Summary Of Principal Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Summary Of Principal Accounting Policies [Line Items]
MPC's ownership percentage of the general partner interest	100.00%
Accounts receivable number of days past-due evaluated for doubtful accounts	180 days
Asset retirement obligation, current	$ 30	$ 6
Asset retirement obligations, noncurrent	$ 222	$ 121
MPLX
Summary Of Principal Accounting Policies [Line Items]
MPC's partnership interest in MLPs (in percentage)	63.60%
ANDX
Summary Of Principal Accounting Policies [Line Items]
MPC's partnership interest in MLPs (in percentage)	63.60%
Minimum
Summary Of Principal Accounting Policies [Line Items]
Estimated useful lives (in years)	3 years
Maximum
Summary Of Principal Accounting Policies [Line Items]
Estimated useful lives (in years)	51 years
Stock Options
Summary Of Principal Accounting Policies [Line Items]
Implied volatility rate weighting (in percentage)	50.00%
Historical volatility rate weighting (in percentage)	50.00%