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Debt - Summary of Outstanding Borrowings (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Oct. 15, 2020
Aug. 18, 2020
Dec. 31, 2019
Sep. 09, 2019
Debt Instrument [Line Items]            
Amortization of deferred financing costs $ 44.0 $ 29.0        
Total 20,757.0       $ 20,119.0  
Unamortized debt issuance costs (118.0)       (106.0)  
Unamortized discount/premium (290.0)       (300.0)  
Amounts due within one year (307.0)       (9.0)  
Total long-term debt due after one year 20,042.0       19,704.0  
Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross           $ 2,000.0
Senior Notes [Member] | MPLX LP            
Debt Instrument [Line Items]            
Long-term Debt, Gross       $ 3,000.0    
Senior Notes [Member] | MarkWest [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross 23.0       23.0  
Senior Notes [Member] | ANDX LP [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross 121.0       190.0  
Variable Rate Senior Notes [Member] | MPLX LP            
Debt Instrument [Line Items]            
Long-term Debt, Gross 1,000.0       2,000.0  
Fixed Rate Senior Notes [Member] | MPLX LP            
Debt Instrument [Line Items]            
Long-term Debt, Gross 19,506.0       16,887.0  
Finance Lease [Member] | Marathon Pipe Line LLC [Member]            
Debt Instrument [Line Items]            
Financing lease obligations(1) [1] 12.0       19.0  
Floating Rate Senior Notes Due September 2021 [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross           1,000.0
Floating Rate Senior Notes Due September 2022 [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross           $ 1,000.0
MPLX Revolving Credit Facility due July 2024 [Member]            
Debt Instrument [Line Items]            
Long-term Line of Credit 95.0       0.0  
MPLX Term Loan [Member]            
Debt Instrument [Line Items]            
Long-term Line of Credit $ 0.0     $ 1,000.0 $ 1,000.0  
Senior Notes Due October 2022 [Member] | Senior Notes [Member] | MPLX LP            
Debt Instrument [Line Items]            
Debt Instrument, Interest Rate, Stated Percentage 6.25%          
Minimum [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Interest Rate, Stated Percentage 1.75%          
Maximum [Member] | Senior Notes [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Interest Rate, Stated Percentage 6.25%          
Subsequent Event | Senior Notes Due October 2022 [Member] | Senior Notes [Member] | MPLX LP            
Debt Instrument [Line Items]            
Long-term Debt, Gross     $ 300.0      
Subsequent Event | Senior Notes Due October 2022 [Member] | Senior Notes [Member] | ANDX LP [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross     $ 34.0      
[1] See Note 20 for lease information.