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Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Related Party Transaction [Line Items]        
Expenses incurred under employee services agreements $ 115 $ 115 $ 232 $ 207
Purchases - related parties [Member]        
Related Party Transaction [Line Items]        
Expenses incurred under employee services agreements 91 88 183 159
General and administrative expenses [Member]        
Related Party Transaction [Line Items]        
Expenses incurred under employee services agreements $ 24 $ 27 $ 49 $ 48