XML 122 R132.htm IDEA: XBRL DOCUMENT v2.4.1.9
Defined Benefit Pension And Other Postretirement Plans (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Feb. 01, 2013
Defined Benefit Plan Disclosure [Line Items]        
Accumulated benefit obligation $ 2,009us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation $ 1,912us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation    
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 86us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount 76us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount 60us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount  
Multiemployer pension plans percentage funded 65.00%mpc_MultiemployerPensionPlansPercentageFunded      
Marathon Petroleum's contributions as a percentage of total contributions to the multi-employer pension plan, maximum 5.00%mpc_MultiemployerPensionPlansPercentageContributionByEntity      
Multiemployer Plans, Pension        
Defined Benefit Plan Disclosure [Line Items]        
Number of multiemployer defined benefit pension or health and welfare plan 1mpc_NumberOfDefinedBenefitPensionOrOtherPostretirementOrHealthPlans
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPensionMember		      
Multi employer pension plans
There have been no significant changes that affect the comparability of 2014, 2013 and 2012 contributions.
      
Multiemployer Plans, Postretirement Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Number of multiemployer defined benefit pension or health and welfare plan 1mpc_NumberOfDefinedBenefitPensionOrOtherPostretirementOrHealthPlans
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		      
MPC contributions 6us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		 5us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		 5us-gaap_MultiemployerPlanPeriodContributions
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		  
Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation 51.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		      
Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation 49.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember		      
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2014 51us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		      
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2014 46us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		      
Pension contributions 16us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		      
Defined benefit plans, estimated future employer contributions in next fiscal year 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		      
Medical Post-65 | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed for the following year: 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mpc_MedicalBenefitsAgeLimitPostSixtyFiveYearsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Unfunded Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plans, estimated future employer contributions in next fiscal year 18us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember		      
Other Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Estimated net gain (loss) that will be amortized from accumulated other comprehensive loss in 2014 9us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		      
Estimated prior service cost that will be amortized from accumulated other comprehensive loss in 2014 4us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		      
Defined benefit plans, estimated future employer contributions in next fiscal year 27us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		      
Galveston Bay Refinery and Related Assets        
Defined Benefit Plan Disclosure [Line Items]        
Decrease in liabilities       (122)us-gaap_DefinedBenefitPlanPlanAmendments
/ us-gaap_BusinessAcquisitionAxis
= mpc_GalvestonBayRefineryAndRelatedAssetsMember
Galveston Bay Refinery and Related Assets | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Percentage increase in discount rates       0.20%mpc_DefinedBenefitPlanPercentageIncreaseInDiscountRate
/ us-gaap_BusinessAcquisitionAxis
= mpc_GalvestonBayRefineryAndRelatedAssetsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Galveston Bay Refinery and Related Assets | Other Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Liability related to retiree medical assumed at the acquisition date       $ 43us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestructuringLiabilities
/ us-gaap_BusinessAcquisitionAxis
= mpc_GalvestonBayRefineryAndRelatedAssetsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember