Related Party Agreements and Transactions - Employee Services Expenses from Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2015 |
Sep. 30, 2014 |
Sep. 30, 2015 |
Sep. 30, 2014 |
|
| Related Party Transaction [Line Items] | ||||
| Expenses incurred under employee services agreements | $ 26.0 | $ 23.3 | $ 76.9 | $ 69.9 |
| Purchases from Related Parties [Member] | ||||
| Related Party Transaction [Line Items] | ||||
| Expenses incurred under employee services agreements | 19.9 | 17.7 | 56.7 | 53.3 |
| General and Administrative Expenses [Member] | ||||
| Related Party Transaction [Line Items] | ||||
| Expenses incurred under employee services agreements | $ 6.1 | $ 5.6 | $ 20.2 | $ 16.6 |