes accruals based on estimates for both revenues and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling MPLX’s records with those of third parties. The delayed information from third parties includes, among other things, actual volumes purchased, transported or sold, adjustments to inventory and invoices for purchases, actual natural gas and NGL deliveries and other operating expenses. MPLX makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and MPLX’s internal records have been reconciled.
Cash and cash equivalents	Cash and Cash Equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with initial maturities of three months or less.
Restricted cash	Restricted Cash – Restricted cash consists of cash and investments that must be maintained as collateral for letters of credit issued to certain third-party producer customers. The balances will be outstanding until certain capital projects are completed and the third party releases the restriction. Restricted cash also consists of cash advances to be used for the operation and maintenance of an operated pipeline. At December 31, 2018 and 2017 , the amount of restricted cash included in “ Other current assets ” on the Consolidated Balance Sheets was $8 million and $4 million , respectively.
Receivables	Receivables – Receivables primarily consist of customer accounts receivable, which are recorded at the invoiced amount and generally do not bear interest. Management reviews the allowance quarterly. Past-due balances over 90 days and other higher- risk amounts are reviewed individually for collectability. Balances that remain outstanding after reasonable collection efforts have been unsuccessful are written off through a charge to the valuation allowance and a credit to accounts receivable.
Inventories	Inventories – Inventories consist primarily of natural gas, propane, other NGLs and materials and supplies to be used in operations. Natural gas, propane, and other NGLs are valued at the lower of cost or market value. Materials and supplies are stated at the lower of cost or market value. Cost for materials and supplies are determined primarily using the weighted-average cost method.
Imbalances	Imbalances – Within our pipelines and storage assets, we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive imbalances are recorded as other current assets and negative imbalances are recorded as accounts payable. Positive and negative product imbalances are settled in cash, settled by physical delivery of gas from a different source, or tracked and settled in the future.
Property, plant and equipment	Property, Plant and Equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis over the estimated useful lives of the assets. Expenditures that extend the useful lives of assets are capitalized. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment assessment is performed and the excess of the book value over the fair value is recorded as an impairment loss. Interest costs for the construction or development of long-lived assets are capitalized and amortized over the related asset’s estimated useful life. When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported on the Consolidated Statements of Income. Gains on the disposal of property, plant and equipment are recognized when they occur, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale.
Intangibles	Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined and compared to the carrying value of the reporting unit. The fair value is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future Net operating margins, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the implied fair value of goodwill is calculated. The excess, if any, of the book value over the implied fair value of goodwill is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. There were no impairments as a result of MPLX’s November 30, 2018 and November 30, 2017 annual goodwill impairment analyses. During 2016, impairment charges of approximately $130 million were recorded.
Goodwill	Goodwill and Intangibles – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The impairment test requires allocating goodwill and other assets and liabilities to reporting units. The fair value of each reporting unit is determined and compared to the carrying value of the reporting unit. The fair value is calculated using the expected present value of future cash flows method. Significant assumptions used in the cash flow forecasts include future Net operating margins, future volumes, discount rates, and future capital requirements. If the fair value of the reporting unit is less than the carrying value, including goodwill, the implied fair value of goodwill is calculated. The excess, if any, of the book value over the implied fair value of goodwill is charged to net income as an impairment expense. Amortization of intangibles with definite lives is calculated using the straight-line method which is reflective of the benefit pattern in which the estimated economic benefit is expected to be received over the estimated useful life of the intangible asset. Intangibles subject to amortization are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the intangible may not be recoverable. If the sum of the expected undiscounted future cash flows related to the asset is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset. Intangibles not subject to amortization are tested for impairment annually and when circumstances indicate that the fair value is less than the carrying amount of the intangible. If the fair value is less than the carrying value, an impairment is recorded for the difference. There were no impairments as a result of MPLX’s November 30, 2018 and November 30, 2017 annual goodwill impairment analyses. During 2016, impairment charges of approximately $130 million were recorded.
Other taxes	Other Taxes – Other taxes primarily include real estate taxes.
Environmental costs	Environmental Costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. MPLX recognizes remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure.
Asset retirement obligations	Asset Retirement Obligations – An ARO is a legal obligation associated with the retirement of tangible long-lived assets that generally result from the acquisition, construction, development or normal operation of the asset. AROs are recorded at fair value in the period in which they are incurred, if a reasonable estimate of fair value can be made, and added to the carrying amount of the associated asset. This additional carrying amount is then depreciated over the life of the asset. The liability is determined using a credit adjusted risk free interest rate and increases due to the passage of time based on the time value of money until the obligation is settled. MPLX recognizes a liability of a conditional ARO as soon as the fair value of the liability can be reasonably estimated. A conditional ARO is defined as an unconditional legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within the control of the entity. AROs have not been recognized for certain assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates.
Investment in unconsolidated affiliates	Investment in Unconsolidated Affiliates – Equity investments in which MPLX exercises significant influence, but does not control and is not the primary beneficiary, are accounted for using the equity method and are reported in “ Equity method investments ” on the accompanying Consolidated Balance Sheets. This includes entities in which we hold majority ownership but the minority shareholders have substantive participating rights. Differences in the basis of the investments and the separate net asset values of the investees, if any, are amortized into net income over the remaining useful lives of the underlying assets and liabilities, except for the excess related to goodwill. MPLX believes the equity method is an appropriate means for it to recognize increases or decreases measured by GAAP in the economic resources underlying the investments. Regular evaluation of these investments is appropriate to evaluate any potential need for impairment. MPLX uses evidence of a loss in value to identify if an investment has an other than a temporary decline.
Deferred financing costs	Deferred Financing Costs – Deferred financing costs are an asset for credit facility costs and netted against debt for senior notes. These costs are amortized over the contractual term of the related obligations using the effective interest method or, in certain circumstances, accelerated if the obligation is refinanced.
Derivative instruments	Derivative Instruments – MPLX uses commodity derivatives to economically hedge a portion of its exposure to commodity price risk. All derivative instruments (including derivatives embedded in other contracts) are recorded at fair value. Certain commodity derivatives are reflected on the consolidated balance sheets on a net basis by counterparty as they are governed by master netting arrangements. MPLX discloses the fair value of all derivative instruments under the captions “ Other noncurrent assets ,” “ Other current liabilities ” and “ Deferred credits and other liabilities ” on the Consolidated Balance Sheets. Changes in the fair value of derivative instruments are reported on the Consolidated Statements of Income in accounts related to the item whose value or cash flows are being managed. All derivative instruments are marked to market through “ Product sales ,” “ Purchased product costs ,” or “Cost of revenues” on the Consolidated Statements of Income. Revenue gains and losses relate to contracts utilized to manage the cash flow for the sale of a product, typically NGLs. Purchased product costs gains and losses relate to contracts utilized to manage the cost of natural gas purchases, typically related to keep-whole arrangements. Cost of revenues gains and losses relate to a contract utilized to manage electricity costs. Changes in risk management for unrealized activities are reported as an adjustment to net income in computing cash flow from operating activities on the accompanying Consolidated Statements of Cash Flows. During the years ended December 31, 2018 , 2017 and 2016 , MPLX did not elect hedge accounting for any derivatives. MPLX has elected the normal purchases and normal sales designation for certain contracts related to the physical purchase of electric power.
Fair value of financial instruments	Fair Value of Financial Instruments – Management believes the carrying amount of financial instruments, including cash and cash equivalents, receivables, receivables from related parties, other current assets, accounts payable, accounts payable to related parties and accrued liabilities approximate fair value because of the short-term maturity of these instruments. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximate fair value due to the variable interest rate that approximates current market rates (see Note 16 ). Derivative instruments are recorded at fair value, based on available market information (see Note 17 ).
Fair value measurement	Fair Value Measurement – Financial assets and liabilities recorded at fair value in the Consolidated Balance Sheets are categorized based upon the fair value hierarchy established by GAAP, which classifies the inputs used to measure fair value into Level 1, Level 2 or Level 3. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The methods and assumptions utilized may produce a fair value that may not be realized in future periods upon settlement. Furthermore, while MPLX believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. For further discussion see Note 16 .
Equity-based compensation arrangements	Equity-Based Compensation Arrangements – MPLX issues phantom units under its share-based compensation plan as described further in Note 22 . A phantom unit entitles the grantee a right to receive a common unit upon the issuance of the phantom unit. The fair value of phantom unit awards granted to employees and non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. The fair value of the units awarded is amortized into earnings using a straight-line amortization schedule over the period of service corresponding with the vesting period. For phantom units that vest immediately and are not forfeitable, equity-based compensation expense is recognized at the time of grant. Performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards and use a Monte Carlo valuation model to calculate a grant date fair value. To satisfy common unit awards, MPLX may issue new common units, acquire common units in the open market or use common units already owned by the general partner.
Tax effects of share-based compensation	Tax Effects of Share-Based Compensation – MPLX elected to adopt the simplified method to establish the beginning balance of the additional paid-in capital pool (“APIC Pool”) related to the tax effects of employee share-based compensation and to determine the subsequent impact on the APIC Pool and Consolidated Statements of Cash Flows of the tax effects of share-based compensation awards that were outstanding upon adoption. Additional paid-in capital is reported as “Common unitholders - public ” on the accompanying Consolidated Balance Sheets.
Income taxes	Income Taxes – MPLX is not a taxable entity for federal income tax purposes. As a result of the MarkWest Merger, MarkWest was the surviving entity for tax purposes. MarkWest is not a taxable entity for federal income tax purposes. As such, MPLX does not directly pay federal income tax. Taxes on MPLX’s net income generally are borne by its partners through the allocation of taxable income. MPLX’s taxable income or loss, which may vary substantially from the net income or loss reported on the Consolidated Statements of Income, is includable in the federal income tax returns of each partner. MPLX and certain legal entities are, however, taxable entities under certain state jurisdictions. MPLX accounts for income taxes under the asset and liability method. Deferred income taxes are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis, capital loss carryforwards and net operating loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect of any tax rate change on deferred taxes is recognized as tax expense/(benefit) from continuing operations in the period that includes the enactment date of the tax rate change. Realizability of deferred tax assets is assessed and, if not more likely than not, a valuation allowance is recorded to reflect the deferred tax assets at net realizable value as determined by management. All deferred tax balances are classified as long-term in the accompanying Consolidated Balance Sheets. All changes in the tax bases of assets and liabilities are allocated among operations and items charged or credited directly to equity.
Distributions	Distributions – In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 9 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued as a liability until declared. However, when distributions related to the eliminated IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in below.
Net income per limited partner unit	Net Income Per Limited Partner Unit – MPLX uses the two-class method when calculating the net income per unit applicable to limited partners, because there is more than one class of participating security. The classes of participating securities include common units, general partner units, preferred units, certain equity-based compensation awards and eliminated IDRs. Class B units are considered to be a separate class of common units that do not participate in distributions. Net income attributable to MPLX LP is allocated to the unitholders differently for preparation of the Consolidated Statements of Equity and the calculation of net income per limited partner unit. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule and subsequently allocated to remaining unitholders in accordance with their respective ownership percentages. However, prior to 2018 when distributions related to the eliminated IDRs were made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders, except Class B unitholders, based on their respective ownership percentages. Subsequent to the conversion of the general partner to a non-economic interest as described in Note 8 , no earnings will be allocated to the general partner. 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 7 . In preparing net income per limited partner units, during periods in which a net loss attributable to MPLX is reported or periods in which the total distributions exceed the reported net income attributable to MPLX’s unitholders, the amount allocable to certain equity-based compensation awards is based on actual distributions to the equity-based compensation awards. Diluted earnings per unit is calculated by dividing net income attributable to MPLX’s common unitholders, after deducting amounts allocable to other participating securities, by the weighted average number of common units and potential common units outstanding during the period. Potential common units are excluded from the calculation of diluted earnings per unit during periods in which net income attributable to MPLX’s unitholders, after deducting amounts that are allocable to the outstanding equity-based compensation awards, preferred units, and eliminated IDRs, is a loss as the impact would be anti-dilutive.
Business combinations	Business Combinations – MPLX recognizes and measures the assets acquired and liabilities assumed in a business combination based on their estimated fair values at the acquisition date, with any remaining difference recorded as goodwill or gain from a bargain purchase. For all material acquisitions, management engages an independent valuation specialist to assist with the determination of fair value of the assets acquired, liabilities assumed, noncontrolling interests, if any, and goodwill, based on recognized business valuation methodologies. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent to the acquisition, and not later than one year from the acquisition date, MPLX will record any material adjustments to the initial estimate based on new information obtained that would have existed as of the acquisition date. An adjustment that arises from information obtained that did not exist as of the date of the acquisition will be recorded in the period of the adjustment. An income, market or cost valuation method may be utilized to estimate the fair value of the assets acquired, liabilities assumed, and noncontrolling interests, if any, in a business combination. The income valuation method represents the present value of future cash flows over the life of the asset using: (i) discrete financial forecasts, which rely on management’s estimates of volumes, NGL prices, revenue and operating expenses; (ii) long-term growth rates; and (iii) appropriate discount rates. The market valuation method uses prices paid for a reasonably similar asset by other purchasers in the market, with adjustments relating to any differences between the assets. The cost valuation method is based on the replacement cost of a comparable asset at prices at the time of the acquisition reduced for depreciation of the asset. Acquisition-related costs are expensed as incurred in connection with each business combination. See Note 4 for more information about the acquisitions.
Accounting for changes in ownership interests in subsidiaries	Accounting for Changes in Ownership Interests in Subsidiaries – MPLX’s ownership interest in a consolidated subsidiary may change if it sells a portion of its interest or acquires additional interest or if the subsidiary issues or repurchases its own shares. If the transaction does not result in a change in control over the subsidiary, the transaction is accounted for as an equity transaction. If a sale results in a loss of control, it would result in the deconsolidation of a subsidiary with a gain or loss recognized on the Consolidated Statements of Income unless the subsidiary meets the definition of in-substance real estate. Deconsolidation of in-substance real estate is recorded at cost with no gain or loss recognized. If the purchase of additional interest occurs which changes the acquirer’s ownership interest from noncontrolling to controlling, the acquirer’s preexisting interest in the acquiree is remeasured to its fair value, with a resulting gain or loss recorded in earnings upon consummation of the business combination. Once an entity has control of a subsidiary, its acquisitions of some or all of the noncontrolling interests in that subsidiary are accounted for as equity transactions and are not considered to be a business combination.

Accounting Standards New Accoun

Accounting Standards New Accounting Pronouncements Not yet Adopted (Tables)	12 Months Ended
	Dec. 31, 2018
ASUs not yet adopted [Abstract]
Description of New Accounting Pronouncements Not yet Adopted [Text Block]	Not Yet Adopted ASU 2017-12, Derivatives and Hedging - Targeted Improvements to Accounting for Hedging Activities In August 2017, the FASB issued an ASU to amend the hedge accounting rules to simplify the application of hedge accounting guidance and better portray the economic results of risk management activities in the financial statements. The guidance expands the ability to hedge nonfinancial and financial risk components, reduces complexity in fair value hedges of interest rate risk, eliminates the requirement to separately measure and report hedge ineffectiveness and eases certain hedge effectiveness assessment requirements. The guidance is effective beginning in 2019 with early adoption permitted. We do not expect the adoption of this ASU to have a material impact on our consolidated financial statements. ASU 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued an ASU which simplifies the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. Under the new guidance, the recognition of an impairment charge is calculated based on the amount by which the carrying amount exceeds the reporting unit’s fair value, which could be different from the amount calculated under the current method using the implied fair value of the goodwill; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The guidance should be applied on a prospective basis, and is effective for annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. ASU 2016-13, Credit Losses - Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued an ASU related to the accounting for credit losses on certain financial instruments. The guidance requires that for most financial assets, losses be based on an expected loss approach which includes estimates of losses over the life of exposure that considers historical, current and forecasted information. Expanded disclosures related to the methods used to estimate the losses as well as a specific disaggregation of balances for financial assets are also required. The change is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years, with early adoption permitted for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. MPLX does not expect application of this ASU to have a material impact on our consolidated financial statements. ASU 2016-02, Leases and related updates In February 2016, the FASB issued an ASU requiring lessees to record virtually all leases on their balance sheets. The ASU also requires expanded disclosures to help financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. For lessors, this amended guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The guidance will be effective for fiscal years beginning after December 15, 2018, and interim periods within those years, with early adoption permitted. As of January 1, 2019, we have transitioned to the new guidance. As part of implementing this standard, MPLX evaluated the impact to our financial statements, disclosures, internal controls and accounting policies. This evaluation process included reviewing all forms of leases, performing a completeness assessment over the lease population and analyzing the practical expedients in order to determine the best path of implementing changes to existing processes and controls. We have implemented a third-party supported lease accounting information system to account for our lease population in accordance with this new standard and established internal controls over the new system. We expect that adoption of the standard will result in the recognition of right-of-use assets and lease liabilities for operating leases in the range of $450 million to $550 million . The adoption of ASC 842 will not have a material impact on our consolidated statements of income or cash flows, except for the potential effects from lease modifications as discussed below. In addition, based on the changes presented in the standard, MPLX, as a lessor, may be required to re-classify existing operating leases to sales-type leases upon modification and related reassessment of the leases. If such a modification were to occur, it may result in the de-recognition of existing assets, recognition of a receivable in the amount of the present value of fixed payments expected to be received by MPLX under the lease, and recognition of a corresponding gain or loss in the period of change. MPLX will evaluate the impact of a lease reassessment as modifications occur.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2018
Business Acquisition [Line Items]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	Based on the fair value estimates of assets acquired and liabilities assumed at the acquisition date, the purchase price was allocated as follows: (In millions) Balance as of September 26, 2018 Receivables, net $ 3 Other current assets 1 Property, plant and equipment, net 336 Intangibles, net 9 Goodwill 126 Accounts payable (17 ) Other current liabilities (7 ) Net assets acquired $ 451
Mt. Airy Terminal [Member]
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information	Assuming the acquisition had occurred on January 1, 2017, the consolidated pro forma results would not have been materially different from the reported results.
Ozark Pipeline
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information	Assuming the acquisition of the Ozark pipeline had occurred on January 1, 2016, the consolidated pro forma results would not have been materially different from reported results.

