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Commitments and Contingencies (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Payments to Acquire Property, Plant, and Equipment [Abstract]    
Commitments to acquire property, plant and equipment $ 747mro_CommitmentsToAcquirePropertyPlantAndEquipment $ 1,270mro_CommitmentsToAcquirePropertyPlantAndEquipment
Water abatement Oil Sands [Member]    
Site Contingency [Line Items]    
Site Contingency, Accrual, Undiscounted Amount 24us-gaap_AccrualForEnvironmentalLossContingenciesGross
/ us-gaap_EnvironmentalRemediationContingencyAxis
= mro_WaterAbatementOilSandsMember		  
Other Guarantee [Member]    
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 57us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= mro_OtherGuaranteeMember