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Debt - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 26, 2019
Sep. 09, 2019
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Oct. 15, 2020
Aug. 31, 2020
Aug. 28, 2020
Aug. 18, 2020
Dec. 31, 2019
Jul. 30, 2019
Debt Instrument [Line Items]                        
Payments of Debt Issuance Costs         $ 23.0 $ 20.0            
Repayments of Long-term Debt         5,372.0 7,423.0            
Payment for Debt Extinguishment or Debt Prepayment Cost     $ (14.0) $ 0.0 (14.0) $ 0.0            
Debt and Lease Obligation     20,757.0   20,757.0           $ 20,119.0  
Unamortized Debt Issuance Expense     118.0   118.0           106.0  
Debt Instrument, Unamortized Discount     290.0   290.0           300.0  
Debt, Current     307.0   307.0           9.0  
Long-term debt     20,042.0   20,042.0           19,704.0  
MPLX Revolving Credit Facility due July 2024 [Member]                        
Debt Instrument [Line Items]                        
Line of Credit Facility, Current Borrowing Capacity                       $ 3,500.0
Long-term Line of Credit     $ 95.0   95.0           0.0  
MPLX Revolving Credit Facility due July 2024 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Proceeds from Lines of Credit         $ 2,995.0              
Debt Instrument, Interest Rate, Effective Percentage     1.51%   1.51%              
Repayments of Long-term Lines of Credit         $ 2,900.0              
Letters of Credit Outstanding     $ 1.0   1.0              
Line of Credit Facility, Remaining Borrowing Capacity     $ 3,405.0   $ 3,405.0              
Line of Credit Facility, Remaining Borrowing Capacity, Percentage     97.30%   97.30%              
MPLX Term Loan [Member]                        
Debt Instrument [Line Items]                        
Line of Credit Facility, Maximum Borrowing Capacity $ 1,000.0                      
Debt Instrument, Description of Variable Rate Basis (i) the Adjusted LIBO Rate (as defined in the Term Loan Agreement) plus a margin ranging from 75.0 basis points to 100.0 basis points per annum, depending on MPLX’s credit ratings, or (ii) the Alternate Base Rate (as defined in the Term Loan Agreement).                      
Long-term Line of Credit     $ 0.0   $ 0.0         $ 1,000.0 1,000.0  
Repayments of Long-term Debt         1,000.0              
Payment for Debt Extinguishment or Debt Prepayment Cost         (1.0)              
Senior Notes [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross   $ 2,000.0                    
Percent of Par   100.00%                    
Senior Notes [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross                   $ 3,000.0    
Senior Notes [Member] | ANDX LP [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     121.0   121.0           190.0  
Senior Notes [Member] | MarkWest [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     23.0   23.0           23.0  
Senior Notes [Member] | Floating Rate Senior Notes Due September 2021 [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross   $ 1,000.0                    
Debt Instrument, Description of Variable Rate Basis   LIBOR plus 0.9 percent per annum                    
Payment for Debt Extinguishment or Debt Prepayment Cost         (3.0)              
Senior Notes [Member] | Floating Rate Senior Notes Due September 2022 [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross   $ 1,000.0                    
Debt Instrument, Description of Variable Rate Basis   LIBOR plus 1.1 percent per annum                    
Senior Notes [Member] | Senior Notes Due March 2026 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     $ 1,500.0   $ 1,500.0              
Debt Instrument, Interest Rate, Stated Percentage     1.75%   1.75%              
Percent of Par                   99.785%    
Senior Notes [Member] | Senior Notes Due August 2030 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     $ 1,500.0   $ 1,500.0              
Debt Instrument, Interest Rate, Stated Percentage     2.65%   2.65%              
Percent of Par                   99.913%    
Senior Notes [Member] | Senior Notes Due October 2022 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage     6.25%   6.25%              
Senior Notes [Member] | Senior Notes Due May 2024 [Member]                        
Debt Instrument [Line Items]                        
Payment for Debt Extinguishment or Debt Prepayment Cost         $ (18.0)              
Senior Notes [Member] | Senior Notes Due May 2024 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross               $ 450.0        
Percent of Par                 103.20%      
Senior Notes [Member] | Senior Notes Due May 2024 [Member] | ANDX LP [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross                 $ 69.0      
Debt Instrument, Interest Rate, Stated Percentage                 6.375%      
Fixed Rate Senior Notes [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     $ 19,506.0   19,506.0           16,887.0  
Variable Rate Senior Notes [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross     1,000.0   1,000.0           2,000.0  
Finance Lease [Member] | Marathon Pipe Line LLC [Member]                        
Debt Instrument [Line Items]                        
Finance Lease, Liability [1]     $ 12.0   $ 12.0           $ 19.0  
Minimum [Member] | Senior Notes [Member]                        
Debt Instrument [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage     1.75%   1.75%              
Maximum [Member] | Senior Notes [Member]                        
Debt Instrument [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage     6.25%   6.25%              
Subsequent Event | Senior Notes [Member] | Senior Notes Due October 2022 [Member] | MPLX LP                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross             $ 300.0          
Subsequent Event | Senior Notes [Member] | Senior Notes Due October 2022 [Member] | ANDX LP [Member]                        
Debt Instrument [Line Items]                        
Long-term Debt, Gross             $ 34.0          
Debt Premium [Member] | Senior Notes [Member] | Senior Notes Due May 2024 [Member]                        
Debt Instrument [Line Items]                        
Payment for Debt Extinguishment or Debt Prepayment Cost         $ (14.0)              
[1] See Note 20 for lease information.