Investments and Noncontrollin_2

Investments and Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2018
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments	The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at December 31, December 31, (In millions, except ownership percentages) 2018 2018 2017 Explorer 25% 90 89 Illinois Extension Pipeline 35% 275 284 LOCAP 59% 27 24 LOOP 41% 226 225 MarEn Bakken 25% 498 520 Centrahoma Processing LLC 40% 160 121 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 236 164 MarkWest Utica EMG, L.L.C. 56% 2,039 2,139 Sherwood Midstream LLC 50% 366 236 Sherwood Midstream Holdings LLC 60% 157 165 Other 100 43 Total $ 4,174 $ 4,010
Summarized Financial Information For Equity Method Investees Table	Summarized financial information for MPLX’s equity method investments for the years ended December 31, 2018 , 2017 and 2016 is as follows: December 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 238 $ 234 $ 1,364 $ 1,836 Costs and expenses 184 95 709 988 Income from operations 54 139 655 848 Net income 53 139 584 776 (Loss)/income from equity method investments (1) $ (10 ) $ 74 $ 176 $ 240 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 187 $ 86 $ 954 $ 1,227 Costs and expenses 97 42 520 659 Income from operations 90 44 434 568 Net income 90 43 345 478 Income from equity method investments (1) $ 10 $ 20 $ 48 $ 78 December 31, 2016 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs (2) Non-VIEs Total Revenues and other income $ 216 $ 18 $ 148 $ 382 Costs and expenses 100 111 117 328 Income/(loss) from operations 116 (93 ) 31 54 Net income/(loss) 114 (93 ) 31 52 Income/(loss) from equity method investments (1) $ 8 $ (89 ) $ 7 $ (74 ) (1) “Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion. (2) Includes an impairment charge of $89 million for the year ended December 31, 2016 related to MPLX’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table. Summarized balance sheet information for MPLX’s equity method investments as of December 31, 2018 and 2017 is as follows: December 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 82 $ 153 $ 379 $ 614 Noncurrent assets 1,939 1,596 4,715 8,250 Current liabilities 28 127 246 401 Noncurrent liabilities $ 3 $ 186 $ 841 $ 1,030 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 65 $ 46 $ 399 $ 510 Noncurrent assets 2,077 930 4,624 7,631 Current liabilities 39 44 220 303 Noncurrent liabilities $ 3 $ 11 $ 904 $ 918 (1) MarkWest Utica EMG, L.L.C (“MarkWest Utica EMG”), noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $750 million and $790 million as of December 31, 2018 and 2017 , respectively.

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Revenue from Related Parties	Revenue received from related parties related to service, rental, and product sales were as follows: (In millions) 2018 2017 2016 Service revenue MPC $ 2,159 $ 1,082 $ 936 Rental income MPC 718 279 235 Product sales (1) MPC $ 49 $ 8 $ 11 (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 2018 , 2017 , and 2016 , these sales totaled $440 million , $254 million and $46 million , respectively
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	MPLX has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets, a fixed annual fee for providing oversight and management services required to run the marine business and is also reimbursed for personnel services. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. The revenue received from these related parties, included in “Other income - related parties” on the Consolidated Statements of Income, was as follows: (In millions) 2018 2017 2016 MPC $ 41 $ 40 $ 45 MarkWest Utica EMG 17 17 16 Ohio Gathering 16 16 15 Jefferson Dry Gas 6 6 3 Sherwood Midstream 12 8 — Other 7 5 7 Total $ 99 $ 92 $ 86
Schedule of Entity Wide Information Allocated Related Party Omnibus Agreement Costs by Income Stmt Line	MPC provides executive management services and certain general and administrative services to MPLX under the terms of an omnibus agreement. Expenses incurred under this agreement are shown in the table below by the income statement line where they were recorded. Charges for services included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Charges for services included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. These charges were as follows: (In millions) 2018 2017 2016 Rental cost of sales - related parties $ 2 $ 1 $ 1 Purchases - related parties 164 67 39 General and administrative expenses 68 37 45 Total $ 234 $ 105 $ 85
Summary of Related Party Costs Added to Property, Plant and Equipment	Also under terms of the omnibus and employee services agreements, some service costs related to engineering services are associated with assets under construction. These costs added to “Property, plant and equipment, net” were as follows: (In millions) 2018 2017 2016 MPC $ 151 $ 42 $ 47
Summary of Allocated Related Party Employee Benefit Costs by Income Statement Line Item	(In millions) 2018 2017 2016 Rental cost of sales - related parties $ 3 $ 1 $ — Purchases - related parties 528 385 349 General and administrative expenses 109 101 100 Total $ 640 $ 487 $ 449
Schedule of Entity Wide Information Purchases from Related Parties	Purchases of products from MPC are classified as “Purchases - related parties.” These purchases include product purchases, payments made to MPC in its capacity as general contractor to MPLX, and certain rent and lease agreements. These purchases were as follows: (In millions) 2018 2017 2016 MPC $ 168 $ 3 $ —
Receivables from Related Parties	Receivables from related parties were as follows: December 31, (In millions) 2018 2017 MPC $ 281 $ 153 Other 8 7 Total $ 289 $ 160
Schedule of Long Term Receivables with Related Parties	Long-term receivables with related parties, which includes straight-line rental income, were as follows: December 31, (In millions) 2018 2017 MPC $ 24 $ 20
Payables to Related Parties	Payables to related parties were as follows: December 31, (In millions) 2018 2017 MPC (1) $ 131 $ 470 MarkWest Utica EMG 51 29 Ohio Gathering 5 8 Sherwood Midstream 16 8 Other — 1 Total $ 203 $ 516 (1) Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017 .
Schedule Of Related Party Deferred Revenue Table	The “Deferred revenue-related parties” balance associated with the minimum volume deficiencies and project reimbursements were as follows: December 31, (In millions) 2018 2017 Minimum volume deficiencies - MPC $ 44 $ 53 Project reimbursements - MPC 50 33 Total $ 94 $ 86

Net Income (Loss) Per Limited_2

Net Income (Loss) Per Limited Partner Unit (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Net Income Per Limited Partner Unit	Potential common units omitted from the diluted earnings per unit calculation for the years ended December 31, 2018 , 2017 and 2016 were less than 1 million . (In millions) 2018 2017 2016 Net income attributable to MPLX LP $ 1,818 $ 794 $ 233 Less: Limited partners’ distributions declared on preferred units (1) 75 65 41 General partner’s distributions declared (includes IDRs) (1)(2) — 328 205 Limited partners’ distributions declared on common units (including common units of general partner) (1) 1,985 895 692 Undistributed net loss attributable to MPLX LP $ (242 ) $ (494 ) $ (705 ) (1) See Note 8 for distribution information. (2) Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared.
Schedule of Basic and Diluted Earnings Per Unit	2018 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared $ 1,985 $ 75 $ 2,060 Undistributed net loss attributable to MPLX LP (242 ) — (242 ) Net income attributable to MPLX LP (1) $ 1,743 $ 75 $ 1,818 Weighted average units outstanding: Basic 761 761 Diluted 761 761 Net income attributable to MPLX LP per limited partner unit: Basic $ 2.29 Diluted $ 2.29 2017 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 328 $ 895 $ 65 $ 1,288 Undistributed net loss attributable to MPLX LP (10 ) (484 ) — (494 ) Net income attributable to MPLX LP (1) $ 318 $ 411 $ 65 $ 794 Weighted average units outstanding: Basic 8 385 393 Diluted 8 388 396 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.07 Diluted $ 1.06 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distribution declared $ 205 $ 692 $ 41 $ 938 Undistributed net loss attributable to MPLX LP (14 ) (691 ) — (705 ) Net income attributable to MPLX LP (1) $ 191 $ 1 $ 41 $ 233 Weighted average units outstanding: Basic 7 331 338 Diluted 7 338 345 Net income attributable to MPLX LP per limited partner unit: Basic $ — Diluted $ — (1) Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Stockholders Equity	The table below summarizes the changes in the number of units outstanding for the years ended December 31, 2016 , 2017 , and 2018 : (In units) Common Class B General Partner (1) Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards 120,989 — 2,470 123,459 Issuance of units under the ATM Program 26,347,887 — 537,710 26,885,597 Contribution of HSM (See Note 4) 22,534,002 — 459,878 22,993,880 Class B Conversion 4,350,057 (3,990,878 ) 7,330 366,509 Class A Reorganization 7,153,177 — (436,758 ) 6,716,419 Balance at December 31, 2016 357,193,288 3,990,878 7,371,105 368,555,271 Unit-based compensation awards 268,167 — 5,472 273,639 Issuance of units under the ATM Program 13,846,998 — 282,591 14,129,589 Contribution of HST/WHC/MPLXT (See Note 4) 12,960,376 — 264,497 13,224,873 Contribution of the Joint-Interest Acquisition (See Note 4) 18,511,134 — 377,778 18,888,912 Class B conversion 4,350,057 (3,990,878 ) 7,330 366,509 Balance at December 31, 2017 407,130,020 — 8,308,773 415,438,793 Unit-based compensation awards 348,387 — 140 348,527 Contribution of Refining Logistics and Fuels Distribution (See Note 4) 111,611,111 — 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 — (10,586,691 ) 264,413,309 Balance at December 31, 2018 794,089,518 — — 794,089,518 (1) Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.
Schedule Of Calculation Of Net Income Applicable to Partners	Net Income Allocation – In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders first and subsequently allocated to the limited partner unitholders in accordance with their respective ownership percentages. Prior to 2018, when distributions related to the IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings are allocated to the unitholders, based on their respective ownership percentages. The following table presents the allocation of the general partner’s GP Interest in net income attributable to MPLX LP, for income statement periods occurring prior to the exchange of the GP economic interests: (In millions) 2017 2016 Net income attributable to MPLX LP $ 794 $ 233 Less: Preferred unit distributions 65 41 General partner's IDRs and other 310 191 Net income attributable to MPLX LP available to general and limited partners 419 1 General partner's two percent GP Interest in net income attributable to MPLX LP 8 — General partner's IDRs and other 310 191 General partner's GP Interest in net income attributable to MPLX LP $ 318 $ 191
Distributions Made to Limited Partner, by Distribution	The allocation of total quarterly cash distributions to general, limited, and preferred unitholders is as follows for the years ended December 31, 2018 , 2017 and 2016 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. (In millions) 2018 2017 2016 General partner's distributions: General partner's distributions on general partner units $ — $ 25 $ 18 General partner's distributions on IDRs (1) — 303 187 Total distribution on general partner units and IDRs — 328 205 Limited partners' distributions: Common unitholders, includes common units of general partner 1,985 895 692 Preferred unit distributions 75 65 41 Total cash distributions declared $ 2,060 $ 1,288 $ 938 (1) Includes distributions of fourth quarter 2017 income declared on general partner common units issued February 1, 2018 in exchange for the economic general partner interest.

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	12 Months Ended
	Dec. 31, 2018
Redeemable Preferred Units Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance for 2018 and 2017 are summarized below: (In millions) 2018 2017 Balance at beginning of period $ 1,000 $ 1,000 Net income allocated 75 65 Distributions received by preferred unitholders (71 ) (65 ) Balance at end of period $ 1,004 $ 1,000

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The table below provides a reconciliation between “Net income” and Segment Adjusted EBITDA. (In millions) 2018 2017 2016 Reconciliation to Net income: L&S Segment Adjusted EBITDA $ 2,057 $ 775 $ 395 G&P Segment Adjusted EBITDA 1,418 1,229 1,024 Total reportable segments 3,475 2,004 1,419 Depreciation and amortization (1) (766 ) (683 ) (591 ) (Provision)/benefit for income taxes (8 ) (1 ) 12 Amortization of deferred financing costs (59 ) (53 ) (46 ) Loss on extinguishment of debt (46 ) — — Non-cash equity-based compensation (19 ) (15 ) (10 ) Impairment expense — — (130 ) Net interest and other financial costs (556 ) (301 ) (215 ) Income/(loss) from equity method investments (2) 240 78 (74 ) Distributions/adjustments related to equity method investments (447 ) (231 ) (150 ) Unrealized derivative gains/(losses) (3) 5 (6 ) (36 ) Acquisition costs (3 ) (11 ) 1 Adjusted EBITDA attributable to noncontrolling interests 18 8 3 Adjusted EBITDA attributable to Predecessor (4) — 47 251 Net income $ 1,834 $ 836 $ 434 (1) Depreciation and amortization attributable to L&S was $240 million , $163 million and $128 million for the years ended 2018 , 2017 and 2016 , respectively. Depreciation and amortization attributable to G&P was $526 million , $520 million and $463 million for 2018 , 2017 and 2016 , respectively. (2) Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016. (3) MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. (4) The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	December 31, (In millions) 2018 2017 Segment Assets Cash and cash equivalents $ 68 $ 5 L&S (1) 6,566 4,611 G&P (1) 16,145 14,884 Total assets $ 22,779 $ 19,500 (1) Equity method investments included in L&S assets were $1.12 billion at December 31, 2018 and $1.15 billion at December 31, 2017 . Equity method investments included in G&P assets were $3.05 billion at December 31, 2018 and $2.86 billion at December 31, 2017 .
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, capital expenditures and total assets for our reportable segments: (In millions) 2018 2017 2016 L&S Service revenue $ 2,289 $ 1,200 $ 1,006 Rental income 725 279 235 Product related revenue 14 — — Income from equity method investments 166 36 — Other income 46 47 53 Total segment revenues and other income (1) 3,240 1,562 1,294 Segment Adjusted EBITDA (2) 2,057 775 395 Maintenance capital expenditures 104 79 58 Growth capital expenditures 452 433 493 G&P Service revenue 1,574 1,038 888 Rental income 342 277 298 Product related revenue 1,135 897 583 Income/(loss) from equity method investments (3) 74 42 (74 ) Other income 60 51 40 Total segment revenues and other income (1) 3,185 2,305 1,735 Segment Adjusted EBITDA (2) 1,418 1,229 1,024 Maintenance capital expenditures 42 24 26 Growth capital expenditures $ 1,432 $ 948 $ 720 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $313 million , $160 million and $77 million for 2018 , 2017 and 2016 , respectively. Third party revenues for the G&P segment were $3,087 million , $2,246 million and $1,684 million for 2018 , 2017 and 2016 , respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (3) Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.

Income Tax (Tables)

Income Tax (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense (Benefit) [Table Text Block]	The components of the “Provision/(benefit) for income taxes” are as follows: December 31, (In millions) 2018 2017 2016 Current income tax expense: Federal $ — $ — $ 4 State — 2 1 Total current — 2 5 Deferred income tax expense/(benefit): Federal — — (16 ) State 8 (1 ) (1 ) Total deferred 8 (1 ) (17 ) Provision/(benefit) for income taxes $ 8 $ 1 $ (12 )
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	A reconciliation of the “Provision/(benefit) for income taxes” and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the year ended December 31, 2016 is as follows: December 31, 2016 (In millions) MarkWest Hydrocarbon (1) Partnership Eliminations Consolidated (Loss)/income before (benefit)/provision for income tax $ (41 ) $ 461 $ 2 $ 422 Federal statutory rate 35 % — % — % Federal income tax at statutory rate (14 ) — — (14 ) State income taxes net of federal benefit (2 ) 1 — (1 ) Provision on income from MPLX LP Class A units 3 — — 3 Change in state statutory rate (1 ) — — (1 ) Other 1 — — 1 (Benefit)/provision for income taxes $ (13 ) $ 1 $ — $ (12 ) (1) MarkWest Hydrocarbon paid tax on its share of MPLX’s income or loss as a result of its ownership of MPLX LP Class A units through September 1, 2016.

