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Income Taxes (Components Of Deferred Tax Assets And Liabilities) (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Deferred tax assets:    
Employee benefits $ 616us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits $ 483us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeBenefits
Environmental 54mpc_DeferredTaxAssetsEnvironmental 37mpc_DeferredTaxAssetsEnvironmental
Investments in subsidiaries and affiliates 24us-gaap_DeferredTaxAssetsInvestmentInSubsidiaries 0us-gaap_DeferredTaxAssetsInvestmentInSubsidiaries
Other 70us-gaap_DeferredTaxAssetsOther 49us-gaap_DeferredTaxAssetsOther
Total deferred tax assets 764us-gaap_DeferredTaxAssetsNet 569us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:    
Property, plant and equipment 2,411us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 2,290us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment
Inventories 614us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedInventoryCosts 614us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedInventoryCosts
Investments in subsidiaries and affiliates 0us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates 267us-gaap_DeferredTaxLiabilitiesInvestmentInNoncontrolledAffiliates
Other 101us-gaap_DeferredTaxLiabilitiesOther 70us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities 3,126us-gaap_DeferredIncomeTaxLiabilities 3,241us-gaap_DeferredIncomeTaxLiabilities
Net deferred tax liabilities $ 2,362us-gaap_DeferredTaxLiabilities $ 2,672us-gaap_DeferredTaxLiabilities