Debt (MPLX Credit Agreement) (Details) - MPLX - MPLX revolving credit facility due July 2024 [Member] $ in Millions	Jul. 30, 2019 USD ($)
Debt Instrument [Line Items]
Total capacity	$ 3,500
Debt instrument, description of variable rate basis	at the Adjusted LIBO Rate or the Alternate Base Rate (both as defined in the MPLX credit agreement) plus an applicable margin
Maximum
Debt Instrument [Line Items]
Line of credit facility additional borrowing capacity	$ 1,000
Number of prior quarterly reporting periods covenant	4
Covenant ratio debt to EBITDA	5.0
Covenant ratio debt to EBITDA post acquisition	5.5
Letter of Credit | Maximum
Debt Instrument [Line Items]
Total capacity	$ 300
Bridge Loan | Maximum
Debt Instrument [Line Items]
Total capacity	$ 150