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Acquisition - Additional Information (Detail) (Celtic, CAD)
0 Months Ended 12 Months Ended
Feb. 26, 2013
Dec. 31, 2013
Feb. 26, 2013
Celtic
     
Business Acquisition [Line Items]      
Consideration paid for acquisition 1,600,000,000us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= imo_CelticExplorationLtdMember
   
Ownership interest acquired 50.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= imo_CelticExplorationLtdMember
  50.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= imo_CelticExplorationLtdMember
Revenues   89,000,000us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual
/ us-gaap_BusinessAcquisitionAxis
= imo_CelticExplorationLtdMember