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Debt (Outstanding Borrowings) (Detail) - USD ($) $ in Millions		12 Months Ended
	Jul. 20, 2016	Dec. 31, 2018	Oct. 02, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total		$ 27,980		$ 13,418
Unamortized debt issuance costs		(128)		(59)
Unamortized (discount) premium, net		(328)		(413)
Amounts due within one year		(544)		(624)
Total long-term debt due after one year		26,980		12,322
Trade receivables securitization facility due July 2019
Debt Instrument [Line Items]
Line of credit facility, expiration date	Jul. 19, 2019
Marathon Petroleum Corporation
Debt Instrument [Line Items]
Commercial paper		0		0
Notes payable		11		0
Marathon Petroleum Corporation \| Line of Credit \| 364-day bank revolving credit facility due September 2019
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		0
Line of credit facility, expiration date		Sep. 30, 2019
Marathon Petroleum Corporation \| Line of Credit \| Bank revolving credit facility due October 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		0
Line of credit facility, expiration date		Oct. 01, 2023
Marathon Petroleum Corporation \| Secured Debt \| Trade receivables securitization facility due July 2019
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		0
Line of credit facility, expiration date		Jul. 19, 2019
Marathon Petroleum Corporation \| Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross			$ 2,905
Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 2.700% due December 2018
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 \| Senior notes, 3.400% due December 2020
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 \| Senior notes, 5.125% due March 2021
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 \| Senior notes, 4.750% due December 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 614	$ 614	0
Debt instrument, maturity date		Dec. 15, 2023
Debt instrument, interest rate		4.75%	4.75%
Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 5.125% due April 2024
Debt Instrument [Line Items]
Long-term debt, gross		$ 241	$ 241	0
Debt instrument, maturity date		Apr. 01, 2024
Debt instrument, interest rate		5.125%	5.125%
Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 3.625%, due September 2024
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 \| Senior notes, 5.125% due December 2026
Debt Instrument [Line Items]
Long-term debt, gross		$ 719	$ 719	0
Debt instrument, maturity date		Dec. 15, 2026
Debt instrument, interest rate		5.125%	5.125%
Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 3.800% due April 2028
Debt Instrument [Line Items]
Long-term debt, gross		$ 496	$ 496	0
Debt instrument, maturity date		Apr. 01, 2028
Debt instrument, interest rate		3.80%	3.80%
Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 6.500% due March 2041
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 \| Senior notes, 4.750% due September 2044
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 \| Senior notes, 5.850% due December 2045
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 \| Senior notes, 5.000%, due September 2054
Debt Instrument [Line Items]
Long-term debt, gross		$ 400		400
Debt instrument, maturity date		Sep. 15, 2054
Debt instrument, interest rate		5.00%
Marathon Petroleum Corporation \| Senior Notes \| MPLX senior notes, 4.700% due April 2048
Debt Instrument [Line Items]
Long-term debt, gross		$ 498	$ 498	0
Debt instrument, maturity date		Apr. 01, 2048
Debt instrument, interest rate		4.50%	4.50%
Marathon Petroleum Corporation \| Senior Notes \| ANDX senior notes, 6.250% due October 2022
Debt Instrument [Line Items]
Long-term debt, gross		$ 337	$ 337	0
Debt instrument, maturity date		Oct. 01, 2022
Debt instrument, interest rate		5.375%	5.375%
Marathon Petroleum Corporation \| Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital lease obligations		$ 629		356
Debt instrument maturity year, start		Jan. 01, 2019
Debt instrument maturity year, end		Mar. 31, 2033
MPLX \| Line of Credit \| MPLX bank revolving credit facility due 2022
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		505
Line of credit facility, expiration date		Jul. 21, 2022
MPLX \| Senior Notes \| MPLX senior notes, 5.500% due February 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		710
Debt instrument, maturity date		Feb. 15, 2023
Debt instrument, interest rate		5.50%
MPLX \| Senior Notes \| MPLX senior notes, 3.375% due March 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		0
Debt instrument, maturity date		Mar. 15, 2023
Debt instrument, interest rate		3.375%
MPLX \| Senior Notes \| MPLX senior notes, 4.500% due July 2023
Debt Instrument [Line Items]
Long-term debt, gross		$ 989		989
Debt instrument, maturity date		Jul. 15, 2023
Debt instrument, interest rate		4.50%
MPLX \| Senior Notes \| MPLX senior notes, 4.875% due December 2024
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 \| Senior Notes \| MPLX senior notes, 4.000% due February 2025
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		500
Debt instrument, maturity date		Feb. 15, 2025
Debt instrument, interest rate		4.00%
MPLX \| Senior Notes \| MPLX senior notes, 4.875% due June 2025
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 \| Senior Notes \| MPLX senior notes, 4.125% due March 2027
