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Dispositions Discontinued Operations (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2014
Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]          
Proceeds from Sale of Oil and Gas Property and Equipment $ 3,760us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment $ 450us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment $ 467us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment    
Angola Blk 31 [Member]          
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]          
Assets of Disposal Group, Including Discontinued Operation, Current   41us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
Disposal Group, Including Discontinued Operation, Other Noncurrent Assets   1,647us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherNoncurrentAssets
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
Total assets   1,688us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperation
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
Liabilities of Disposal Group, Including Discontinued Operation, Current   25us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
Liabilities of Disposal Group, Including Discontinued Operation, Noncurrent   43us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
Total liabilities   68us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperation
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= mro_AngolaBlk31Member
     
ANGOLA          
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]          
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax 532us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
       
Disposal Group, Including Discontinued Operation, Revenue 58us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
361us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
0us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
   
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, before Income Tax 51us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
247us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
(17)us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
   
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax 426us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
0us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
0us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
   
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]          
Proceeds from Sale of Oil and Gas Property and Equipment       2,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
 
Interest Percentage       10.00%mro_InterestPercentage
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_AO
 
NORWAY          
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]          
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax         976us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
Disposal Group, Including Discontinued Operation, Revenue 1,981us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
3,176us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
3,726us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
   
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, before Income Tax 1,693us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
2,537us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
3,026us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
   
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax 1,406us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
0us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
0us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
   
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]          
Proceeds from Sale of Oil and Gas Property and Equipment         2,100us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO
Cash Transferred To Buyer         $ 589mro_CashTransferredToBuyer
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= country_NO