Inventories (Tables)

Inventories (Tables)	12 Months Ended
	Dec. 31, 2018
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: December 31, (In millions) 2018 2017 NGLs $ 9 $ 4 Line fill 9 8 Spare parts, materials and supplies 59 53 Total inventories $ 77 $ 65

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: Estimated Useful Lives December 31, (In millions) 2018 2017 Natural gas gathering and NGL transportation pipelines and facilities 5 - 30 years $ 5,926 $ 5,178 Processing, fractionation and storage facilities 10 - 40 years 5,336 3,893 Pipelines and related assets 15 - 51 years 2,560 2,253 Barges and towing vessels 20 years 620 490 Terminals and related assets 4 - 30 years 1,178 821 Refinery related assets 5 - 30 years 938 — Land, building, office equipment and other 3 - 35 years 957 770 Construction-in-progress 801 1,057 Total 18,316 14,462 Less accumulated depreciation 3,677 2,275 Property, plant and equipment, net $ 14,639 $ 12,187

Goodwill and Intangibles (Table

Goodwill and Intangibles (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The changes in carrying amount of goodwill were as follows for the periods presented: (In millions) L&S G&P Total Gross goodwill as of December 31, 2016 $ 162 $ 2,213 $ 2,375 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2016 162 2,083 2,245 Impairment losses — — — Acquisitions — — — Balance as of December 31, 2017 162 2,083 2,245 Impairment losses — — — Acquisitions 341 — 341 Balance as of December 31, 2018 $ 503 $ 2,083 $ 2,586 Gross goodwill as of December 31, 2018 $ 503 $ 2,213 $ 2,716 Accumulated impairment losses — (130 ) (130 ) Balance as of December 31, 2018 $ 503 $ 2,083 $ 2,586
Schedule of Acquired Finite-Lived Intangible Assets by Major Class	MPLX’s intangible assets are comprised of customer contracts and relationships, gross intangible assets with accumulated amortization as of December 31, 2018 and 2017 is shown below: December 31, 2018 December 31, 2017 (In millions) Useful Life Gross Accumulated Amortization (1) Net Gross Accumulated Amortization (1) Net L&S 4-6 years $ 9 $ — $ 9 $ — $ — $ — G&P 11-25 years 533 (118 ) 415 533 (80 ) 453 $ 542 $ (118 ) $ 424 $ 533 $ (80 ) $ 453
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	Estimated future amortization expense related to the intangible assets at December 31, 2018 is as follows: (In millions) 2019 $ 40 2020 40 2021 40 2022 39 2023 39 Thereafter 226 Total $ 424

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value on a recurring basis as of December 31, 2018 and 2017 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. December 31, 2018 December 31, 2017 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Commodity contracts $ — $ — $ — $ (2 ) Embedded derivatives in commodity contracts — (61 ) — (64 ) Total carrying value in Consolidated Balance Sheets $ — $ (61 ) $ — $ (66 )
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 agreement embedded in a keep-whole processing agreement. The fair value calculation for Level 3 instruments at December 31, 2018 used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.58 to $1.01 and (2) the probability of renewal of 90 percent for the first five -year term and 80 percent for the second five -year term of the gas purchase agreement and related keep-whole processing agreement. For commodity contracts, increases in forward NGL prices result in a decrease in the fair value of the derivative assets and an increase in the fair value of derivative liabilities. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability.
Schedule of changes in Level 3 fair value measurements [Table Text Block]	The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. 2018 2017 (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 $ (2 ) $ (64 ) $ (6 ) $ (54 ) Total gains/(losses) (realized and unrealized) included in earnings (1) 6 (9 ) (5 ) (19 ) Settlements (4 ) 12 9 9 Fair value at end of period — (61 ) (2 ) (64 ) The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period $ — $ (8 ) $ (2 ) $ (6 ) (1) Gains and losses on commodity derivatives classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.
Fair Value, by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of the long-term debt, excluding capital leases, and SMR liability. December 31, 2018 2017 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 13,169 $ 13,484 $ 7,718 $ 6,966 SMR liability $ 92 $ 86 $ 104 $ 91

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of December 31, 2018 , MPLX had no outstanding commodity contracts.
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) December 31, 2018 December 31, 2017 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets /Other current liabilities $ — $ (7 ) $ — $ (14 ) Other noncurrent assets /Deferred credits and other liabilities — (54 ) — (52 ) Total $ — $ (61 ) $ — $ (66 ) (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: December 31, (In millions) 2018 2017 2016 Product sales Realized gains/(losses) $ 4 $ (9 ) $ 2 Unrealized gains/(losses) 2 4 (15 ) Total derivative gains/(losses) related to product sales 6 (5 ) (13 ) Purchased product costs Realized losses (12 ) (9 ) (5 ) Unrealized gains/(losses) 3 (10 ) (22 ) Total derivative loss related to purchased product costs (9 ) (19 ) (27 ) Cost of revenues Realized losses — — (3 ) Unrealized gains — — 1 Total derivative losses related to cost of revenues — — (2 ) Total derivative losses $ (3 ) $ (24 ) $ (42 )

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Extinguishment of Debt [Line Items]
Schedule of Extinguishment of Debt [Table Text Block]	On December 10, 2018, MPLX redeemed all of the $750 million 5.5 percent senior notes due February 15, 2023, $40 million of which was issued by the MarkWest subsidiary. These notes were redeemed at 101.833 percent of the principal amount, which resulted in a payment of $14 million related to the note premium and the immediate recognition of $46 million of unamortized debt issuance costs.
Summary of Outstanding Borrowings	MPLX’s outstanding borrowings at December 31, 2018 and 2017 consisted of the following: December 31, (In millions) 2018 2017 MPLX LP: Bank revolving credit facility due 2022 $ — $ 505 5.500% senior notes due February 2023 — 710 3.375% senior notes due March 2023 500 — 4.500% senior notes due July 2023 989 989 4.875% senior notes due December 2024 1,149 1,149 4.000% senior notes due February 2025 500 500 4.875% senior notes due June 2025 1,189 1,189 4.125% senior notes due March 2027 1,250 1,250 4.000% senior notes due March 2028 1,250 — 4.800% senior notes due February 2029 750 — 4.500% senior notes due April 2038 1,750 — 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 — 5.500% senior notes due February 2049 1,500 — 4.900% senior notes due April 2058 500 — Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 63 Capital lease obligations due 2020 6 7 Total 13,856 7,362 Unamortized debt issuance costs (97 ) (27 ) Unamortized discount (366 ) (389 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 13,392 $ 6,945
Schedule of Debt Payments	The following table shows five years of scheduled debt payments. (In millions) 2019 $ 1 2020 5 2021 — 2022 — 2023 $ 1,500
Schedule of interest payable dates	Interest on each series of MPLX LP and MarkWest senior notes is payable semi-annually in arrears, according to the table below. Senior Notes Interest payable semi-annually in arrears 3.375% senior notes due 2023 March 15 th and September 15 th 4.500% senior notes due 2023 January 15 th and July 15 th 4.875% senior notes due 2024 June 1 st and December 1 st 4.000% senior notes due 2025 February 15 th and August 15 th 4.875% senior notes due 2025 June 1 st and December 1 st 4.125% senior notes due 2027 March 1 st and September 1 st 4.000% senior notes due 2028 March 15 th and September 15 th 4.800% senior notes due 2029 February 15 th and August 15 th 4.500% senior notes due 2038 April 15 th and October 15 th 5.200% senior notes due 2047 March 1 st and September 1 st 4.700% senior notes due 2048 April 15 th and October 15 th 5.500% senior notes due 2049 February 15 th and August 15 th 4.900% senior notes due 2058 April 15 th and October 15 th
Schedule of amounts related to the SMR included in the Balance Sheet	As of December 31, 2018 and 2017 , the following amounts related to the SMR are included in the accompanying Consolidated Balance Sheets: (In millions) December 31, 2018 December 31, 2017 Assets Property, plant and equipment, net $ 51 $ 56 Liabilities Accrued liabilities 5 5 Deferred credits and other liabilities $ 81 $ 86

Revenue Revenue (Tables)

Revenue Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue, Initial Application Period Cumulative Effect Transition [Table Text Block]	The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of ASC 606 was as follows: (In millions) Balance at December 31, 2017 ASC 606 Adjustment Balance at January 1, 2018 Assets Inventories $ 65 $ 1 $ 66 Property, plant and equipment, net 12,187 (3 ) 12,184 Liabilities Long-term deferred revenue 42 (3 ) 39 Equity Common unitholders - public $ 8,379 $ 1 $ 8,380 Aside from the adjustments to the opening balances noted above, the impact of adoption on the Consolidated Balance Sheets for the year ended December 31, 2018 was approximately a $2 million adjustment to “Inventories.” The disclosure of the impact of adoption on the Consolidated Statements of Income for the year ended December 31, 2018 was as follows: December 31, 2018 (In millions) ASC 606 Balance ASC 605 Balance Effect of Change Higher/ (Lower) Revenues and other income: Service revenue $ 1,704 $ 1,342 $ 362 Service revenue - related parties 2,159 2,166 (7 ) Service revenue - product related 198 — 198 Rental income 349 284 65 Product sales (1) 897 982 (85 ) Product sales - related parties 49 42 7 Costs and expenses: Cost of revenues (2) 948 579 369 Purchased product costs 845 740 105 Rental cost of sales 135 70 65 Depreciation and amortization 766 767 (1 ) Net income $ 1,834 $ 1,832 $ 2 (1) G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments. (2) Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”
Disaggregation of Revenue [Table Text Block]	The following table represents a disaggregation of revenue for each reportable segment for the year ended December 31, 2018 : December 31, 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 130 $ 1,574 $ 1,704 Service revenue - related parties 2,159 — 2,159 Service revenue - product related — 198 198 Product sales (1) 7 890 897 Product sales - related parties 7 42 49 Total revenues from contracts with customers $ 2,303 $ 2,704 5,007 Non-ASC 606 revenue (2) 1,418 Total revenues and other income $ 6,425 (1) G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
Contract with Customer, Asset and Liability [Table Text Block]	The table below reflects the changes in our contract balances for the year ended December 31, 2018 : (In millions) Balance at January 1, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at December 31, 2018 Contract assets $ 4 $ — $ — $ 4 Deferred revenue 5 8 (9 ) 4 Deferred revenue - related parties 42 40 (32 ) 50 Long-term deferred revenue 5 5 — 10 Long-term deferred revenue - related parties $ 43 $ (1 ) $ — $ 42 (1) Balance represents ASC 606 portion of each respective line item. (2) $1 million revenue was recognized related to past performance obligations in the current year.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	The table below includes estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period. As of December 31, 2018 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $105 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 25 years . Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2019 $ 1,146 2020 1,152 2021 1,166 2022 1,151 2023 and thereafter 5,524 Total revenue on remaining performance obligations (1)(2)(3) $ 10,139 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information	(In millions) December 31, 2018 December 31, 2017 Cash and cash equivalents $ 68 $ 5 Restricted cash (1) 8 4 Cash, cash equivalents and restricted cash (2) $ 76 $ 9 (1) The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets. (2) As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.” (In millions) 2018 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 484 $ 263 $ 213 Income taxes paid 1 3 4 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 2 6 (3 ) Contribution - fixed assets to joint venture (1) — 337 — Contribution - common units issued (2) $ 4,236 $ 1,133 $ 669 (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5 . (2) For 2016, includes limited partner units issued to MPC as consideration in the acquisition of HSM. For 2017, includes limited and general partner units issued to MPC as consideration in the acquisitions of the joint-interests, HST, WHC and MPLXT. For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. See Note 4 .
Summary of Changes in Capital Accruals	The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: (In millions) 2018 2017 2016 Increase/(decrease) in capital accruals $ 104 $ 71 $ (22 )

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 through December 31, 2018 : (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2016 $ — $ — $ — Joint-Interest Acquisition (13 ) (1 ) (14 ) Balance at December 31, 2017 (1) (13 ) (1 ) (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance as of December 31, 2018 (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.” (2) Components of other comprehensive loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Phantom Units
Equity-Based Compensation [Line Items]
Summary of Unit Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units in 2018 : Phantom Units Number of Units Weighted Average Fair Value Aggregate Intrinsic Value (In millions) Outstanding at December 31, 2017 1,351,523 $ 34.53 Granted 437,092 33.84 Settled (509,570 ) 34.38 Forfeited (124,710 ) 34.50 Outstanding at December 31, 2018 1,154,335 34.34 Vested and expected to vest at December 31, 2018 1,139,877 34.34 $ 35 Non-forfeitable at December 31, 2018 (1) 321,638 $ 34.59 $ 10
Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested and Expected to Vest	The following is a summary of the values related to phantom units: Phantom Units Intrinsic Value of Units Issued During the Period (in millions) Weighted Average Grant Date Fair Value of Units Granted During the Period 2018 $ 18 $ 33.84 2017 15 36.26 2016 $ 5 $ 29.42
Performance Shares
Equity-Based Compensation [Line Items]
Summary of Unit Award Activity	The following table presents a summary of the 2018 activity for performance unit awards to be settled in MPLX LP common units: Performance Units Number of Units Weighted Outstanding at December 31, 2017 2,536,594 $ 0.85 Granted — — Settled (538,594 ) 1.04 Forfeited (56,250 ) 0.90 Outstanding at December 31, 2018 1,941,750 0.80
Schedule of Share-based Compensation, Performance Unit Awards, Valuation Assumptions	The following table provides a summary of the weighted average inputs used for these assumptions: 2018 2017 2016 Risk-free interest rate N/A 1.52% 0.96% Look-back period N/A 2.83 years 2.83 years Expected volatility N/A 49.34% 47.59% Grant date fair value of performance units granted N/A $0.90 $0.63

Lease Operations (Tables)

Lease Operations (Tables)	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Schedule of Future Minimum Rental Payments Receivable for Operating Leases [Table Text Block]	The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2018 : (In millions) Related Party Third Party Total 2019 $ 748 $ 160 $ 908 2020 750 159 909 2021 627 150 777 2022 627 148 775 2023 616 142 758 2024 and thereafter 2,321 1,111 3,432 Total minimum future rentals $ 5,689 $ 1,870 $ 7,559
Schedule of Property Subject to or Available for Operating Lease [Table Text Block]	The following schedule summarizes MPLX’s investment in assets held for operating lease by major classes as of December 31, 2018 and 2017 : December 31, (In millions) 2018 2017 Natural gas gathering and NGL transportation pipelines and facilities $ 964 $ 851 Processing, fractionation and storage facilities 1,398 573 Pipelines and related assets 266 253 Barges and towing vessels 619 491 Terminals and related assets 1,178 822 Refinery related assets 938 — Land, building, office equipment and other 162 44 Construction-in-progress 189 85 Total 5,714 3,119 Less accumulated depreciation 2,038 1,056 Property, plant and equipment, net $ 3,676 $ 2,063

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]
Reconciliation of Changes in Asset Retirement Obligations [Table Text Block]	The following is a reconciliation of the changes in the ARO from January 1, 2017 to December 31, 2018 : (In millions) 2018 2017 AROs at beginning of period $ 28 $ 25 Liabilities incurred 1 2 Accretion expense 1 1 AROs at end of period $ 30 $ 28

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Contractual Obligation, Fiscal Year Maturity Schedule	MPLX executed transportation and terminalling agreements that obligate us to minimum volume, throughput or payment commitments over the terms of the agreements, which range from 9 to 11 years . After the minimum volume commitments are met in the transportation and terminalling agreements, MPLX pays additional amounts based on throughput. There are escalation clauses in the transportation and terminalling agreements, which are based on Consumer Price Index adjustments. The minimum future payments under these agreements as of December 31, 2018 are as follows: (In millions) 2019 $ 52 2020 52 2021 48 2022 46 2023 46 2024 and thereafter 180 Total $ 424
Schedule Of Future Minimum Lease Payments For Operating And Capital Leases Table	MPLX has various non-cancellable operating lease agreements, most of these leases include renewal options. MPLX also leases certain pipelines under a capital lease that has a fixed price purchase option in 2020. Future minimum commitments as of December 31, 2018 , for capital lease obligations and for operating lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Capital Obligations Operating Obligations 2019 $ 2 $ 73 2020 5 70 2021 — 67 2022 — 64 2023 — 58 2024 and thereafter — 719 Total minimum lease payments 7 $ 1,051 Less: imputed interest costs 1 Present value of net minimum lease payments $ 6
Schedule of Rent Expense	Operating lease rental expense was: (In millions) 2018 2017 2016 Minimum rental expense $ 85 $ 64 $ 57
Schedule of Minimum Amounts Payable Annually Under the Product Supply Agreement	On September 1, 2009, MarkWest entered into a product supply agreement creating a long-term contractual obligation for the payment of processing fees in exchange for the entire product processed by the SMR. See Note 18 for additional discussion. The product received under this agreement is sold to a refinery customer pursuant to a corresponding long-term agreement. The minimum amounts payable annually under the product supply agreement, excluding the potential impact of inflation adjustments per the agreement, are as follows: (In millions) 2019 $ 17 2020 17 2021 17 2022 17 2023 17 2024 and thereafter 110 Total minimum payments 195 Less: Services element 75 Less: Interest 34 Total SMR liability 86 Less: Current portion of SMR liability 5 Long-term portion of SMR liability $ 81

Select Quarterly Financial Da_2

Select Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Data	2018 2017 (In millions, except per unit data) 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. 1st Qtr. 2nd Qtr. 3rd Qtr. 4th Qtr. Total revenues and other income $ 1,420 $ 1,578 $ 1,712 $ 1,715 $ 886 $ 916 $ 980 $ 1,085 Income from operations 557 608 672 666 265 280 311 335 Net income 423 456 516 439 187 191 217 241 Net income attributable to MPLX LP 421 453 510 434 150 190 216 238 Net income attributable to MPLX LP per limited partner unit: Common - basic 0.61 0.55 0.62 0.52 0.20 0.26 0.29 0.31 Common - diluted 0.61 0.55 0.62 0.52 0.19 0.26 0.29 0.31 Subordinated - basic and diluted — — — — — — — — Cash distributions declared per limited partner common unit 0.6175 0.6275 0.6375 0.6475 0.5400 0.5625 0.5875 0.6075 Distributions declared: Limited partner units - Public 179 181 185 187 149 162 170 175 Limited partner units - MPC 288 316 322 327 49 56 62 171 General partner units - MPC — — — — 5 6 7 — IDRs - MPC — — — — 60 70 81 — Redeemable preferred units 16 20 19 20 16 17 16 16 Total distributions declared $ 483 $ 517 $ 526 $ 534 $ 279 $ 311 $ 336 $ 362

Description of the Business a_3

Description of the Business and Basis of Presentation (Detail)	Oct. 31, 2012	Mar. 27, 2012	Dec. 31, 2018
Description Of Business And Basis Of Presentation [Line Items]
Date of partnership formation		Mar. 27, 2012
Date for initial public offering completed	Oct. 31, 2012
Number of reportable segments			2

Summary of Principal Accounti_3

Summary of Principal Accounting Policies (Details) - USD ($)		3 Months Ended		12 Months Ended
		Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Restricted Cash	[1]			$ 8,000,000	$ 4,000,000
Restricted cash					4,000,000
Impairment losses				0	0
Impairment expense		$ 1,000,000	$ 129,000,000	$ 0	$ 0	$ 130,000,000
[1]	The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets.