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 \| Senior Notes \| MPLX senior notes, 4.000% due March 2028
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 \| Senior Notes \| MPLX senior notes, 4.800% due February 2029
Debt Instrument [Line Items]
Long-term debt, gross		$ 750		0
Debt instrument, maturity date		Feb. 15, 2019
Debt instrument, interest rate		4.80%
MPLX \| Senior Notes \| MPLX senior notes, 4.500% due April 2038
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 \| Senior Notes \| MPLX senior notes, 5.200% due March 2047
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 \| Senior Notes \| MPLX senior notes, 4.700% due April 2048
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 \| Senior Notes \| MPLX senior notes, 5.500% due February 2049
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,500		0
Debt instrument, maturity date		Feb. 15, 2049
Debt instrument, interest rate		5.50%
MPLX \| Senior Notes \| MPLX senior notes, 4.900% due April 2058
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		0
Debt instrument, maturity date		Apr. 15, 2058
Debt instrument, interest rate		4.90%
MPLX \| Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital lease obligations		$ 6		7
Debt instrument maturity year, start		Jan. 01, 2019
Debt instrument maturity year, end		Mar. 31, 2020
ANDX \| Line of Credit \| ANDX revolving and dropdown credit facilities due 2021
Debt Instrument [Line Items]
Long-term debt, gross		$ 1,245		0
Line of credit facility, expiration date		Jan. 29, 2021
ANDX \| Senior Notes \| ANDX senior notes, 5.500% due October 2019
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		0
Debt instrument, maturity date		Oct. 01, 2019
Debt instrument, interest rate		5.50%
ANDX \| Senior Notes \| ANDX senior notes, 6.250% due October 2022
Debt Instrument [Line Items]
Long-term debt, gross		$ 300		0
Debt instrument, maturity date		Oct. 15, 2022
Debt instrument, interest rate		6.25%
ANDX \| Senior Notes \| ANDX senior notes, 3.500% due December 2022
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		0
Debt instrument, maturity date		Dec. 01, 2022
Debt instrument, interest rate		3.50%
ANDX \| Senior Notes \| ANDX senior notes, 6.375% due May 2024
Debt Instrument [Line Items]
Long-term debt, gross		$ 450		0
Debt instrument, maturity date		May 01, 2024
Debt instrument, interest rate		6.375%
ANDX \| Senior Notes \| ANDX senior notes, 5.250% due January 2025
Debt Instrument [Line Items]
Long-term debt, gross		$ 750		0
Debt instrument, maturity date		Jan. 15, 2025
Debt instrument, interest rate		5.25%
ANDX \| Senior Notes \| ANDX senior notes, 4.250% due December 2027
Debt Instrument [Line Items]
Long-term debt, gross		$ 750		0
Debt instrument, maturity date		Dec. 01, 2027
Debt instrument, interest rate		4.25%
ANDX \| Senior Notes \| ANDX senior notes, 5.200% due December 2047
Debt Instrument [Line Items]
Long-term debt, gross		$ 500		0
Debt instrument, maturity date		Dec. 01, 2047
Debt instrument, interest rate		5.20%
ANDX \| Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital lease obligations		$ 15		0
Andeavor \| Marathon Petroleum Corporation \| Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 469		0
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 4.750% due December 2023
Debt Instrument [Line Items]
Debt instrument, maturity date		Dec. 15, 2023
Debt instrument, interest rate		4.75%
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 5.125% due April 2024
Debt Instrument [Line Items]
Debt instrument, maturity date		Apr. 01, 2024
Debt instrument, interest rate		5.125%
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 5.125% due December 2026
Debt Instrument [Line Items]
Debt instrument, maturity date		Dec. 15, 2026
Debt instrument, interest rate		5.125%
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| Senior notes, 3.800% due April 2028
Debt Instrument [Line Items]
Debt instrument, maturity date		Apr. 01, 2028
Debt instrument, interest rate		3.80%
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| MPLX senior notes, 4.700% due April 2048
Debt Instrument [Line Items]
Debt instrument, maturity date		Apr. 01, 2048
Debt instrument, interest rate		4.50%
Andeavor \| Marathon Petroleum Corporation \| Senior Notes \| ANDX senior notes, 6.250% due October 2022
Debt Instrument [Line Items]
Debt instrument, maturity date		Oct. 01, 2022
Debt instrument, interest rate		5.375%
MarkWest \| MPLX \| Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 23		$ 63
MarkWest \| MPLX \| Senior Notes \| MPLX senior notes, 5.500% due February 2023
Debt Instrument [Line Items]
Debt instrument, maturity date		Feb. 15, 2023
Debt instrument, interest rate		5.50%
MarkWest \| MPLX \| Senior Notes \| MPLX senior notes, 4.500% due July 2023
Debt Instrument [Line Items]
Debt instrument, maturity date		Jul. 15, 2023
Debt instrument, interest rate		4.50%
MarkWest \| MPLX \| Senior Notes \| MPLX senior notes, 4.875% due December 2024
Debt Instrument [Line Items]
Debt instrument, maturity date		Dec. 01, 2024
Debt instrument, interest rate		4.875%
MarkWest \| MPLX \| Senior Notes \| MPLX senior notes, 4.875% due June 2025
Debt Instrument [Line Items]
Debt instrument, maturity date		Jun. 01, 2025
Debt instrument, interest rate		4.875%