| Schedule of Stockholders Equity [Table Text Block] |
The changes in the number of units outstanding from December 31, 2014 through September 30, 2015 are summarized below: | | | | | | | | | | | | | (In units) | Common | | Subordinated | | General Partner | | Total | Balance at December 31, 2014 | 43,341,098 |
| | 36,951,515 |
| | 1,638,625 |
| | 81,931,238 |
| Unit-based compensation awards(1) | 18,932 |
| | — |
| | 386 |
| | 19,318 |
| Issuance of units under the ATM Program(2) | 25,166 |
| | — |
| | 514 |
| | 25,680 |
| Subordinated unit conversion | 36,951,515 |
| | (36,951,515 | ) | | — |
| | — |
| Balance at September 30, 2015 | 80,336,711 |
|
| — |
|
| 1,639,525 |
|
| 81,976,236 |
|
| | (1) | As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $0.1 million in exchange for 386 general partner units to maintain its two percent general partner interest. |
| | (2) | As a result of common units issued under the ATM Program during the period, MPLX GP contributed less than $0.1 million in exchange for 514 general partner units to maintain its two percent general partner interest. |
|
| Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block] |
Net Income Allocation - In preparing our consolidated statements of equity, net income attributable to MPLX LP is allocated to the unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are 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 based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP: | | | | | | | | | | | | | | | | | | Three Months Ended September 30, | | Nine Months Ended September 30, | (In millions) | 2015 | | 2014 | | 2015 | | 2014 | Net income attributable to MPLX LP | $ | 41.5 |
| | $ | 29.1 |
| | $ | 138.3 |
| | $ | 92.1 |
| Less: General partner's incentive distribution rights and other | 7.9 |
| | 1.0 |
| | 16.9 |
| | 1.9 |
| Net income attributable to MPLX LP available to general and limited partners | $ | 33.6 |
| | $ | 28.1 |
| | $ | 121.4 |
| | $ | 90.2 |
| | | | | | | | | General partner's two percent interest in net income attributable to MPLX LP | $ | 0.7 |
| | $ | 0.5 |
| | $ | 2.5 |
| | $ | 1.8 |
| General partner's incentive distribution rights and other | 7.9 |
| | 1.0 |
| | 16.9 |
| | 1.9 |
| General partner's interest in net income attributable to MPLX LP | $ | 8.6 |
| | $ | 1.5 |
| | $ | 19.4 |
| | $ | 3.7 |
|
|
| Distributions Made to Limited Partner, by Distribution [Table Text Block] |
Our distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. | | | | | | | | | | | | | | | | | | Three Months Ended September 30, | | Nine Months Ended September 30, | (In millions) | 2015 | | 2014 | | 2015 | | 2014 | General partner's distributions: | | | | | | | | General partner's distributions | $ | 1.0 |
| | $ | 0.5 |
| | $ | 2.5 |
| | $ | 1.5 |
| General partner's incentive distribution rights distributions | 7.8 |
| | 1.0 |
| | 16.5 |
| | 1.9 |
| Total general partner's distributions | $ | 8.8 |
| | $ | 1.5 |
| | $ | 19.0 |
| | $ | 3.4 |
| Limited partners' distributions: | | | | | | | | Common unitholders | $ | 37.7 |
| | $ | 13.2 |
| | $ | 74.6 |
| | $ | 38.0 |
| Subordinated unitholders | — |
| | 13.2 |
| | 31.4 |
| | 38.0 |
| Total limited partners' distributions | 37.7 |
| | 26.4 |
| | 106.0 |
| | 76.0 |
| Total cash distributions declared | $ | 46.5 |
| | $ | 27.9 |
| | $ | 125.0 |
| | $ | 79.4 |
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