Accounting Standards ASU 2016-0

Accounting Standards ASU 2016-02 Leases and related updates (Details) - Accounting Standards Update 2016-02 [Member] $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Minimum
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 450
Maximum
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 550

Acquisitions Acquisitions (Acqu

Acquisitions Acquisitions (Acquisition Mt. Airy Terminal) (Details) bbl / d in Thousands, bbl in Millions, $ in Millions		Sep. 26, 2018USD ($)bbl / dbbl	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)shares
Business Acquisition [Line Items]
Intangibles, net			$ 424	$ 453
Goodwill			2,586	2,245	$ 2,245
Income/(loss) from equity method investments	[1],[2]		$ 240	$ 78	$ (74)
Partners' Capital Account, Units, Acquisitions | shares					22,993,880
Mt. Airy Terminal [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables		$ 3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other		1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		336
Intangibles, net		9
Goodwill		126
Payments to Acquire Businesses, Gross		$ 451
Storage Capacity | bbl		4
Barrels Handled | bbl / d		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable		$ (17)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other		(7)
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest		$ 451
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.

Acquisitions Acquisitions (Refi

Acquisitions Acquisitions (Refining Logistics and Fuels Distribution) (Details) bbl in Millions, $ in Millions		Feb. 01, 2018USD ($)Tanksharesbbl	Sep. 01, 2017USD ($)shares	Mar. 01, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)shares
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares										22,993,880
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%		2.00%	2.00%			2.00%
Contribution from MPC								$ 1,046	$ 675	$ 225
Property, plant and equipment, net						$ 12,187	$ 14,639	14,639	12,187
Goodwill						2,245	2,586	$ 2,586	2,245	$ 2,245
Number of storage tanks | Tank		619
Number of rail and truck racks		32
Number of docks and gasoline blenders		18
Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual							1,359
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual							$ 874
Payments to Acquire Businesses, Gross		$ 4,100
Partners' Capital Account, Units, Acquisitions | shares								113,888,889
Equity interest issued or issuable, fair value assigned		4,300
Business Acquisition, Total Consideration, Fair Value Assigned		8,400
Property, plant and equipment, net		830
Refining Logistics [Member]
Business Acquisition [Line Items]
Goodwill		85
Fuels Distribution [Member]
Business Acquisition [Line Items]
Goodwill		$ 130
Limited Partners Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares										22,534,002
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares								111,611,111
Limited Partners Common Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		111,611,111
Limited Partners Common Units | MPLX Logistics LLC | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		18,176,666
Limited Partners Common Units | MPLX Holdings Inc | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		7,824,167
General Partner Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]									459,878
General Partner Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]							2,277,778
General Partner Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		2,277,778
General Partner Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		85,610,278
LOOP LOCAP SAX and Explorer
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares								264,413,309
Equity interest issued or issuable, fair value assigned			$ 653
Business Acquisition, Total Consideration, Fair Value Assigned			$ 1,070
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest			2.00%
LOOP LOCAP SAX and Explorer | Limited Partners Common Units | MPLX Logistics LLC
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			3,350,893
LOOP LOCAP SAX and Explorer | Limited Partners Common Units | MPLX Holdings Inc
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			1,441,224
LOOP LOCAP SAX and Explorer | General Partner Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			377,778
LOOP LOCAP SAX and Explorer | General Partner Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			13,719,017
Marathon Petroleum Corporation [Member]
Business Acquisition [Line Items]
Contribution from MPC				$ 6	$ 15
Marathon Petroleum Corporation [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Contribution from MPC		$ 23.7
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Contribution from MPC								$ 0	$ 0	$ 84
Marathon Petroleum Corporation [Member] | LOOP LOCAP SAX and Explorer
Business Acquisition [Line Items]
Contribution from MPC						$ 10
Crude Oil
Business Acquisition [Line Items]
Storage Capacity | bbl		56
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Acquisitions (Joint-Interest Ac

Acquisitions (Joint-Interest Acquisition) (Details) $ in Millions		Feb. 01, 2018	Sep. 01, 2017USD ($)inmishares	Mar. 01, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)shares
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares									22,993,880
Aggregate percentage of general partner interest		2.00%		2.00%	2.00%		2.00%
Income/(loss) from equity method investments	[1],[2]						$ 240	$ 78	$ (74)
Contribution from MPC							1,046	675	$ 225
Limited Partners Common Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares									22,534,002
General Partner Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares	[3]								459,878
MPC
Schedule of Equity Method Investments [Line Items]
Contribution from MPC				$ 6	$ 15
MPC | Limited Partners Common Units
Schedule of Equity Method Investments [Line Items]
Contribution from MPC							0	0	$ 84
MPC | General Partner
Schedule of Equity Method Investments [Line Items]
Contribution from MPC							$ 0	0	$ 141
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage			35.00%				35.00%
Pipeline length | mi			168
Pipeline diameter | in			24
Number of pump stations			2
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage			41.00%				41.00%
LOCAP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage			59.00%				59.00%
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage			25.00%				25.00%
Pipeline length | mi			1,830
LOOP LOCAP SAX and Explorer
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture			$ 420
Equity interest issued or issuable, value assigned			630
Total consideration, value assigned			1,050
Equity interest issued or issuable, fair value assigned			653
Business Acquisition, Total Consideration, Fair Value Assigned			$ 1,070
Units, acquisitions | shares							264,413,309
Aggregate percentage of general partner interest			2.00%
Income/(loss) from equity method investments							$ 118	$ 21
LOOP LOCAP SAX and Explorer | General Partner Common Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares			13,719,017
LOOP LOCAP SAX and Explorer | General Partner Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares			377,778
LOOP LOCAP SAX and Explorer | MPLX Holdings Inc | Limited Partners Common Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares			1,441,224
LOOP LOCAP SAX and Explorer | MPLX Logistics LLC | Limited Partners Common Units
Schedule of Equity Method Investments [Line Items]
Units, acquisitions | shares			3,350,893
LOOP LOCAP SAX and Explorer | MPC
Schedule of Equity Method Investments [Line Items]
Contribution from MPC						$ 10
LOOP LOCAP SAX and Explorer | MPC | General Partner
Schedule of Equity Method Investments [Line Items]
Payments of Distributions to Affiliates							$ 32
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Acquisitions (Acquisition of HS

Acquisitions (Acquisition of HST, WHC & MPLXT) (Details) bbl in Millions, $ in Millions		Feb. 01, 2018	Mar. 01, 2017USD ($)misharesbbl	Mar. 31, 2016USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)shares		Dec. 31, 2016USD ($)shares
Business Acquisition [Line Items]
Units, acquisitions								22,993,880
Aggregate percentage of general partner interest		2.00%	2.00%	2.00%	2.00%
Contribution from MPC | $					$ 1,046	$ 675		$ 225
Number of natural gas liquids storage caverns			8
Number of light product terminals			62
Wholly Owned Properties
Business Acquisition [Line Items]
Number of light product terminals			59
Property Subject to Operating Lease
Business Acquisition [Line Items]
Number of light product terminals			1
Partially Owned Properties
Business Acquisition [Line Items]
Number of light product terminals			2
Natural gas liquids storage caverns
Business Acquisition [Line Items]
Storage Capacity | bbl			1.8
Light-product terminal
Business Acquisition [Line Items]
Storage Capacity | bbl			23.6
Crude Oil
Business Acquisition [Line Items]
Miles of pipeline | mi			174
Refined products
Business Acquisition [Line Items]
Miles of pipeline | mi			430
MPC
Business Acquisition [Line Items]
Contribution from MPC | $			$ 6	$ 15
Limited Partners Common Units
Business Acquisition [Line Items]
Units, acquisitions								22,534,002
Limited Partners Common Units | MPC
Business Acquisition [Line Items]
Contribution from MPC | $					$ 0	$ 0		$ 84
General Partner Units
Business Acquisition [Line Items]
Units, acquisitions	[1]							459,878
HST & WHC & MPLXT
Business Acquisition [Line Items]
Equity interest issued or issuable, value assigned | $			504
Equity interest issued or issuable, fair value assigned | $			$ 503
Units, acquisitions						13,224,873
HST & WHC & MPLXT | General Partner Common Units
Business Acquisition [Line Items]
Units, acquisitions			9,197,900
HST & WHC & MPLXT | Limited Partners Common Units
Business Acquisition [Line Items]
Units, acquisitions						12,960,376
HST & WHC & MPLXT | Limited Partners Common Units | MPLX Logistics LLC
Business Acquisition [Line Items]
Units, acquisitions			2,630,427
HST & WHC & MPLXT | Limited Partners Common Units | MPLX Holdings Inc
Business Acquisition [Line Items]
Units, acquisitions			1,132,049
HST & WHC & MPLXT | General Partner Units
Business Acquisition [Line Items]
Units, acquisitions			264,497			264,497	[1]
HST & WHC & MPLXT | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross | $			$ 1,500
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Acquisitions (Acquisition of Oz

Acquisitions (Acquisition of Ozark Pipeline) (Details) bbl / d in Thousands, $ in Millions	Mar. 01, 2017USD ($)bbl / dinmi	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Business Acquisition [Line Items]
Payments to acquire property, plant, and equipment			$ 1,919	$ 1,411	$ 1,313
Ozark Pipeline
Business Acquisition [Line Items]
Payments to acquire property, plant, and equipment	$ 219
Pipeline length | mi	433
Pipeline diameter | in	22
Pipeline barrels handled | bbl / d	230
Revenues and other income		$ 64
Income from operations		$ 20

Acquisitions (Acquisition of Ma

Acquisitions (Acquisition of MarEn Bakken) (Details) bbl / d in Thousands, $ in Millions		Feb. 15, 2017USD ($)Quarterly_reporting_periodbbl / d	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments				$ 341	$ 761	$ 87
Income/(loss) from equity method investments	[1],[2]			240	$ 78	$ (74)
MPC
Schedule of Equity Method Investments [Line Items]
Incentive distribution rights forfeited per quarter		$ 1.6
Number of quarters IDRs forfeited | Quarterly_reporting_period		12
Prorated incentive distribution rights forfeited		$ 0.8
MarEn Bakken
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments		$ 2,000
MarEn Bakken
Schedule of Equity Method Investments [Line Items]
Barrels Handled | bbl / d		520
Payments to acquire equity method investments		$ 500
Percentage of ownership interest in joint venture acquired		25.00%
Income/(loss) from equity method investments			$ 15	$ 48
Bakken Pipeline System
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in joint venture acquired		37.00%
Equity method investment, ownership percentage		9.00%
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.

Acquisitions (Acquisition of Ha

Acquisitions (Acquisition of Hardin Street Marine LLC) (Details) $ in Millions	Feb. 01, 2018	Mar. 01, 2017USD ($)	Mar. 31, 2016USD ($)shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Business Acquisition [Line Items]
Aggregate percentage of general partner interest	2.00%	2.00%	2.00%	2.00%
Contribution from MPC				$ 1,046	$ 675	$ 225
Number of tow boats			18
Number of barges			219
MPC
Business Acquisition [Line Items]
Contribution from MPC		$ 6	$ 15
Percent of volumes shipped			60.00%
MPC | General Partner
Business Acquisition [Line Items]
Contribution from MPC				$ 0	$ 0	$ 141
Hardin Street Marine
Business Acquisition [Line Items]
Equity interest issued or issuable, value assigned			$ 600
Effective date of acquisition			Mar. 31, 2016
Hardin Street Marine | Limited Partner
Business Acquisition [Line Items]
Equity interest issued or issuable, number of shares | shares			22,534,002
Equity interest issued or issuable, fair value assigned			$ 669
Hardin Street Marine | General Partner
Business Acquisition [Line Items]
Equity interest issued or issuable, number of shares | shares			459,878
Equity interest issued or issuable, fair value assigned			$ 14

Investments and Noncontrollin_3

Investments and Noncontrolling Interest (Equity Method Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Sep. 01, 2017	Jan. 01, 2017
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 4,174	$ 4,010
Centrahoma Processing LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	40.00%
Equity method investments	$ 160	121
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	25.00%		25.00%
Equity method investments	$ 90	89
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	35.00%		35.00%
Equity method investments	$ 275	284
LOCAP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	59.00%		59.00%
Equity method investments	$ 27	24
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	41.00%		41.00%
Equity method investments	$ 226	225
MarEn Bakken
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	25.00%
Equity method investments	$ 498	520
Jefferson Dry Gas
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	67.00%
Equity method investments	$ 236	164
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	56.00%
Equity method investments	$ 2,039	2,139
Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%
Equity method investments	$ 366	236
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	80.00%
Equity method investments	$ 157	165
Sherwood Midstream Holdings | Direct Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	60.00%			79.00%
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 100	$ 43

Investments and Noncontrollin_4

Investments and Noncontrolling Interests (Summary of Equity Method Investment Financial Information) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Revenue and other income		$ 1,836	$ 1,227		$ 382
Costs and expenses		988	659		328
Income (loss) from operations		848	568		54
Net income (loss)		776	478		52
Income/(loss) from equity method investments	[1],[2]	240	78		(74)
Current assets		614	510
Noncurrent assets		8,250	7,631
Current liabilities		401	303
Noncurrent liabilities		1,030	918
Ohio Condensate
Schedule of Equity Method Investments [Line Items]
Impairment related to equity method investment from asset impairment and elimination of basis differential					89
Ohio Gathering
Schedule of Equity Method Investments [Line Items]
Investment in subsidiary		750	790
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Revenue and other income		238	187		216
Costs and expenses		184	97		100
Income (loss) from operations		54	90		116
Net income (loss)		53	90		114
Income/(loss) from equity method investments		(10)	10		8	[2]
Current assets		82	65
Noncurrent assets		1,939	2,077	[3]
Current liabilities		28	39
Noncurrent liabilities		3	3
Other VIEs
Schedule of Equity Method Investments [Line Items]
Revenue and other income		234	86		18
Costs and expenses		95	42		111
Income (loss) from operations		139	44		(93)
Net income (loss)		139	43		(93)
Income/(loss) from equity method investments		74	20		(89)	[2],[4]
Current assets		153	46
Noncurrent assets		1,596	930
Current liabilities		127	44
Noncurrent liabilities		186	11
Non-VIEs
Schedule of Equity Method Investments [Line Items]
Revenue and other income		1,364	954		148
Costs and expenses		709	520		117
Income (loss) from operations		655	434		31
Net income (loss)		584	345		31
Income/(loss) from equity method investments		176	48		$ 7	[2]
Current assets		379	399
Noncurrent assets		4,715	4,624
Current liabilities		246	220
Noncurrent liabilities		$ 841	$ 904
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	MarkWest Utica EMG, L.L.C (“MarkWest Utica EMG”), noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $750 million and $790 million as of December 31, 2018 and 2017, respectively.
[4]	Includes an impairment charge of $89 million for the year ended December 31, 2016 related to MPLX’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table.

Investments and Noncontrollin_5

Investments and Noncontrolling Interests (Narrative) (Details) bbl / d in Thousands, $ in Millions		Jan. 01, 2017USD ($)bbl / d	Jun. 30, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Feb. 28, 2013USD ($)
Schedule of Equity Method Investments [Line Items]
Total segment revenues and other income(1)				$ 99	$ 92	$ 86
Contribution - fixed assets to joint venture	[1]			0	337	0
Current assets				897	559
Current liabilities				1,186	1,304
Contribution - common units issued	[2]			4,236	1,133	669
Distributions from unconsolidated affiliates - return of capital				$ 16	26	0
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
NCI owners total funding commitment							$ 950
Percentage of capital required by reporting entity after NCI minimum contribution							100.00%
Aggregate contributions to VIE threshold							$ 2,000
Maximum percentage ownership interest in joint ventures				70.00%
Threshold percentage for noncontrolling owners to make pro rata contributions				30.00%
Maximum noncontrolling percentage of capital contributions until pro rata ownership threshhold is reached				10.00%
Noncontrolling interest owners actual contribution				$ 1,200
Contribution by the partnership				1,500
Noncontrolling interest owners capital contribution preference threshold				500
Noncontrolling owners accrual of preference amount				$ 0		16
Percentage of aggregate investment balance				56.00%
Total segment revenues and other income(1)				$ 17	17	16
Equity method investment, ownership percentage				56.00%
Ohio Gathering
Schedule of Equity Method Investments [Line Items]
Total segment revenues and other income(1)				$ 16	16	$ 15
Equity method investment, ownership percentage				34.00%
Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Total segment revenues and other income(1)				$ 12	8
Equity method investment, ownership percentage				50.00%
Contribution - fixed assets to joint venture		$ 134
Payments to acquire interest in joint venture		20
Sherwood Midstream LLC | Antero Midstream Partners L.P.
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture		$ 154
MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Capacity | bbl / d		20
Current assets				$ 132
Noncurrent assets				550
Current liabilities				75
Liabilities, Noncurrent				1
MarkWest Ohio Fractionation Company, L.L.C. [Member] | Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture		$ 126
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Proceeds From Sale Of Ownership Interest In Assets Sold By Company In Affiliate				$ 15
Equity method investment, ownership percentage				80.00%
Contribution - fixed assets to joint venture		203
Contribution - common units issued		209	$ 10
Basis difference				$ 14
Gain (loss) on disposition of assets		2
Distributions from unconsolidated affiliates - return of capital		$ 45
Ownership Interest In Assets Sold By Company In Affiliate				6.00%
Sherwood Midstream Holdings | Sherwood Midstream LLC
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		21.00%
Payments to acquire interest in joint venture		$ 44	$ 4
Sherwood Midstream Holdings | Direct Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		79.00%		60.00%
Sherwood Midstream Holdings | Indirect Ownership Interest
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage				20.20%
Gathering and Processing [Member]
Schedule of Equity Method Investments [Line Items]
Basis difference				$ 1,000	1,000
Basis difference, not amortized				459
Logistics and Storage [Member]
Schedule of Equity Method Investments [Line Items]
Basis difference				114	$ 118
Basis difference, not amortized				$ 39
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.
[2]	For 2016, includes limited partner units issued to MPC as consideration in the acquisition of HSM. For 2017, includes limited and general partner units issued to MPC as consideration in the acquisitions of the joint-interests, HST, WHC and MPLXT. For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. See Note 4.

