Acquisitions (Purchases of Pipe Line Holdings) (Details) - USD ($) $ in Millions |
11 Months Ended | 12 Months Ended | |||
|---|---|---|---|---|---|
Dec. 04, 2015 |
Dec. 03, 2015 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Business Acquisition [Line Items] | |||||
| Payments to acquire businesses, gross | $ 0 | $ 0 | $ 12 | ||
| Purchase of additional interest in Pipe Line Holdings | (1,931) | (12) | |||
| General Partner | |||||
| Business Acquisition [Line Items] | |||||
| Parent ownership interest, changes, purchase of interest by parent | $ 6 | ||||
| General Partner | MPC | |||||
| Business Acquisition [Line Items] | |||||
| Purchase of additional interest in Pipe Line Holdings | $ (1,394) | (6) | |||
| Noncontrolling Interest | |||||
| Business Acquisition [Line Items] | |||||
| Purchase of additional interest in Pipe Line Holdings | $ 6 | $ (6) | |||
| Noncontrolling Interest | MPC | |||||
| Business Acquisition [Line Items] | |||||
| Ownership interest | 0.50% | ||||
| Pipe Line Holding | |||||
| Business Acquisition [Line Items] | |||||
| Effective date of acquisition | Dec. 04, 2015 | ||||
| Percentage of partners interest acquired | 0.50% | ||||
| Payments to acquire businesses, gross | $ 12 | ||||