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Debt - Outstanding Borrowings (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Feb. 08, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Commercial paper $ 0    
Total 17,807   $ 13,418
Unamortized debt issuance costs (109)   (59)
Unamortized discount [1] (440)   (413)
Amounts due within one year (26)   (624)
Total long-term debt due after one year 17,232   12,322
MPLX LP      
Debt Instrument [Line Items]      
Amounts due within one year (1)   (1)
Total long-term debt due after one year 11,861   6,945
Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 5,500  
Senior Notes | MarkWest | MPLX LP      
Debt Instrument [Line Items]      
Unamortized discount (362)   (374)
Trade receivables securitization facility due July 2019      
Debt Instrument [Line Items]      
Long-term debt, gross 0    
MPLX bank revolving credit facility due 2022 | Line of Credit | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross 0    
MPLX senior notes, 3.375%, due March 2023 | Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 500  
Debt instrument, interest rate   3.375%  
MPLX senior notes, 4.000%, due March 2028 | Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 1,250  
Debt instrument, interest rate   4.00%  
MPLX senior notes, 4.500%, due April 2038 | Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 1,750  
Debt instrument, interest rate   4.50%  
MPLX senior notes, 4.700%, due April 2048 | Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 1,500  
Debt instrument, interest rate   4.70%  
MPLX senior notes, 4.900%, due April 2058 | Senior Notes | MPLX LP      
Debt Instrument [Line Items]      
Long-term debt, gross   $ 500  
Debt instrument, interest rate   4.90%  
Marathon Petroleum Corporation      
Debt Instrument [Line Items]      
Commercial paper 0   0
Marathon Petroleum Corporation | Capital Lease Obligations      
Debt Instrument [Line Items]      
Capital lease obligations $ 350   356
Debt instrument maturity year, start Jan. 01, 2018    
Debt instrument maturity year, end Mar. 31, 2033    
Marathon Petroleum Corporation | 364-day bank revolving credit facility due July 2018 | Line of Credit      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   0
Line of credit facility, expiration date Jul. 20, 2018    
Marathon Petroleum Corporation | Trade receivables securitization facility due July 2019 | Secured Debt      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   0
Line of credit facility, expiration date Jul. 19, 2019    
Marathon Petroleum Corporation | Bank revolving credit facility due 2022 | Line of Credit      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   0
Line of credit facility, expiration date Jul. 21, 2022    
Marathon Petroleum Corporation | Senior notes, 2.700% due December 2018 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   600
Debt instrument, maturity date Dec. 14, 2018    
Debt instrument, interest rate 2.70%    
Marathon Petroleum Corporation | Senior notes, 3.400% due December 2020 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 650   650
Debt instrument, maturity date Dec. 15, 2020    
Debt instrument, interest rate 3.40%    
Marathon Petroleum Corporation | Senior notes, 5.125% due March 2021 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,000   1,000
Debt instrument, maturity date Mar. 01, 2021    
Debt instrument, interest rate 5.125%    
Marathon Petroleum Corporation | Senior notes, 3.625%, due September 2024 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 750   750
Debt instrument, maturity date Sep. 15, 2024    
Debt instrument, interest rate 3.625%    
Marathon Petroleum Corporation | Senior notes, 6.500%, due March 2041 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,250   1,250
Debt instrument, maturity date Mar. 01, 2041    
Debt instrument, interest rate 6.50%    
Marathon Petroleum Corporation | Senior notes, 4.750%, due September 2044 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 800   800
Debt instrument, maturity date Sep. 15, 2044    
Debt instrument, interest rate 4.75%    
Marathon Petroleum Corporation | Senior notes, 5.850% due December 2045 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 250   250
Debt instrument, maturity date Dec. 15, 2045    
Debt instrument, interest rate 5.85%    
Marathon Petroleum Corporation | Senior notes, 5.000%, due September 2054 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 400   400
Debt instrument, maturity date Sep. 15, 2054    