Related Party Agreements and _3

Related Party Agreements and Transactions (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Apr. 26, 2018USD ($)
Loan Agreement [Abstract]
Revenue from related parties	$ 5	$ 11
Purchases from related parties	8	44
MPC
Loan Agreement [Abstract]
Prepaid insurance	1	8
Purchases from related parties	168	3	$ 0
MPC Investment | Related Party Revolving Credit Agreement
Loan Agreement [Abstract]
Current borrowing capacity	$ 1,000			$ 500
Expiration date	Dec. 4, 2020
Description of variable rate basis	LIBOR plus 1.50 percent
Proceeds from lines of credit	$ 4,000	2,400
Repayments of lines of credit	4,300	2,000
Outstanding balance	$ 0	$ 386
Interest rate during period	3.473%	2.777%
Commercial Agreements | MPC | Transportation Services Agreements [Member] | Minimum | Crude oil and product pipeline
Commercial Agreements
Term Of Agreements	5 years
Automatic renewal term agreement	2 years
Commercial Agreements | MPC | Transportation Services Agreements [Member] | Maximum | Crude oil and product pipeline
Commercial Agreements
Term Of Agreements	15 years
Automatic renewal term agreement	5 years
Commercial Agreements | MPC | Five Year Transportation Services Agreement [Member] | Handling crude oil and products
Commercial Agreements
Term Of Agreements	5 years
Commercial Agreements | MPC | Six Year Transportation Services Agreement [Member] | Providing marine transportation of crude oil, feedstocks and refined petroleum products, and related services
Commercial Agreements
Term Of Agreements	6 years
Commercial Agreements | MPC | Ten Year Storage Services Agreement [Member] | Storage services tank farms
Commercial Agreements
Term Of Agreements	10 years
Commercial Agreements | MPC | Seventeen Year Storage Services Agreement [Member] | Storage services tank farms
Commercial Agreements
Term Of Agreements	17 years
Commercial Agreements | MPC | Three Year Storage Services Agreement [Member] | Storage services tank farms
Commercial Agreements
Renewal term agreement	5 years
Term Of Agreements	3 years
Commercial Agreements | MPC | Three Year Storage Services Agreement [Member] | Crude oil and product pipeline
Commercial Agreements
Term Of Agreements	3 years
Commercial Agreements | MPC | Storage Services Agreements | Storage services tank farms
Commercial Agreements
Renewal term agreement	5 years
Automatic renewal term agreement	1 year
Commercial Agreements | MPC | Storage Services Agreements | Minimum | Storage services tank farms
Commercial Agreements
Automatic renewal term agreement	0 years
Commercial Agreements | MPC | Storage Services Agreements | Maximum | Storage services tank farms
Commercial Agreements
Automatic renewal term agreement	10 years
Commercial Agreements | MPC | Ten Year Terminal Services Agreement [Member] | Refined products
Commercial Agreements
Term Of Agreements	10 years
Commercial Agreements | MPC | Terminal Services Agreements | Minimum | Refined products
Commercial Agreements
Automatic renewal term agreement	2 years
Commercial Agreements | MPC | Terminal Services Agreements | Maximum | Refined products
Commercial Agreements
Automatic renewal term agreement	5 years
Commercial Agreements | MPC | Management Services Agreements [Member] | Co-location Agreements [Member]
Commercial Agreements
Term Of Agreements	50 years
Commercial Agreements | MPC | Management Services Agreements [Member] | Marine Management Services [Member]
Commercial Agreements
Automatic renewal term agreement	5 years
Number of renewals	2
Commercial Agreements | MPC | Lease Agreements [Member] | Management Services Agreements [Member]
Commercial Agreements
Term Of Agreements	50 years
Operating Agreements | MPC | Minimum
Commercial Agreements
Term Of Agreements	1 year
Operating Agreements | MPC | Maximum
Commercial Agreements
Term Of Agreements	5 years
MarkWest Utica EMG
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	56.00%
Ohio Gathering
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	34.00%
Sherwood Midstream LLC
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	50.00%
Sherwood Midstream Holdings
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	80.00%
Jefferson Dry Gas
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	67.00%

Related Party Agreements and _4

Related Party Agreements and Transactions (Revenue from Related Parties) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Rental income - related parties		$ 718	$ 279	$ 235
MPC
Related Party Transaction [Line Items]
Service revenue - related parties		2,159	1,082	936
Rental income - related parties		718	279	235
Product sales - related parties	[1]	49	8	11
Sales revenue, goods, related party, net zero		$ 440	$ 254	$ 46
[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 2018, 2017, and 2016, these sales totaled $440 million, $254 million and $46 million, respectively

Related Party Agreements and _5

Related Party Agreements and Transactions (Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	$ 99	$ 92	$ 86
MPC
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	41	40	45
MarkWest Utica EMG
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	17	17	16
Ohio Gathering
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	16	16	15
Jefferson Dry Gas
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	6	6	3
Sherwood Midstream
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	12	8	0
Other
Related Party Transaction [Line Items]
Total segment revenues and other income(1)	$ 7	$ 5	$ 7

Related Party Agreements and _6

Related Party Agreements and Transactions Related Party Agreements and Transactions (Charges related to Omnibus Agreement) (Details) - MPC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Charges for services received under the omnibus agreement	$ 234	$ 105	$ 85
Rental cost of sales - related parties [Member]
Related Party Transaction [Line Items]
Charges for services received under the omnibus agreement	2	1	1
Purchases - related parties
Related Party Transaction [Line Items]
Charges for services received under the omnibus agreement	164	67	39
General and Administrative Expense - Related Party [Member]
Related Party Transaction [Line Items]
Charges for services received under the omnibus agreement	$ 68	$ 37
General and administrative expenses
Related Party Transaction [Line Items]
Charges for services received under the omnibus agreement			$ 45

Related Party Agreements and _7

Related Party Agreements and Transactions (Summary of Related Party Costs Added to Property, Plant and Equipment) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
MPC | Construction-in-progress
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 151	$ 42	$ 47

Related Party Agreements and _8

Related Party Agreements and Transactions (Summary of Employee Services Expenses by Income Statement Line Item) (Detail) - MPC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 640	$ 487	$ 449
Rental cost of sales - related parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	3	1	0
Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	528	385	349
General and Administrative Expense - Related Party [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 109	$ 101
General and administrative expenses
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements			$ 100

Related Party Agreements and _9

Related Party Agreements and Transactions (Purchases - Related Party) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Purchases from related parties	$ 8	$ 44
MPC
Related Party Transaction [Line Items]
Purchases from related parties	$ 168	$ 3	$ 0

Related Party Agreements and_10

Related Party Agreements and Transactions (Receivables from Related Parties) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Receivables - related parties	$ 289	$ 160
MPC
Related Party Transaction [Line Items]
Receivables - related parties	281	153
Other
Related Party Transaction [Line Items]
Receivables - related parties	$ 8	$ 7

Related Party Agreements and_11

Related Party Agreements and Transactions Related Party Agreements and Transactions (Long Term Receivables from Related Parties) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 24	$ 20
MPC
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 24	$ 20

Related Party Agreements and_12

Related Party Agreements and Transactions (Payables to Related Parties) (Detail) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payables - related parties		$ 203	$ 516
MPC
Related Party Transaction [Line Items]
Payables - related parties	[1]	131	470
MarkWest Utica EMG
Related Party Transaction [Line Items]
Payables - related parties		51	29
Ohio Gathering
Related Party Transaction [Line Items]
Payables - related parties		5	8
Sherwood Midstream
Related Party Transaction [Line Items]
Payables - related parties		16	8
Other
Related Party Transaction [Line Items]
Payables - related parties		0	1
MPC Investment | Related Party Revolving Credit Agreement
Related Party Transaction [Line Items]
Outstanding balance		$ 0	$ 386
[1]	Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017. There was no outstanding balance on the MPC Loan Agreement as of December 31, 2018.

Related Party Agreements and_13

Related Party Agreements and Transactions (Deferred Revenue from Related Parties) (Details) - MPC - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 94	$ 86
Minimum Committed Volume Contracts [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	44	53
Reimbursable Projects
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 50	$ 33

Net Income (Loss) Per Limited_3

Net Income (Loss) Per Limited Partner Unit (Schedule of Distributions by Partner Class) (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Net Income Per Share [Line Items]
Potentially dilutive securities										1		1		1
Net income attributable to MPLX LP		$ 434	$ 510	$ 453	$ 421	$ 238	$ 216	$ 190	$ 150	$ 1,818	[1]	$ 794	[1]	$ 233	[1]
Total cash distributions declared										2,060		1,288		938
Undistributed net loss attributable to MPLX LP										(242)		(494)		(705)
Preferred Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP										75	[1]	65	[1]	41
Total cash distributions declared		$ 20	$ 19	$ 20	$ 16	$ 16	$ 16	$ 17	$ 16	75	[2]	65	[2]	41	[2]
General Partner
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]											318		191
Total cash distributions declared	[2]									0	[3]	328		205
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]									1,743		411		1
Total cash distributions declared	[2]									$ 1,985		$ 895		$ 692
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.
[2]	See Note 8 for distribution information.
[3]	Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared.

Net Income (Loss) Per Limited_4

Net Income (Loss) Per Limited Partner Unit (Basic and Diluted Earnings Per Unit) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Net income attributable to MPLX LP:
Total cash distributions declared										$ 2,060		$ 1,288		$ 938
Undistributed net income loss attributable to MPLX LP										(242)		(494)		(705)
Net income attributable to MPLX LP		$ 434	$ 510	$ 453	$ 421	$ 238	$ 216	$ 190	$ 150	$ 1,818	[1]	$ 794	[1]	$ 233	[1]
Weighted average units outstanding:
Basic (shares)										761		393		338
Diluted (shares)										761		396		345
General Partner
Net income attributable to MPLX LP:
Total cash distributions declared	[3]									$ 0	[2]	$ 328		$ 205
Undistributed net income loss attributable to MPLX LP														(14)
Net income attributable to MPLX LP	[1]											318		$ 191
Weighted average units outstanding:
Basic (shares)														7
Diluted (shares)														7
Limited Partners Common Units
Net income attributable to MPLX LP:
Total cash distributions declared	[3]									1,985		895		$ 692
Undistributed net income loss attributable to MPLX LP										(242)		(484)		(691)
Net income attributable to MPLX LP	[1]									$ 1,743		$ 411		$ 1
Weighted average units outstanding:
Basic (shares)										761		385		331
Diluted (shares)										761		388		338
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)		$ 0.52	$ 0.62	$ 0.55	$ 0.61	$ 0.31	$ 0.29	$ 0.26	$ 0.20	$ 2.29		$ 1.07		$ 0
Diluted (in USD per unit)		$ 0.52	$ 0.62	$ 0.55	$ 0.61	$ 0.31	$ 0.29	$ 0.26	$ 0.19	$ 2.29		$ 1.06		$ 0
Preferred Units
Net income attributable to MPLX LP:
Total cash distributions declared		$ 20	$ 19	$ 20	$ 16	$ 16	$ 16	$ 17	$ 16	$ 75	[3]	$ 65	[3]	$ 41	[3]
Undistributed net income loss attributable to MPLX LP										0		0		0
Net income attributable to MPLX LP										$ 75	[1]	65	[1]	$ 41
Marathon Petroleum Corporation [Member] | General Partner
Net income attributable to MPLX LP:
Total cash distributions declared												328
Undistributed net income loss attributable to MPLX LP												$ (10)
Weighted average units outstanding:
Basic (shares)												8
Diluted (shares)												8
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Net income attributable to MPLX LP:
Total cash distributions declared		$ 327	$ 322	$ 316	$ 288	$ 171	$ 62	$ 56	$ 49
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.
[2]	Distributions declared on January 25, 2018 on general partner common units issued on February 1, 2018 in exchange for the economic general partner interest, including IDRs, are shown as general partner distributions declared.
[3]	See Note 8 for distribution information.

Equity (Units Outstanding) (Det

Equity (Units Outstanding) (Details) - shares	Feb. 01, 2018	Mar. 01, 2017	Mar. 31, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Line Items]
Number of units outstanding				794,089,518		415,438,793		368,555,271	311,469,407
Aggregate percentage of general partner interest	2.00%	2.00%	2.00%	2.00%
Limited Partners Common Units
Stockholders Equity [Line Items]
Number of units outstanding				794,089,518		407,130,020		357,193,288	296,687,176
Limited Partners Common Units | MPC
Stockholders Equity [Line Items]
Common units outstanding				505,000,000		118,000,000
General Partner And Limited Partner Units | MPC
Stockholders Equity [Line Items]
Common units outstanding				504,701,934
General Partner Units
Stockholders Equity [Line Items]
Number of units outstanding				0	[1]	8,308,773	[1]	7,371,105	6,800,475	[1]
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Equity (Reorganization Transact

Equity (Reorganization Transactions) (Details) $ / shares in Units, $ in Millions		Feb. 01, 2018USD ($)shares	Jul. 01, 2017Agreement$ / sharesshares	Sep. 01, 2016USD ($)shares	Jul. 01, 2016Agreement$ / sharesshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted							366,509	366,509
Partners' Capital Account, Exchanges and Conversions | $						$ 0	$ 0	$ 0
Contributions from MPC | $						$ 0	$ 0	$ 225
Units contributed								6,716,419
Limited Partners Common Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted		275,000,000				275,000,000	4,350,057	4,350,057
Partners' Capital Account, Exchanges and Conversions | $		$ 10,400
Units contributed				21,401,137				7,153,177
Limited Partners Common Units | MPC
Class of Stock [Line Items]
Partners' Capital Account, Exchanges and Conversions | $						$ 7,926	$ 0	$ 0
Contributions from MPC | $				$ 84
Class B Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted							(3,990,878)	(3,990,878)
General Partner Units
Class of Stock [Line Items]
Partners' Capital Account, Units, Converted	[1]					(10,586,691)	7,330	7,330
Units contributed				436,758				(436,758)	[1]
General Partner | MPC
Class of Stock [Line Items]
Partners' Capital Account, Exchanges and Conversions | $						$ (7,926)	$ 0	$ 0
Contributions from MPC | $				$ 141
MarkWest
Class of Stock [Line Items]
Common Units Conversion Ratio			1.09		1.09
Cash consideration to unitholders | $ / shares			$ 6.20		$ 6.20
Maximum | M&R MWE Liberty LLC [Member]
Class of Stock [Line Items]
Number of Underwritten Offerings Allowable to be Demanded | Agreement			3		3
Number of Underwritten Offerings Allowable to be Demanded in 12 Month Period | Agreement			1		1
Maximum | M&R MWE Liberty LLC [Member] | Limited Partners Common Units
Class of Stock [Line Items]
Percentage of Limited Partners' Capital Account, Units Outstanding			5.00%		5.00%
Maximum | M&R MWE Liberty LLC [Member] | Class B Units
Class of Stock [Line Items]
Percentage of Units Sold in Public Offering			20.00%		20.00%
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Equity (ATM Program) (Details)

Equity (ATM Program) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders Equity [Line Items]
Common Stock, Monetary Value of Shares Authorized	$ 1,700
Units issued		14,129,589	26,885,597
ATM Program
Stockholders Equity [Line Items]
Net proceeds		$ 473	$ 776
Limited Partners Common Units
Stockholders Equity [Line Items]
Units issued	0	13,846,998	26,347,887

Equity (Changes in Partners Cap

Equity (Changes in Partners Capital, Unit Rollforward) (Details) - shares		Feb. 01, 2018	Sep. 01, 2017	Mar. 01, 2017	Sep. 01, 2016	Mar. 31, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Stockholders Equity [Line Items]
Beginning balance, number of units							415,438,793		368,555,271		311,469,407
Unit-based compensation awards, in units							348,527		273,639		123,459
Units issued									14,129,589		26,885,597
Unit conversion									366,509		366,509
Units, acquisitions											22,993,880
Units contributed											6,716,419
Ending balance, number of units							794,089,518		415,438,793		368,555,271
Aggregate percentage of general partner interest		2.00%		2.00%		2.00%	2.00%
LOOP LOCAP SAX and Explorer
Stockholders Equity [Line Items]
Units, acquisitions							264,413,309
Aggregate percentage of general partner interest			2.00%
Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions							113,888,889
HST & WHC & MPLXT
Stockholders Equity [Line Items]
Units, acquisitions									13,224,873
LOOP LOCAP SAX and Explorer
Stockholders Equity [Line Items]
Units, acquisitions									18,888,912
Limited Partners Common Units
Stockholders Equity [Line Items]
Beginning balance, number of units							407,130,020		357,193,288		296,687,176
Unit-based compensation awards, in units							348,387		268,167		120,989
Units issued							0		13,846,998		26,347,887
Unit conversion		275,000,000					275,000,000		4,350,057		4,350,057
Units, acquisitions											22,534,002
Units contributed					21,401,137						7,153,177
Ending balance, number of units							794,089,518		407,130,020		357,193,288
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions							111,611,111
Limited Partners Common Units | HST & WHC & MPLXT
Stockholders Equity [Line Items]
Units, acquisitions									12,960,376
Limited Partners Common Units | LOOP LOCAP SAX and Explorer
Stockholders Equity [Line Items]
Units, acquisitions									18,511,134
Class B Units
Stockholders Equity [Line Items]
Beginning balance, number of units							0		3,990,878		7,981,756
Unit conversion									(3,990,878)		(3,990,878)
Ending balance, number of units							0		0		3,990,878
General Partner Units
Stockholders Equity [Line Items]
Beginning balance, number of units							8,308,773	[1]	7,371,105		6,800,475	[1]
Unit-based compensation awards, in units	[1]						140		5,472		2,470
Units issued	[1]								282,591		537,710
Unit conversion	[1]						(10,586,691)		7,330		7,330
Units, acquisitions	[1]										459,878
Units contributed					436,758						(436,758)	[1]
Ending balance, number of units							0	[1]	8,308,773	[1]	7,371,105
General Partner Units | LOOP LOCAP SAX and Explorer
Stockholders Equity [Line Items]
Units, acquisitions			377,778
General Partner Units | Refining Logistics & Fuels Distribution [Member]
Stockholders Equity [Line Items]
Units, acquisitions	[1]						2,277,778
General Partner Units | HST & WHC & MPLXT
Stockholders Equity [Line Items]
Units, acquisitions				264,497					264,497	[1]
General Partner Units | LOOP LOCAP SAX and Explorer
Stockholders Equity [Line Items]
Units, acquisitions	[1]								377,778
[1]	Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.

