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Equity-Based Compensation (Tables) | 12 Months Ended
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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 Average Fair Value
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

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- Definition Tabular disclosure of number, weighted-average exercise price or conversion ratio, aggregate intrinsic value, and weighted-average remaining contractual term for outstanding restricted stock units that are fully vested and expected to vest. Includes, but is not limited to, unvested restricted stock units for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited during the year. + 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)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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