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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

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- Definition Convertible units as of the balance sheet date. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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