Equity (Net Income Allocation)

Equity (Net Income Allocation) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Equity [Abstract]
Net income attributable to MPLX LP	$ 434	$ 510	$ 453	$ 421	$ 238	$ 216	$ 190	$ 150	$ 1,818	[1]	$ 794	[1]	$ 233	[1]
Distributions to preferred unitholders									(75)		(65)		(41)
General partner's IDRs and other											310		191
Net income attributable to MPLX LP available to general and limited partners											419		1
General partner's two percent GP Interest in net income attributable to MPLX LP											8		0
General partner's GP Interest in net income attributable to MPLX LP									$ 0		$ 318		$ 191
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.

Equity (Cash Distributions) (De

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

Jan. 25, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

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

Distributions to preferred units

$ 2,060

$ 1,288

$ 938

Subsequent Event

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

Distribution made to limited partner, declaration date

Jan. 25, 2019

Distributions to preferred units

$ 514

Cash distributions declared per limited partner common unit

$ 0.6475

Distribution made to limited partner, distribution date

Feb. 14, 2019

Distribution made to limited partner, date of record

Feb. 5, 2019

Limited Partners Common Units

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

Distributions to preferred units

[1]

$ 1,985

$ 895

$ 692

Cash distributions declared per limited partner common unit

$ 0.6475

$ 0.6375

$ 0.6275

$ 0.6175

$ 0.6075

$ 0.5875

$ 0.5625

$ 0.5400

$ 2.5300

$ 2.2975

[1]

See Note 8 for distribution information.

Equity (Cash Distributions Appl

Equity (Cash Distributions Applicable to Period the Distributions Were Earned) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
General partner's distributions on general partner units	$ 0	$ 25	$ 18
General partner's distributions on IDRs	0	303	187
Total distribution on general partner units and IDRs	0	328	205
Total cash distributions declared	2,060	1,288	938
Limited Partners Common Units
Class of Stock [Line Items]
Limited partners' distributions	1,985	895	692
Preferred Units
Class of Stock [Line Items]
Limited partners' distributions	$ 75	$ 65	$ 41

Redeemable Preferred Units (Nar

Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	May 13, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units			14,129,589	26,885,597
Issuance of redeemable preferred units		$ 0	$ 0	$ 984
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units	30,800,000
Preferred units, dividend rate, percentage	6.50%
Units issued, price per unit	$ 32.50
Issuance of redeemable preferred units	$ 984
Preferred units, dividend rate, per-dollar-amount	$ 0.528125
Preferred units, description	The holders may convert their preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the 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 LP common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the 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 preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the 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 preferred units to common units at the then applicable change of control conversion rate.

Redeemable Preferred Units (Rol

Redeemable Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	May 13, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Redeemable preferred units, beginning balance		$ 1,000
Issuance of redeemable preferred units		0	$ 0	$ 984
Distributions to unitholders and general partner		(1,819)	(1,120)	(845)
Redeemable preferred units, ending balance		1,004	1,000
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Redeemable preferred units, beginning balance		1,000	1,000
Issuance of redeemable preferred units	$ 984
Net income allocated		75	65
Distributions to unitholders and general partner		(71)	(65)
Redeemable preferred units, ending balance		$ 1,004	$ 1,000	$ 1,000

Segment Information (Details)

Segment Information (Details)	12 Months Ended
	Dec. 31, 2018
Segment Reporting Information [Line Items]
Number of reportable segments	2

Segment Information - Revenues

Segment Information - Revenues and Adjusted EBITDA (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income		$ 1,715	$ 1,712	$ 1,578	$ 1,420	$ 1,085	$ 980	$ 916	$ 886	$ 6,425	$ 3,867	$ 3,029
Revenue from Contract with Customer, Excluding Assessed Tax										5,007
Rental income										349	277	298
Income (Loss) from Equity Method Investments	[1],[2]									240	78	(74)
Other income										7	6	7
Revenues	[3]									5	(6)	(36)
Ohio Condensate
Segment Reporting, Revenue Reconciling Item [Line Items]
Impairment related to equity method investment from asset impairment and elimination of basis differential												89
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA										3,475	2,004	1,419
Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,303
Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income	[4]									3,240	1,562	1,294
Rental income										725	279	235
Income (Loss) from Equity Method Investments										166	36	0
Other income										46	47	53
Adjusted EBITDA	[5]									2,057	775	395
Growth Capital Expenditures										452	433	493
Maintenance Capital Expenditures										104	79	58
Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,704
Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income	[4]									3,185	2,305	1,735
Rental income										342	277	298
Income (Loss) from Equity Method Investments	[6]									74	42	(74)
Other income										60	51	40
Adjusted EBITDA	[5]									1,418	1,229	1,024
Growth Capital Expenditures										1,432	948	720
Maintenance Capital Expenditures										42	24	26
Service [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,704	1,156	958
Service [Member] | Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										130
Service [Member] | Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,289	1,200	1,006
Service [Member] | Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,574
Service [Member] | Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,574	1,038	888
Product [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										902	889	572
Product [Member] | Logistics and Storage [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										14	0	0
Product [Member] | Gathering and Processing [Member] | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,135	897	583
Third Party | Logistics and Storage [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income										313	160	77
Third Party | Gathering and Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total segment revenues and other income										$ 3,087	$ 2,246	$ 1,684
[1]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[2]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.
[4]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $313 million, $160 million and $77 million for 2018, 2017 and 2016, respectively. Third party revenues for the G&P segment were $3,087 million, $2,246 million and $1,684 million for 2018, 2017 and 2016, respectively.
[5]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
[6]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Cash and cash equivalents		$ 68	$ 5
Total assets		22,779	19,500
Equity method investments		4,174	4,010
Operating Segments | Logistics and Storage [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	[1]	6,566	4,611
Equity method investments		1,120	1,150
Operating Segments | Gathering and Processing [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	[1]	16,145	14,884
Equity method investments		$ 3,050	$ 2,860
[1]	Equity method investments included in L&S assets were $1.12 billion at December 31, 2018 and $1.15 billion at December 31, 2017. Equity method investments included in G&P assets were $3.05 billion at December 31, 2018 and $2.86 billion at December 31, 2017.

Segment Information - Reconcili

Segment Information - Reconciliation Adjusted EBITDA to Net income (Details) - USD ($) $ in Millions		3 Months Ended										12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]											$ 766	$ 683	$ 591
Provision/(benefit) for income taxes												8	1	(12)
Amortization of deferred financing costs												59	53	46
Extinguishment of Debt, Gain (Loss), Net of Tax												(46)	0	0
Total compensation expense												(19)	(15)	(10)
Goodwill and Intangible Asset Impairment										$ (1)	$ (129)	0	0	(130)
Interest and Other Financial Costs												556	301	215
Income (Loss) from Equity Method Investments	[2],[3]											240	78	(74)
Distributions from unconsolidated affiliates												400	241	148
Unrealized gains/(losses)	[4]											5	(6)	(36)
Acquisition Costs, Period Cost												3	11	1
Net income		$ 439	$ 516	$ 456	$ 423	$ 241	$ 217	$ 191	$ 187			1,834	836	434
Logistics and Storage [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization												(240)	(163)	(128)
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization												(526)	(520)	(463)
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA												3,475	2,004	1,419
Amortization of deferred financing costs												59	53	46
Operating Segments | Logistics and Storage [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	[5]											2,057	775	395
Income (Loss) from Equity Method Investments												166	36	0
Operating Segments | Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	[5]											1,418	1,229	1,024
Income (Loss) from Equity Method Investments	[6]											74	42	(74)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Distributions from unconsolidated affiliates												447	231	150
Adjusted EBITDA attributable to noncontrolling interests												18	8	3
Adjusted EBITDA attributable to Predecessor	[7]											$ 0	$ 47	$ 251
[1]	Depreciation and amortization attributable to L&S was $240 million, $163 million and $128 million for the years ended 2018, 2017 and 2016, respectively. Depreciation and amortization attributable to G&P was $526 million, $520 million and $463 million for 2018, 2017 and 2016, respectively.
[2]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[3]	“Income/(loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[4]	MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.
[5]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”
[6]	Includes an impairment expense of $89 million related to one of MPLX’s equity method investments for the year ended December 31, 2016.
[7]	The Adjusted EBITDA adjustments related to Predecessor are excluded from Adjusted EBITDA attributable to MPLX LP prior to the acquisition date.

Major Customers and Concentra_2

Major Customers and Concentration of Credit Risk (Details) - First Major Customer - MPC	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues And Other Income]
Concentration Risk [Line Items]
Concentration risk, percentage	46.00%	36.00%	41.00%
Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	48.00%	37.00%	39.00%

Income Tax (Narrative) (Detail)

Income Tax (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 01, 2016
Income Taxes [Line Items]
Provision/(benefit) for income taxes	$ 8	$ 1	$ (12)
Income tax rate	1.00%	1.00%	5.00%
Deferred tax liabilities, net	$ 13	$ 5
Net penalties and interest expense (benefits)	(1)		$ 1
Penalties and interest accrued	$ 0	$ 0	$ 0
Federal | Minimum
Income Taxes [Line Items]
Open tax year	2,015
Federal | Maximum
Income Taxes [Line Items]
Open tax year	2,016
State | Minimum
Income Taxes [Line Items]
Open tax year	2,013
State | Maximum
Income Taxes [Line Items]
Open tax year	2,017
MPC
Income Taxes [Line Items]
Deferred tax liabilities assumed				$ 377

Income Tax (Components of Tax E

Income Tax (Components of Tax Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax expense:
Federal	$ 0	$ 0	$ 4
State	0	2	1
Total current	0	2	5
Deferred income tax expense (benefit):
Federal	0	0	(16)
State	8	(1)	(1)
Total deferred	8	(1)	(17)
Provision/(benefit) for income taxes	$ 8	$ 1	$ (12)

Income Tax (Reconciliation of F

Income Tax (Reconciliation of Federal Statutory Income Tax Rate) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity Information [Line Items]
(Loss) income before (benefit) provision for income tax		$ 1,842	$ 837	$ 422
Federal statutory rate	[1]			35.00%
Federal income tax at statutory rate				$ (14)
State income taxes net of federal benefit				(1)
Provision on income from MPLX LP Class A units				3
Change in state statutory rate				(1)
Other				1
Provision/(benefit) for income taxes		$ 8	$ 1	(12)
Reportable Legal Entities [Member] | MarkWest Hydrocarbon [Member]
Entity Information [Line Items]
(Loss) income before (benefit) provision for income tax	[1]			(41)
Federal income tax at statutory rate	[1]			(14)
State income taxes net of federal benefit	[1]			(2)
Provision on income from MPLX LP Class A units	[1]			3
Change in state statutory rate	[1]			(1)
Other	[1]			1
Provision/(benefit) for income taxes	[1]			(13)
Reportable Legal Entities [Member] | Partnership [Member]
Entity Information [Line Items]
(Loss) income before (benefit) provision for income tax				461
Federal income tax at statutory rate				0
State income taxes net of federal benefit				1
Provision on income from MPLX LP Class A units				0
Change in state statutory rate				0
Other				0
Provision/(benefit) for income taxes				1
Consolidation, Eliminations [Member]
Entity Information [Line Items]
(Loss) income before (benefit) provision for income tax				2
Federal income tax at statutory rate				0
State income taxes net of federal benefit				0
Provision on income from MPLX LP Class A units				0
Change in state statutory rate				0
Other				0
Provision/(benefit) for income taxes				$ 0
[1]	MarkWest Hydrocarbon paid tax on its share of MPLX’s income or loss as a result of its ownership of MPLX LP Class A units through September 1, 2016.

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
NGLs	$ 9	$ 4
Line fill	9	8
Spare parts, materials and supplies	59	53
Total inventories	$ 77	$ 65

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 18,316	$ 14,462
Less accumulated depreciation	3,677	2,275
Property, plant and equipment, net	14,639	12,187
Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,926	5,178
Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,336	3,893
Pipelines and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,560	2,253
Barges and towing vessels
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years
Property, plant and equipment, gross	$ 620	490
Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,178	821
Refineries and related assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	938	0
Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	957	770
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 801	$ 1,057
Minimum | Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Minimum | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years
Minimum | Pipelines and related assets
Property, Plant and Equipment [Line Items]
Estimated useful lives	15 years
Minimum | Terminals and related assets
Property, Plant and Equipment [Line Items]
Estimated useful lives	4 years
Minimum | Refineries and related assets [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Minimum | Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Maximum | Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Estimated useful lives	30 years
Maximum | Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Maximum | Pipelines and related assets
Property, Plant and Equipment [Line Items]
Estimated useful lives	51 years
Maximum | Terminals and related assets
Property, Plant and Equipment [Line Items]
Estimated useful lives	30 years
Maximum | Refineries and related assets [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	30 years
Maximum | Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Estimated useful lives	35 years

Property Plant and Equipment (N

Property Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 3,677	$ 2,275
Assets Held under Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Gross assets	25	25
Accumulated depreciation	$ 9	$ 9

Goodwill and Intangibles (Goodw

Goodwill and Intangibles (Goodwill) (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Goodwill [Line Items]
Total Number of Reporting Units			12
Impairment expense	$ 1	$ 129	$ 0	$ 0	$ 130
Number of reporting units			8		3
Goodwill			$ 2,586	$ 2,245	$ 2,245
Number of reporting units impaired					2
Goodwill | Income Approach Valuation Technique | Minimum
Goodwill [Line Items]
Attrition rate					5.00%
Goodwill | Income Approach Valuation Technique | Maximum
Goodwill [Line Items]
Attrition rate					10.00%
Measurement Input, Discount Rate [Member] | Equity Method Investments | Income Approach Valuation Technique | Minimum
Goodwill [Line Items]
Fair Value Inputs					10.50%
Measurement Input, Discount Rate [Member] | Equity Method Investments | Income Approach Valuation Technique | Maximum
Goodwill [Line Items]
Fair Value Inputs					11.50%
Measurement Input, Discount Rate [Member] | Goodwill | Income Approach Valuation Technique | Minimum
Goodwill [Line Items]
Fair Value Inputs					11.50%
Measurement Input, Discount Rate [Member] | Goodwill | Income Approach Valuation Technique | Maximum
Goodwill [Line Items]
Fair Value Inputs					12.80%

Goodwill and Intangibles (Recon

Goodwill and Intangibles (Reconciliation of Goodwill) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Gross goodwill	$ 2,716,000,000		$ 2,375,000,000
Accumulated impairment losses	(130,000,000)		(130,000,000)
Goodwill, beginning balance	2,245,000,000	$ 2,245,000,000
Impairment losses	0	0
Acquisitions	341,000,000	0
Goodwill, ending balance	2,586,000,000	2,245,000,000
Logistics and Storage [Member]
Goodwill [Line Items]
Gross goodwill	503,000,000		162,000,000
Accumulated impairment losses	0		0
Goodwill, beginning balance	162,000,000	162,000,000
Impairment losses	0	0
Acquisitions	341,000,000	0
Goodwill, ending balance	503,000,000	162,000,000
Gathering and Processing [Member]
Goodwill [Line Items]
Gross goodwill	2,213,000,000		2,213,000,000
Accumulated impairment losses	(130,000,000)		$ (130,000,000)
Goodwill, beginning balance	2,083,000,000	2,083,000,000
Impairment losses	0	0
Acquisitions	0	0
Goodwill, ending balance	$ 2,083,000,000	$ 2,083,000,000

Goodwill and Intangibles (Intan

Goodwill and Intangibles (Intangible Assets) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		$ 38	$ 38
Gross		542	533
Accumulated amortization	[1]	(118)	(80)
Intangibles, net		424	453
Logistics and Storage [Member]
Finite-Lived Intangible Assets [Line Items]
Gross		9	0
Accumulated amortization	[1]	0	0
Intangibles, net		$ 9	0
Logistics and Storage [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life		4 years
Logistics and Storage [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life		6 years
Gathering and Processing [Member]
Finite-Lived Intangible Assets [Line Items]
Gross		$ 533	533
Accumulated amortization	[1]	(118)	(80)
Intangibles, net		$ 415	$ 453
Gathering and Processing [Member] | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life		11 years
Gathering and Processing [Member] | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life		25 years
[1]	Amortization expense attributable to the G&P segment for the years ended December 31, 2018 and 2017 was $38 million in both years.

