Acquisitions - Summary Changes in MPLX LP's Ownership Interest in Pipe Line Holdings (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||||||
|---|---|---|---|---|---|---|---|---|
Dec. 01, 2014 |
Mar. 01, 2014 |
Sep. 30, 2015 |
Sep. 30, 2014 |
Sep. 30, 2015 |
Sep. 30, 2014 |
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| Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items] | ||||||||
| Net income attributable to MPLX LP | [1] | $ 41.5 | $ 29.1 | $ 138.3 | $ 92.1 | |||
| Transfer to noncontrolling interest retained by MPC: | ||||||||
| Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC | 41.5 | 29.1 | 138.3 | (80.4) | ||||
| General Partner [Member] | ||||||||
| Transfer to noncontrolling interest retained by MPC: | ||||||||
| Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings | $ (465.2) | $ (172.5) | $ 0.0 | $ 0.0 | $ 0.0 | $ (172.5) | ||
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