Debt (Oustanding Borrowings) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Capital lease obligations due 2012-2027	$ 342	[1]	$ 299	[1]
Total	3,342		3,299
Unamortized discount	(10)		(11)
Fair value adjustments	17	[2]	19	[2]
Amounts due within one year	(20)		(15)
Total long-term debt due after one year	3,329		3,292
Revolving Credit Agreement due 2017 [Member]
Debt Instrument [Line Items]
Borrowings outstanding at period end
Revolving Credit Agreement due 2017 [Member] | MPLX Operations LLC [Member]
Debt Instrument [Line Items]
Borrowings outstanding at period end
Revolving Credit Agreement due 2017 [Member]
Debt Instrument [Line Items]
Borrowings outstanding at period end
Senior Notes [Member] | 3.500% Senior Notes Due March 1, 2016 [Member]
Debt Instrument [Line Items]
Senior notes	750		750
Senior Notes [Member] | 5.125% Senior Notes Due March 1, 2021 [Member]
Debt Instrument [Line Items]
Senior notes	1,000		1,000
Senior Notes [Member] | 6.500% Senior Notes Due March 1, 2041 [Member]
Debt Instrument [Line Items]
Senior notes	1,250		1,250
Trade Receivables Securitization [Member]
Debt Instrument [Line Items]
Trade receivables securitization facility due 2014

[1] These obligations as of September 30, 2012 include $147 million related to assets under construction at that date for which a capital lease will commence upon completion of construction. The amounts currently reported are based upon the percent of construction completed as of September 30, 2012 and therefore do not reflect future lease obligations of $166 million related to the assets.

[2] See Notes 14 and 15 for information on interest rate swaps.