Goodwill and Intangibles (Futur

Goodwill and Intangibles (Future Amortization Expense) (Details) $ in Millions	Dec. 31, 2018USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,018	$ 40
2,019	40
2,020	40
2,021	39
2,022	39
Thereafter	226
Total future intangibles amortization expenses	$ 424

Fair Value Measurements - Recur

Fair Value Measurements - Recurring (Financial Instruments by Valuation Hierarchy) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 0	$ 0
Liabilities	(61)	(66)
Commodity contracts | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Liabilities	0	(2)
Embedded derivatives in commodity contracts | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Liabilities	$ (61)	$ (64)

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring (Significant Unobservable Inputs in Level 3 Valuation) (Details) - Significant unobservable inputs (Level 3)	12 Months Ended
	Dec. 31, 2018USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Term of counterparty option to renew gas purchase agreement	5 years
Commodity contracts | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value inputs commodity price	$ 0.58
Commodity contracts | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value inputs commodity price	$ 1.01
Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Probability of renewal, first term	90.00%
Probability of renewal, second term	80.00%

Fair Value Measurements - Rec_3

Fair Value Measurements - Recurring (Changes in Level 3 Measurements) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Commodity Derivative Contracts (net)
Derivative assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy [Roll Forward]
Fair value at beginning of period		$ (2)	$ (6)
Total gains/(losses) (realized and unrealized) included in earnings(1)	[1]	6	(5)
Settlements		(4)	9
Fair value at end of period		0	(2)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period		0	(2)
Embedded Derivatives in Commodity Contracts (net)
Derivative assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy [Roll Forward]
Fair value at beginning of period		(64)	(54)
Total gains/(losses) (realized and unrealized) included in earnings(1)	[1]	(9)	(19)
Settlements		12	9
Fair value at end of period		(61)	(64)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to liabilities still held at end of period		$ (8)	$ (6)
[1]	Gains and losses on commodity derivatives classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 13,169	$ 7,718
SMR liability	92	104
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	13,484	6,966
SMR liability	$ 86	$ 91

Derivative Financial Instrume_3

Derivative Financial Instruments (Embedded Derivatives in Commodity Contracts) (Details) - Embedded derivative in natural gas processing and purchase contract [Member] - Embedded derivatives in commodity contracts $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Derivative [Line Items]
Number of renewals	2
Term of counterparty option to renew gas purchase agreement	5 years
Embedded derivative fair value of embedded derivative liability including inception value allocable to host contract	$ 61	$ 64

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivatives Balance Sheet Location) (Details) - Commodity contracts - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	[1]	$ 0	$ 0
Derivative liability, fair value, gross liability	[1]	(61)	(66)
Not Designated as Hedging Instrument | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset		0	0
Not Designated as Hedging Instrument | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability		(7)	(14)
Not Designated as Hedging Instrument | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset		0	0
Not Designated as Hedging Instrument | Deferred credits and other liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability		$ (54)	$ (52)
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume_5

Derivative Financial Instruments (Derivatives Income Statement Location) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains/(losses)	[1]	$ 5	$ (6)	$ (36)
Total derivative loss		(3)	(24)	(42)
Product sales
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		4	(9)	2
Unrealized gains/(losses)		2	4	(15)
Total derivative loss		6	(5)	(13)
Purchased product costs
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		(12)	(9)	(5)
Unrealized gains/(losses)		3	(10)	(22)
Total derivative loss		(9)	(19)	(27)
Cost of revenues
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		0	0	(3)
Unrealized gains/(losses)		0	0	1
Total derivative loss		$ 0	$ 0	$ (2)
[1]	MPLX makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Debt (Summary of Outstanding Bo

Debt (Summary of Outstanding Borrowings) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 10, 2018	Nov. 15, 2018	Feb. 08, 2018	Dec. 31, 2017	Feb. 10, 2017
Debt Instrument [Line Items]
Debt and capital lease obligations	$ 13,856				$ 7,362
Unamortized debt issuance costs	97				27
Unamortized discount	366				389
Amounts due within one year	1				1
Total long-term debt due after one year	13,392				6,945
MPLX LP | Line of Credit | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Long-term debt, gross	0				505
MPLX LP | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross			$ 2,250	$ 5,500
MPLX LP | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross	0	$ 750			710
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	500				0
MPLX LP | Senior Notes | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Long-term debt, gross	989				989
MPLX LP | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Long-term debt, gross	1,149				1,149
MPLX LP | Senior Notes | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Long-term debt, gross	500				500
MPLX LP | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Long-term debt, gross	1,189				1,189
MPLX LP | Senior Notes | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Long-term debt, gross	1,250				1,250	$ 1,250
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,250				0
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	750				0
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,750				0
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Long-term debt, gross	1,000				1,000	$ 1,000
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,500				0
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,500				0
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	500				0
MarkWest | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 40
MarkWest | Senior Notes | MarkWest senior notes
Debt Instrument [Line Items]
Long-term debt, gross	23				63
Marathon Pipe Line LLC | Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease obligations due 2020	$ 6				$ 7

Debt (Summary of Outstanding _2

Debt (Summary of Outstanding Borrowings Interest Rates and Table Due Dates) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 10, 2018	Feb. 10, 2017
MPLX LP | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Proceeds from long-term lines of credit	$ 1,410	$ 670
MPLX LP | Line of Credit | MPLX Revolver due July 2022
Debt Instrument [Line Items]
Expiration date	Jul. 21, 2022
MPLX LP | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%		5.50%
Maturity date	Feb. 15, 2023
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.375%
Maturity date	Mar. 15, 2023
MPLX LP | Senior Notes | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Jul. 15, 2023
MPLX LP | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Dec. 1, 2024
MPLX LP | Senior Notes | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Maturity date	Feb. 15, 2025
MPLX LP | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Jun. 1, 2025
MPLX LP | Senior Notes | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Interest rate, stated percentage	4.125%			4.125%
Maturity date	Mar. 1, 2027
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.00%
Maturity date	Mar. 15, 2028
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.80%
Maturity date	Feb. 15, 2029
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Apr. 15, 2038
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Interest rate, stated percentage	5.20%			5.20%
Maturity date	Mar. 1, 2047
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.70%
Maturity date	Apr. 15, 2048
MPLX LP | Senior Notes | Senior Notes Due November 2049 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.50%
Maturity date	Feb. 15, 2049
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.90%
Maturity date	Apr. 15, 2058
MarkWest | Senior Notes | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%
Maturity date	Jul. 15, 2023
MarkWest | Senior Notes | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Dec. 1, 2024
MarkWest | Senior Notes | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Interest rate, stated percentage	4.875%
Maturity date	Jun. 1, 2025
Marathon Pipe Line LLC | Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease due date year	2,020

Debt (Schedule of Debt Payments

Debt (Schedule of Debt Payments) (Detail) $ in Millions	Dec. 31, 2018USD ($)
Debt Disclosure [Abstract]
2,018	$ 1
2,019	5
2,020	0
2,021	0
2,022	$ 1,500

Debt (Credit Agreements) (Detai

Debt (Credit Agreements) (Detail) $ in Millions	Jul. 21, 2017USD ($)	Oct. 27, 2015	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Feb. 19, 2017
Debt Instrument [Line Items]
Long-term debt - repayments			$ 6,780	$ 416	$ 1,312
MPLX Revolver | MPLX LP
Debt Instrument [Line Items]
Maximum borrowing capacity					2,000
Term		5 years
Description of variable rate basis	Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA	4
MPLX Revolver | MPLX LP | Maximum
Debt Instrument [Line Items]
Additional borrowing capacity	$ 500
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)	5
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)	5.5
MPLX Revolver | MPLX LP | Minimum
Debt Instrument [Line Items]
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)	1
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)	1
MPLX Revolver | MPLX LP | Swingline Loan
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100
Term loan facility due 2019
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 250
Term loan facility due 2019 | MPLX LP
Debt Instrument [Line Items]
Interest rate, effective percentage						2.407%
MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 2,250
Proceeds from long-term lines of credit			$ 1,410	$ 670
Term	5 years
Interest rate, effective percentage			3.464%	2.748%
Repayments of long-term lines of credit			$ 1,915	$ 165
Letters of credit outstanding, amount			3	3
Remaining borrowing capacity			$ 2,200	$ 1,700
Remaining borrowing capacity, percentage			99.90%	77.40%
MPLX Revolver due July 2022 | MPLX LP | Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 222
Number of renewals	2
Debt Instrument, Renewal Term	1 year
Line of Credit | MPLX Revolver | MPLX LP
Debt Instrument [Line Items]
Proceeds from long-term lines of credit				$ 0
Line of Credit | MPLX Revolver due July 2022 | MPLX LP
Debt Instrument [Line Items]
Expiration date			Jul. 21, 2022
Long-term debt, outstanding			$ 0	$ 505

Debt (Senior Notes) (Details)

Debt (Senior Notes) (Details) - USD ($) $ in Millions	Feb. 08, 2018	Mar. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 10, 2018	Nov. 15, 2018	Feb. 10, 2017
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax			$ 46	$ 0	$ 0
Ozark Pipeline
Debt Instrument [Line Items]
Business Combination, Cash Consideration Transferred Net of Liabilities Assumed		$ 1,500
MPLX LP | MPLX 364-Day Term Loan [Member]
Debt Instrument [Line Items]
Repayments of Short-term Debt	$ 4,100
MPLX LP | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 5,500						$ 2,250
MPLX LP | Senior Notes | Senior Notes Due February 2029 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 750	0
Interest rate, stated percentage			4.80%
Percent of par							99.432%
MPLX LP | Senior Notes | Senior Notes Due February 2049 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,500	0
Interest rate, stated percentage			5.50%
Percent of par							98.031%
MPLX LP | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross			$ 0	710		$ 750
Interest rate, stated percentage			5.50%			5.50%
Percent of par						101.833%
MPLX LP | Senior Notes | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 500	0
Interest rate, stated percentage			3.375%
Percent of par	99.931%
MPLX LP | Senior Notes | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,250	1,250				$ 1,250
Interest rate, stated percentage			4.125%					4.125%
Percent of par								99.834%
MPLX LP | Senior Notes | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,000	1,000				$ 1,000
Interest rate, stated percentage			5.20%					5.20%
Percent of par								99.304%
MPLX LP | Senior Notes | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,250	0
Interest rate, stated percentage			4.00%
Percent of par	99.551%
MPLX LP | Senior Notes | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,750	0
Interest rate, stated percentage			4.50%
Percent of par	98.811%
MPLX LP | Senior Notes | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,500	0
Interest rate, stated percentage			4.70%
Percent of par	99.348%
MPLX LP | Senior Notes | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Long-term debt, gross			$ 500	$ 0
Interest rate, stated percentage			4.90%
Percent of par	99.289%
MarkWest | Senior Notes | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Long-term debt, gross						$ 40
Debt Premium [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax			$ 14

Debt (SMR Transaction) (Details

Debt (SMR Transaction) (Details) - USD ($) $ in Millions	Sep. 01, 2009	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net		$ 14,639	$ 12,187
Accrued liabilities		250	231
Deferred credits and other liabilities		197	188
Steam Methane Reformer Held Under Financing Arrangement [Member]
Property, Plant and Equipment [Line Items]
Proceeds from liability	$ 73
Imputed interest rate on liability	6.39%
Property, plant and equipment, net		51	56
Accrued liabilities		5	5
Deferred credits and other liabilities		$ 81	$ 86

Revenue Impact of Adoption (Det

Revenue Impact of Adoption (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rental income	$ 349	$ 277	$ 298
Contract with Customer, Liability	$ 105
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	25 years
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized	$ (1)
Revenue from Contract with Customer, Excluding Assessed Tax	5,007
Rental cost of sales	135	62	$ 57
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Rental income	65
Rental cost of sales	65
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Third Party Reimbursements [Member]
Rental income	65
Cost of Goods and Services Sold	369
Revenue from Contract with Customer, Excluding Assessed Tax	369
Rental cost of sales	65
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Non-cash Consideration [Member]
Cost of Goods and Services Sold	50
Sales Revenue Services, Product Related	52
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Percent-of-Proceeds [Member]
Sales Revenue Services, Product Related	146
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Imbalances [Member]
Cost of Goods and Services Sold	55
Revenue from Contract with Customer, Excluding Assessed Tax	55
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Service Oil Allowances [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	7
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Oil Allowances [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	7
Inventories [Member] | Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 2	$ 1

Revenue ASC 606 Initial Applica

Revenue ASC 606 Initial Application Period Cumulative Effect (Details) - USD ($) $ in Millions		3 Months Ended									12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											$ 5,007
Rental income											349		$ 277		$ 298
Inventories		$ 77				$ 65					77		65
Long-term deferred revenue		80				42					80		42
Long-term deferred revenue - related parties		43				43					43		43
Property, plant and equipment, net		14,639				12,187					14,639		12,187
Deferred revenue - related parties		51				43					51		43
Total MPLX LP partners’ capital		6,708				9,827					6,708		9,827
Rental cost of sales											135		62		57
Depreciation and amortization	[1]										766		683		591
Net income		439	$ 516	$ 456	$ 423	241		$ 217	$ 191	$ 187	1,834		836		434
Other income											7		6		7
Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract with Customer, Asset, Gross		4				4	[2]				4		4	[2]
Long-term deferred revenue		10				5	[2]				10		5	[2]
Long-term deferred revenue - related parties		42				43	[2]				42		43	[2]
Deferred Revenue, Current		4				5	[2]				4		5	[2]
Deferred revenue - related parties		50				42	[2]				50		42	[2]
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Rental income											284
Rental cost of sales											70
Depreciation and amortization											767
Net income											1,832
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Rental income											65
Rental cost of sales											65
Depreciation and amortization											1
Net income											2
Cumulative Adjusted 606 Balance [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Inventories						66							66
Long-term deferred revenue						39							39
Property, plant and equipment, net						12,184							12,184
Inventories [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		2				1					2		1
Property, Plant and Equipment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						(3)							(3)
Deferred Revenue, Noncurrent [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						(3)							(3)
Public | Limited Partners Common Units
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total MPLX LP partners’ capital		$ 8,336				8,379					8,336		8,379
Public | Limited Partners Common Units | Cumulative Adjusted 606 Balance [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total MPLX LP partners’ capital						8,380							8,380
Public | Limited Partners Common Units | Equity [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 1							1
Service [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											1,704		1,156		958
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											2,159		1,082		936
Service [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											1,342
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											2,166
Service [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											362
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											(7)
Service, Other [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											198		0		0
Service, Other [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											0
Service, Other [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											198
Product [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax											902		889		572
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[3]										897
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											49		8		11
Product [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue											982
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											42
Product [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue											(85)
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties											7
Oil and Gas, Refining and Marketing [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold											948	[4]	528		454
Oil and Gas, Refining and Marketing [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold	[4]										579
Oil and Gas, Refining and Marketing [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold	[4]										369
Natural Gas, Midstream [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold											845		$ 651		$ 448
Natural Gas, Midstream [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold											740
Natural Gas, Midstream [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cost of Goods and Services Sold											105
Other Income [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other income	[5]										$ 1,418
[1]	Depreciation and amortization attributable to L&S was $240 million, $163 million and $128 million for the years ended 2018, 2017 and 2016, respectively. Depreciation and amortization attributable to G&P was $526 million, $520 million and $463 million for 2018, 2017 and 2016, respectively.
[2]	Balance represents ASC 606 portion of each respective line item.
[3]	G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments.
[4]	Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”
[5]	Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										$ 5,007
Rental income										349	$ 277	$ 298
Other Operating Income										(3)	(24)	(42)
Total revenues and other income		$ 1,715	$ 1,712	$ 1,578	$ 1,420	$ 1,085	$ 980	$ 916	$ 886	6,425	3,867	3,029
Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,704	1,156	958
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										2,159	1,082	936
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										198	0	0
Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										902	889	572
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[1]									897
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										49	$ 8	$ 11
Logistics and Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,303
Logistics and Storage [Member] | Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										130
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										2,159
Logistics and Storage [Member] | Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										0
Logistics and Storage [Member] | Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue										7
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										7
Gathering and Processing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										2,704
Gathering and Processing [Member] | Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										1,574
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										0
Gathering and Processing [Member] | Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax										198
Gathering and Processing [Member] | Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	[1]									890
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties										42
Other Income [Member]
Disaggregation of Revenue [Line Items]
Other Operating Income										$ 5
[1]	G&P “Product sales” for the year ended December 31, 2018 excludes approximately $5 million of impact related to derivative gains and mark-to-market adjustments.

