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Transactions With related Parties - Additional Information (Detail) (CAD)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Rail Loading Joint Venture    
Related Party Transaction [Line Items]    
Outstanding obligations due to a rail loading joint venture 123us-gaap_DueToRelatedPartiesNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= imo_RailLoadingJointVentureMember		   
Long-Term Loans    
Related Party Transaction [Line Items]    
Outstanding loans from an affiliate 4,746imo_OutstandingLoansFromAffiliate
/ us-gaap_DebtInstrumentAxis
= imo_LongTermLoansMember		 4,316imo_OutstandingLoansFromAffiliate
/ us-gaap_DebtInstrumentAxis
= imo_LongTermLoansMember
Short-Term Loans    
Related Party Transaction [Line Items]    
Outstanding loans from an affiliate 75imo_OutstandingLoansFromAffiliate
/ us-gaap_DebtInstrumentAxis
= imo_ShortTermLoansMember		 75imo_OutstandingLoansFromAffiliate
/ us-gaap_DebtInstrumentAxis
= imo_ShortTermLoansMember