Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | |||||||
|---|---|---|---|---|---|---|---|---|---|
Sep. 30, 2015 |
Sep. 30, 2014 |
Sep. 30, 2015 |
Sep. 30, 2014 |
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| Net Income Per Share [Line Items] | |||||||||
| Net income attributable to MPLX LP | [1] | $ 41.5 | $ 29.1 | $ 138.3 | $ 92.1 | ||||
| Less: Distribution declared | (46.5) | (27.9) | (125.0) | (79.4) | |||||
| Undistributed net income attributable to MPLX LP | (5.0) | 1.2 | 13.3 | 12.7 | |||||
| General Partner [Member] | MPC [Member] | |||||||||
| Net Income Per Share [Line Items] | |||||||||
| Net income attributable to MPLX LP | [1] | 8.7 | 1.8 | 25.6 | 6.6 | ||||
| Less: Distribution declared | [2] | (8.8) | (1.5) | (19.0) | (3.4) | ||||
| Undistributed net income attributable to MPLX LP | (0.1) | 0.3 | 6.6 | 3.2 | |||||
| Limited Partners Common Units [Member] | |||||||||
| Net Income Per Share [Line Items] | |||||||||
| Net income attributable to MPLX LP | [1] | 32.8 | 13.7 | 79.2 | 42.8 | ||||
| Less: Distribution declared | [2] | (37.7) | (13.2) | (74.6) | (38.0) | ||||
| Undistributed net income attributable to MPLX LP | (4.9) | 0.5 | 4.6 | 4.8 | |||||
| Limited Partners Subordinated Units [Member] | MPC [Member] | |||||||||
| Net Income Per Share [Line Items] | |||||||||
| Net income attributable to MPLX LP | [1] | 0.0 | 13.6 | 33.5 | 42.7 | ||||
| Less: Distribution declared | [2] | 0.0 | (13.2) | (31.4) | (38.0) | ||||
| Undistributed net income attributable to MPLX LP | $ 0.0 | $ 0.4 | $ 2.1 | $ 4.7 | |||||
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