Revenue Contract Balance Rollfo

Revenue Contract Balance Rollforward (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Beginning Balance Deferred revenue - related parties	$ 43
Ending Balance Deferred Revenue, Current, Related Parties	51
Beginning Balance Deferred Revenue, Noncurrent	42
Ending Balance Deferred Revenue, Noncurrent	80
Beginning Balance Deferred Revenue, Noncurrent, Related Parties	43
Ending Balance Deferred Revenue, Noncurrent, Related Parties	43
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized	(1)
Accounting Standards Update 2014-09 [Member]
Beginning Balance Contract with Customer, Asset, Gross	4	[1]
Contract with Customer, Asset Increase (Decrease)	0
Contract with Customer, Asset, Reclassified to Receivable	0	[2]
Ending Balance Contract with Customer, Asset, Gross	4
Beginning Balance Deferred Revenue, Current	5	[1]
Deferred Revenue, Period Increase (Decrease)	8
Deferred Revenue, Revenue Recognized	(9)	[2]
Ending Balance Deferred Revenue, Current	4
Beginning Balance Deferred revenue - related parties	42	[1]
Deferred Revenue - Related Party, Period Increase (Decrease)	40
Deferred Revenue - Related Parties, Revenue Recognized	(32)	[2]
Ending Balance Deferred Revenue, Current, Related Parties	50
Beginning Balance Deferred Revenue, Noncurrent	5	[1]
Deferred Revenue, Noncurrent, Period Increase (Decrease)	5
Deferred Revenue, Noncurrent, Revenue Recognized	0	[2]
Ending Balance Deferred Revenue, Noncurrent	10
Beginning Balance Deferred Revenue, Noncurrent, Related Parties	43	[1]
Deferred Revenue, Noncurrent, Related Party, Period Increase (Decrease)	(1)
Deferred Revenue, Noncurrent, Related Parties, Revenue Recognized	0	[2]
Ending Balance Deferred Revenue, Noncurrent, Related Parties	$ 42
[1]	Balance represents ASC 606 portion of each respective line item.
[2]	$1 million revenue was recognized related to past performance obligations in the current year.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	Dec. 31, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with Customer, Liability	$ 105
Revenue, Remaining Performance Obligation, Expected to be Recognized in Current Fiscal Year	1,146
Revenue, Remaining Performance Obligation, Expected to be Recognized in Next Fiscal Year	1,152
Revenue, Remaining Performance Obligation, Expected to be Recognized in Third Fiscal Year	1,166
Revenue, Remaining Performance Obligation, Expected to be Recognized in Fourth Fiscal Year	1,151
Revenue, Remaining Performance Obligation, Expected to be Recognized in Fifth Fiscal Year and Thereafter	5,524
Revenue, Remaining Performance Obligation, Amount	$ 10,139	[1],[2],[3]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	25 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		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents		$ 68		$ 5
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)		484		263		$ 213
Income taxes paid		1		3		4
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories		2		6		(3)
Contribution - fixed assets to joint venture	[1]	0		337		0
Contribution - common units issued	[2]	4,236		1,133		669
Acquisitions [Abstract]
Restricted Cash	[3]	8		4
Cash, cash equivalents and restricted cash at end of period		$ 76	[4]	$ 9	[4]	$ 239	$ 52
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.
[2]	For 2016, includes limited partner units issued to MPC as consideration in the acquisition of HSM. For 2017, includes limited and general partner units issued to MPC as consideration in the acquisitions of the joint-interests, HST, WHC and MPLXT. For 2018, includes limited and general partner units issued to MPC as consideration in the acquisition of Refining Logistics and Fuels Distribution. See Note 4.
[3]	The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets.
[4]	As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”

Supplemental Cash Flow Inform_4

Supplemental Cash Flow Information Supplemental Cash Flow Information (Details) $ in Millions	Dec. 31, 2017USD ($)
LOOP LOCAP SAX and Explorer
Suppplemental Cash Flow [Line Items]
Accounts payable, related parties	$ 11

Supplemental Cash Flow Inform_5

Supplemental Cash Flow Information (Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Increase/(decrease) in capital accruals	$ 104	$ 71	$ (22)

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (16)	$ (14)	$ 0
Other Comprehensive Income (Loss), Net of Tax	(2)	(14)
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	(14)	(13)	0
Other Comprehensive Income (Loss), Net of Tax	(1)	(13)
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	(2)	(1)	$ 0
Other Comprehensive Income (Loss), Net of Tax	$ (1)	$ (1)

Equity-Based Compensation (Narr

Equity-Based Compensation (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation Expense
Total compensation expense	$ (19)	$ (15)	$ (10)
Phantom Units
Unit-based awards under the Plan
Grant date fair value of performance units granted	$ 33.84	$ 36.26	$ 29.42
Outstanding Phantom and Performance Unit Awards
Non-forfeitable at December 31, 2018(1)	321,638
Unrecognized compensation cost	$ 17
Cost not yet recognized, period for recognition	1 year 10 months 3 days
Vested and expected to vest at December 31, 2018	1,139,877
Performance Shares
Unit-based awards under the Plan
Grant date fair value of performance units granted		$ 0.90	$ 0.63
Outstanding Phantom and Performance Unit Awards
Unrecognized compensation cost	$ 1
Cost not yet recognized, period for recognition	1 year
Vested and expected to vest at December 31, 2018	64,084
Mplx 2012 Incentive Compensation Plan [Member]
Compensation Expense
Total compensation expense	$ 22	$ 18	$ 10
Maximum | Mplx Two Thousand Eighteen Incentive Compensation Plan [Member]
Equity-Based Compensation [Line Items]
Stock or Units Available for Distributions	16,000,000
Maximum | Mplx 2012 Incentive Compensation Plan [Member]
Equity-Based Compensation [Line Items]
Stock or Units Available for Distributions	2,750,000
Officer | Mplx 2012 Incentive Compensation Plan [Member] | Phantom Units
Unit-based awards under the Plan
Award requisite service period	3 years
Accrued distributions	$ 4	4
Officer | Mplx 2012 Incentive Compensation Plan [Member] | Performance Shares
Unit-based awards under the Plan
Award percentage paid out in cash	75.00%
Award percentage paid out in stock	25.00%
General and administrative expenses
Compensation Expense
Stock-based compensation plans expenses	$ 6	$ 2	$ 5

Equity-Based Compensation (Summ

Equity-Based Compensation (Summary of Phantom Unit Award Activity) (Detail) - Phantom Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Units
Outstanding at December 31, 2017	1,351,523
Granted	437,092
Settled	(509,570)
Forfeited	(124,710)
Outstanding at December 31, 2018	1,154,335	1,351,523
Vested and expected to vest at December 31, 2018	1,139,877
Non-forfeitable at December 31, 2018(1)	321,638
Weighted Average Fair Value
Outstanding at December 31, 2016 (usd per share)	$ 34.53
Granted (usd per share)	33.84	$ 36.26	$ 29.42
Settled (usd per share)	34.38
Forfeited (usd per share)	34.50
Outstanding at December 31, 2017 (usd per share)	34.34	$ 34.53
Vested and expected to vest at December 31, 2017 (usd per share)	34.34
Convertible at December 31, 2017 (usd per share)	$ 34.59
Aggregate Intrinsic Value
Vested and expected to vest at December 31, 2018	$ 35
Non-forfeitable at December 31, 2018(1)	$ 10

Equity-Based Compensation (Su_2

Equity-Based Compensation (Summary of Values Related To Vested and Unvested Restricted Stock Awards) (Details) - Phantom Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity-Based Compensation [Line Items]
Intrinsic value of units issued during the period	$ 18	$ 15	$ 5
Grant date fair value of performance units granted	$ 33.84	$ 36.26	$ 29.42

Equity-Based Compensation (Su_3

Equity-Based Compensation (Summary of Performance Unit Award Activity) (Details) - Performance Shares	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Number of Units
Outstanding at December 31, 2017 | shares	2,536,594
Granted | shares	0
Settled | shares	(538,594)
Forfeited | shares	(56,250)
Outstanding at December 31, 2018 | shares	1,941,750
Weighted Average Fair Value
Outstanding at December 31, 2016 (usd per share) | $ / shares	$ 0.85
Granted (usd per share) | $ / shares	0
Settled (usd per share) | $ / shares	1.04
Forfeited (usd per share) | $ / shares	0.90
Outstanding at December 31, 2017 (usd per share) | $ / shares	$ 0.80

Equity-Based Compensation (Assu

Equity-Based Compensation (Assumptions) (Details) - Performance Shares - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity-Based Compensation [Line Items]
Risk-free interest rate	1.52%	0.96%
Look-back period	2 years 9 months 30 days	2 years 9 months 30 days
Expected volatility	49.34%	47.59%
Grant date fair value of performance units granted	$ 0.90	$ 0.63

Lease Operations Lease Operatio

Lease Operations Lease Operations (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Operating lease revenue, excluding executory costs	$ 928	$ 601	$ 586
Operating leases of lessor contingent rentals received	$ 9	$ 9	$ 7

Lease Operations (Schedule of M

Lease Operations (Schedule of Minimum Future Rentals on Non-cancellable Leases) (Details) $ in Millions	Dec. 31, 2018USD ($)
2,018	$ 908
2,019	909
2,020	777
2,021	775
2,022	758
2023 and thereafter	3,432
Total minimum future rentals	7,559
Related Party
2,018	748
2,019	750
2,020	627
2,021	627
2,022	616
2023 and thereafter	2,321
Total minimum future rentals	5,689
Third Party
2,018	160
2,019	159
2,020	150
2,021	148
2,022	142
2023 and thereafter	1,111
Total minimum future rentals	$ 1,870

Lease Operations (Schedule of I

Lease Operations (Schedule of Investment in Assets Held for Operating Lease) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Property Subject to or Available for Operating Lease [Line Items]
Total	$ 5,714	$ 3,119
Less accumulated depreciation	2,038	1,056
Property, plant and equipment, net	3,676	2,063
Natural gas gathering and NGL transportation pipelines and facilities
Property Subject to or Available for Operating Lease [Line Items]
Total	964	851
Processing, fractionation and storage facilities
Property Subject to or Available for Operating Lease [Line Items]
Total	1,398	573
Pipelines and related assets
Property Subject to or Available for Operating Lease [Line Items]
Total	266	253
Barges and towing vessels
Property Subject to or Available for Operating Lease [Line Items]
Total	619	491
Terminals and related assets
Property Subject to or Available for Operating Lease [Line Items]
Total	1,178	822
Refineries and related assets [Member]
Property Subject to or Available for Operating Lease [Line Items]
Total	938	0
Land, building, office equipment and other
Property Subject to or Available for Operating Lease [Line Items]
Total	162	44
Construction-in-progress
Property Subject to or Available for Operating Lease [Line Items]
Total	$ 189	$ 85

Asset Retirement Obligations (R

Asset Retirement Obligations (Reconciliation of Changes in AROs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]
AROs at beginning of period	$ 28	$ 25
Liabilities incurred	1	2
Accretion expense	1	1
AROs at end of period	$ 30	$ 28

Asset Retirement Obligations (N

Asset Retirement Obligations (Narrative) (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]
Assets legally restricted for purposes of settling AROs	$ 0	$ 0

Commitments and Contingencies_2

Commitments and Contingencies (Detail) - USD ($) $ in Millions		Sep. 06, 2016	Dec. 31, 2018	Dec. 31, 2017
Commitments And Contingencies [Line Items]
Accrual for environmental loss contingencies			$ 14	$ 13
Payable to MPC for indemnification of environmental costs			203	516
Contractual commitments to acquire property, plant and equipment			746
MPC
Commitments And Contingencies [Line Items]
Payable to MPC for indemnification of environmental costs	[1]		131	470
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Receivable from MPC for indemnification of environmental costs			0
Payable to MPC for indemnification of environmental costs				$ 1
MarkWest [Member]
Commitments And Contingencies [Line Items]
Damages sought			10
Apex
Commitments And Contingencies [Line Items]
Litigation settlement, amount		$ 10
Bilfinger Westcon, Inc. [Member]
Commitments And Contingencies [Line Items]
Damages sought			40
MarkWest, MarkWest Liberty Midstream, Ohio Fractionation, MarkWest Utica EMG [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty			0.9
Estimated cost of proposed supplemental environmental projects			3.3
Markwest Liberty Midstream [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty			0.6
Estimated cost of proposed supplemental environmental projects			$ 2.4
[1]	Balance includes $386 million related to the MPC Loan Agreement as of December 31, 2017. There was no outstanding balance on the MPC Loan Agreement as of December 31, 2018.

Commitments and Contingencies_3

Commitments and Contingencies (Minimum Future Payments) (Details) - Transportation And Terminalling Agreements $ in Millions	Dec. 31, 2018USD ($)
Other Commitments [Line Items]
2,018	$ 52
2,019	52
2,020	48
2,021	46
2,022	46
2023 and thereafter	180
Total	$ 424

Commitments and Contingencies_4

Commitments and Contingencies (Future Minimum Commitments) (Details) $ in Millions	Dec. 31, 2018USD ($)
Capital Lease Obligations
2,018	$ 2
2,019	5
2,020	0
2,021	0
2,022	0
Later years	0
Total minimum lease payments	7
Less: imputed interest costs	1
Present value of net minimum lease payments	6
Operating Lease Obligations
2,018	73
2,019	70
2,020	67
2,021	64
2,022	58
Later years	719
Total minimum lease payments	$ 1,051

Commitments and Contingencies_5

Commitments and Contingencies (Operating Lease Rental Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Minimum rental expense	$ 85	$ 64	$ 57
Minimum Committed Volume Contracts [Member] | Minimum
Other Commitments [Line Items]
Term Of Agreements	9 years
Minimum Committed Volume Contracts [Member] | Maximum
Other Commitments [Line Items]
Term Of Agreements	11 years

Commitments and Contingencies_6

Commitments and Contingencies (SMR Liability) (Details) - SMR Transaction $ in Millions	Dec. 31, 2018USD ($)
Recorded Unconditional Purchase Obligation [Line Items]
2,018	$ 17
2,019	17
2,020	17
2,021	17
2,022	17
2023 and thereafter	110
Total minimum payments	195
Less: Services element	75
Less: Interest	34
Total SMR liability	86
Less: Current portion of SMR liability	5
Long-term portion of SMR liability	$ 81

Select Quarterly Financial Da_3

Select Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Quarterly Financial Data [Line Items]
Total revenues and other income		$ 1,715	$ 1,712	$ 1,578	$ 1,420	$ 1,085	$ 980	$ 916	$ 886	$ 6,425		$ 3,867		$ 3,029
Income from operations		666	672	608	557	335	311	280	265	2,503		1,191		683
Net income		439	516	456	423	241	217	191	187	1,834		836		434
Net income (loss) attributable to MPLX LP		434	510	453	421	238	216	190	150	1,818	[1]	794	[1]	233	[1]
Distributions declared:
Total cash distributions declared										2,060		1,288		938
Distribution declared, general partner												(32)
Incentive distribution, subsequent distribution amount										0		303		187
Total distributions declared		$ 534	$ 526	$ 517	$ 483	$ 362	$ 336	$ 311	$ 279
Limited Partners Common Units
Quarterly Financial Data [Line Items]
Net income (loss) attributable to MPLX LP	[1]									$ 1,743		$ 411		$ 1
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.52	$ 0.62	$ 0.55	$ 0.61	$ 0.31	$ 0.29	$ 0.26	$ 0.20	$ 2.29		$ 1.07		$ 0
Common - diluted (in USD per unit)		0.52	0.62	0.55	0.61	0.31	0.29	0.26	0.19	2.29		1.06		$ 0
Cash distributions declared per limited partner common unit		0.6475	0.6375	0.6275	0.6175	0.6075	0.5875	0.5625	0.5400	$ 2.5300		$ 2.2975
Distributions declared:
Total cash distributions declared	[2]									$ 1,985		$ 895		$ 692
Limited Partners Subordinated Units
Net income (loss) attributable to MPLX LP per limited partner unit:
Subordinated - basic and diluted (in USD per unit)		$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Redeemable Preferred Units
Quarterly Financial Data [Line Items]
Net income (loss) attributable to MPLX LP										75	[1]	65	[1]	41
Distributions declared:
Total cash distributions declared		$ 20	$ 19	$ 20	$ 16	$ 16	$ 16	$ 17	$ 16	$ 75	[2]	65	[2]	$ 41	[2]
Public | Limited Partners Common Units
Distributions declared:
Total cash distributions declared		187	185	181	179	175	170	162	149
Distribution declared, general partner												0
MPC
Distributions declared:
Distribution declared, general partner		0	0	0	0	0	7	6	5
Incentive distribution, subsequent distribution amount		0	0	0	0	0	81	70	60
MPC | Limited Partners Common Units
Distributions declared:
Total cash distributions declared		$ 327	$ 322	$ 316	$ 288	$ 171	$ 62	$ 56	$ 49
Distribution declared, general partner												$ 0
[1]	Allocation of net income/(loss) attributable to MPLX LP assumes all earnings for the period were distributed based on the current period distribution priorities.
[2]	See Note 8 for distribution information.