Segment Information (Income From Operations Attributable To Operating Segments) (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes	[1],[2]									$ 95,750
Sales and other operating revenue, including excise taxes	[2]										$ 74,104		$ 63,277
Sales to related parties	[3]									754	629		62
Income from operations		$ 2,017	$ 1,403	$ 1,711	$ 440	$ 1,168	$ 1,577	$ 982	$ 291	5,571	4,018		2,386
Income from equity method investments										373	306		(185)
Depreciation and amortization										2,490	2,114		2,001
Inventory market valuation adjustment										0	0		(370)
Refining & Marketing
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										68,939
Sales and other operating revenue, including excise taxes											52,761		43,167
Sales to related parties										746	621		61
Inventory market valuation adjustment	[4]												345
Retail
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										23,538
Sales and other operating revenue, including excise taxes											19,021		18,282
Sales to related parties										8	8		1
Inventory market valuation adjustment	[4]												25
Midstream
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										3,273
Sales and other operating revenue, including excise taxes											2,322		1,828
Sales to related parties										0	0		0
Reportable Segment [Member]
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										95,750
Sales and other operating revenue, including excise taxes											74,104		63,277
Intersegment Eliminations
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										16,307
Sales and other operating revenue, including excise taxes											12,756		11,854
Intersegment Eliminations | Refining & Marketing
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										12,914
Sales and other operating revenue, including excise taxes	[5]										11,309		10,589
Intersegment Eliminations | Retail
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										6
Sales and other operating revenue, including excise taxes	[5]										4		3
Intersegment Eliminations | Midstream
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										3,387
Sales and other operating revenue, including excise taxes	[5]										1,443		1,262
Operating Segments
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										112,811
Sales and other operating revenue, including excise taxes											87,489		75,193
Income from operations										6,261	4,389		3,138
Income from equity method investments	[6]									363	283		171
Depreciation and amortization	[6]									2,412	2,056		1,941
Segment capital expenditures and investments	[7]									4,147	2,968		2,915
Operating Segments | Refining & Marketing
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										82,599
Sales and other operating revenue, including excise taxes											64,691		53,817
Income from operations										2,481	2,321	[8]	1,357	[8]
Income from equity method investments										15	17		24
Depreciation and amortization										1,174	1,082		1,063
Segment capital expenditures and investments	[7]									1,057	832		1,054
Operating Segments | Retail
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										23,552
Sales and other operating revenue, including excise taxes											19,033		18,286
Income from operations										1,028	729	[8]	733	[8]
Income from equity method investments										74	69		5
Depreciation and amortization										353	275		273
Segment capital expenditures and investments	[7]									460	381		303
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Sales and other operating revenues, excluding consumer excise taxes										6,660
Sales and other operating revenue, including excise taxes											3,765		3,090
Income from operations										2,752	1,339		1,048
Income from equity method investments										274	197		142
Depreciation and amortization										885	699		605
Segment capital expenditures and investments	[7]									$ 2,630	$ 1,755		$ 1,558

[1] The 2018 period reflects an election to present certain taxes on a net basis concurrent with our adoption of ASC 606.

[2] The 2018 period reflects an election to present certain taxes on a net basis concurrent with our adoption of ASU 2014-09, Revenue - Revenue from Contracts with Customers (“ASC 606”). See Notes 2 and 3 for further information.

[3] Sales to related parties consists primarily of sales of refined products to PFJ Southeast, an equity affiliate which owns and operates travel plazas primarily in the Southeast region of the United States.

[4] 2017 includes MPC’s share of gains from the the sale of assets remaining from the canceled Sandpiper pipeline project. 2016 includes impairments of goodwill and equity method investments. See Note 17.

[5] Management believes intersegment transactions were conducted under terms comparable to those with unaffiliated parties.

[6] Differences between segment totals and MPC totals represent amounts related to unallocated items and are included in “Items not allocated to segments” in the reconciliation below

[7] Capital expenditures include changes in capital accruals and investments in affiliates

[8] In 2016, the Refining & Marketing and Retail segments include an inventory LCM benefit of $345 million and $25 million, respectively.