Defined Benefit Pension And Other Postretirement Plans (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 1,912	$ 2,035
Defined benefit plan investment goals	The overall expected long-term return on plan assets assumption is determined based on an asset rate-of-return modeling tool developed by a third-party investment group. The tool utilizes underlying assumptions based on actual returns by asset category and inflation and takes into account our asset allocation to derive an expected long-term rate of return on those assets. Capital market assumptions reflect the long-term capital market outlook. The assumptions for equity and fixed income investments are developed using a building-block approach, reflecting observable inflation information and interest rate information available in the fixed income markets. Long-term assumptions for other asset categories are based on historical results, current market characteristics and the professional judgment of our internal and external investment teams.
Defined Benefit Plan, Diversification	The asset allocation strategy will change over time in response to changes primarily in funded status, which is dictated by current and anticipated market conditions, the independent actions of our investment committee, required cash flows to and from the plans and other factors deemed appropriate. Such changes in asset allocation are intended to allocate additional assets to the fixed income asset class should the funded status improve. The fixed income asset class shall be invested in such a manner that its interest rate sensitivity correlates highly with that of the plans' liabilities. Other asset classes are intended to provide additional return with associated higher levels of risk.
Plan investment policies and strategies	Investment performance and risk is measured and monitored on an ongoing basis through quarterly investment meetings and periodic asset and liability studies.
Contributions to defined contribution plans	76	60	60
Multiemployer Plans, Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
The Company's contributions to multiemployer plans	3	4	3
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation	62.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Targeted asset allocation	38.00%
Pension And Postretirement Healthcare Plan [Member] | Galveston Bay Refinery And Related Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in laibilities	(122)
Percentage increase in discount rate	0.20%
Liability related to retiree medical assumed at the acquisition date	43
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in benefit liabilities		537
Increase in retiree medical liabilities resulting from a reduction in discount rates	183	(117)
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2014	52
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2014	46
Pension Contributions	161
Pension Benefits [Member] | Funded Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plans, estimated future employer contributions in next fiscal year	0
Pension Benefits [Member] | Unfunded Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plans, estimated future employer contributions in next fiscal year	18
Health Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in benefit liabilities		40
Increase in retiree medical liabilities resulting from a reduction in discount rates		57
Health Plan [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum increase in tax-free contributions to health reimbursement account (in percentage)		4.00%
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase in retiree medical liabilities resulting from a reduction in discount rates	(17)	(53)
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2014	3
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2014	4
Other Benefits [Member] | Unfunded Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plans, estimated future employer contributions in next fiscal year	26
Other Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Multi employer pension plans	There have been no significant changes that affect the comparability of 2013, 2012 and 2011 contributions.
Other Pension Plans [Member] | Multiemployer Plans, Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of multiemployer defined benefit pension or health and welfare plan		1
Other Pension Plans [Member] | Maximum [Member] | Multiemployer Plans, Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded percentage of plans in red zone	65.00%
Marathon Petroleum's contributions as a percentage of total contributions to the multi-employer pension plan, maximum	5.00%
Health Plans [Member] | Multiemployer Health And Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of multiemployer defined benefit pension or health and welfare plan		1
The Company's contributions to multiemployer plans	$ 5	$ 5	$ 4