Debt instrument, interest rate 5.00%    
MPLX LP | Senior Notes | MarkWest      
Debt Instrument [Line Items]      
Long-term debt, gross $ 63   63
MPLX LP | Capital Lease Obligations      
Debt Instrument [Line Items]      
Capital lease obligations $ 7   7
Debt instrument maturity year, start Jan. 01, 2018    
Debt instrument maturity year, end Mar. 31, 2020    
MPLX LP | MPLX 364-day term loan | Unsecured Debt      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   0
Line of credit facility, expiration date Dec. 31, 2018    
MPLX LP | MPLX term loan facility due 2019 | Unsecured Debt      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   0
Line of credit facility, expiration date Nov. 20, 2019    
MPLX LP | MPLX bank revolving credit facility due 2022 | Line of Credit      
Debt Instrument [Line Items]      
Long-term debt, gross $ 0   505
Line of credit facility, expiration date Jul. 21, 2022    
MPLX LP | MPLX senior notes, 5.500%, due February 2023 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 710   710
Debt instrument, maturity date Feb. 15, 2023    
Debt instrument, interest rate 5.50%    
MPLX LP | MPLX senior notes, 5.500%, due February 2023 | Senior Notes | MarkWest      
Debt Instrument [Line Items]      
Debt instrument, maturity date Feb. 15, 2023    
Debt instrument, interest rate 5.50%    
MPLX LP | MPLX senior notes, 3.375%, due March 2023 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 500   0
Debt instrument, maturity date Mar. 15, 2023    
Debt instrument, interest rate 3.375%    
MPLX LP | MPLX senior notes, 4.500%, due July 2023 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 989   989
Debt instrument, maturity date Jul. 15, 2023    
Debt instrument, interest rate 4.50%    
MPLX LP | MPLX senior notes, 4.500%, due July 2023 | Senior Notes | MarkWest      
Debt Instrument [Line Items]      
Debt instrument, maturity date Jul. 15, 2023    
Debt instrument, interest rate 4.50%    
MPLX LP | MPLX senior notes, 4.875%, due December 2024 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,149   1,149
Debt instrument, maturity date Dec. 01, 2024    
Debt instrument, interest rate 4.875%    
MPLX LP | MPLX senior notes, 4.875%, due December 2024 | Senior Notes | MarkWest      
Debt Instrument [Line Items]      
Debt instrument, maturity date Dec. 01, 2024    
Debt instrument, interest rate 4.875%    
MPLX LP | MPLX senior notes, 4.000%, due February 2025 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 500   500
Debt instrument, maturity date Feb. 15, 2025    
Debt instrument, interest rate 4.00%    
MPLX LP | MPLX senior notes, 4.875%, due June 2025 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,189   1,189
Debt instrument, maturity date Jun. 01, 2025    
Debt instrument, interest rate 4.875%    
MPLX LP | MPLX senior notes, 4.875%, due June 2025 | Senior Notes | MarkWest      
Debt Instrument [Line Items]      
Debt instrument, maturity date Jun. 01, 2025    
Debt instrument, interest rate 4.875%    
MPLX LP | MPLX senior notes, 4.125%, due March 2027 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,250   1,250
Debt instrument, maturity date Mar. 01, 2027    
Debt instrument, interest rate 4.125%    
MPLX LP | MPLX senior notes, 4.000%, due March 2028 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,250   0
Debt instrument, maturity date Mar. 15, 2028    
Debt instrument, interest rate 4.00%    
MPLX LP | MPLX senior notes, 4.500%, due April 2038 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,750   0
Debt instrument, maturity date Apr. 15, 2038    
Debt instrument, interest rate 4.50%    
MPLX LP | MPLX senior notes, 5.200%, due March 2047 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,000   1,000
Debt instrument, maturity date Mar. 01, 2047    
Debt instrument, interest rate 5.20%    
MPLX LP | MPLX senior notes, 4.700%, due April 2048 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 1,500   0
Debt instrument, maturity date Apr. 15, 2048    
Debt instrument, interest rate 4.70%    
MPLX LP | MPLX senior notes, 4.900%, due April 2058 | Senior Notes      
Debt Instrument [Line Items]      
Long-term debt, gross $ 500   $ 0
Debt instrument, maturity date Apr. 15, 2058    
Debt instrument, interest rate 4.90%    
[1] Includes $362 million and $374 million of unamortized discount as of March 31, 2018 and December 31, 2017, respectively, related to the difference between the fair value and the principal amount of assumed MarkWest debt.