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Income Tax (Reconciliation of Federal Statutory Income Tax Rate) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Entity Information [Line Items]      
(Loss) income before (benefit) provision for income tax $ 1,842 $ 837 $ 422
Federal statutory rate [1]     35.00%
Federal income tax at statutory rate     $ (14)
State income taxes net of federal benefit     (1)
Provision on income from MPLX LP Class A units     3
Change in state statutory rate     (1)
Other     1
Provision/(benefit) for income taxes $ 8 $ 1 (12)
Reportable Legal Entities [Member] | MarkWest Hydrocarbon [Member]      
Entity Information [Line Items]      
(Loss) income before (benefit) provision for income tax [1]     (41)
Federal income tax at statutory rate [1]     (14)
State income taxes net of federal benefit [1]     (2)
Provision on income from MPLX LP Class A units [1]     3
Change in state statutory rate [1]     (1)
Other [1]     1
Provision/(benefit) for income taxes [1]     (13)
Reportable Legal Entities [Member] | Partnership [Member]      
Entity Information [Line Items]      
(Loss) income before (benefit) provision for income tax     461
Federal income tax at statutory rate     0
State income taxes net of federal benefit     1
Provision on income from MPLX LP Class A units     0
Change in state statutory rate     0
Other     0
Provision/(benefit) for income taxes     1
Consolidation, Eliminations [Member]      
Entity Information [Line Items]      
(Loss) income before (benefit) provision for income tax     2
Federal income tax at statutory rate     0
State income taxes net of federal benefit     0
Provision on income from MPLX LP Class A units     0
Change in state statutory rate     0
Other     0
Provision/(benefit) for income taxes     $ 0
[1] MarkWest Hydrocarbon paid tax on its share of MPLX’s income or loss as a result of its ownership of MPLX LP Class A units through September 1, 2016.