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Fair Value of Pension Plan Assets Including Level Within Fair Value Hierarchy (Detail) (CAD)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 6,807us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 5,872us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Canadian Equity Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 460us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_CanadianEquitySecuritiesMember
932us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_CanadianEquitySecuritiesMember
Non-Canadian Equity Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 2,153us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_NonCanadianEquitySecuritiesMember
1,911us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_NonCanadianEquitySecuritiesMember
Corporate Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 922us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CorporateDebtSecuritiesMember
654us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CorporateDebtSecuritiesMember
Government Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 3,033us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_GovernmentDebtSecuritiesMember
2,161us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_GovernmentDebtSecuritiesMember
Asset Backed Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 5us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Mortgage Funds    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value   1us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Equities - Venture Capital    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 211us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_VentureCapitalEquitySecuritiesMember
188us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_VentureCapitalEquitySecuritiesMember
Cash    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 23us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
25us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
Quoted Prices In Active Markets For Identical Assets (Level 1)    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
12us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Quoted Prices In Active Markets For Identical Assets (Level 1) | Cash    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
12us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Significant Other Observable Inputs (Level 2)    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 6,588us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
5,671us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant Other Observable Inputs (Level 2) | Canadian Equity Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 460us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_CanadianEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[1] 932us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_CanadianEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[1]
Significant Other Observable Inputs (Level 2) | Non-Canadian Equity Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 2,153us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_NonCanadianEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[1] 1,911us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_NonCanadianEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[1]
Significant Other Observable Inputs (Level 2) | Corporate Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 922us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[2] 654us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[2]
Significant Other Observable Inputs (Level 2) | Government Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 3,033us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_GovernmentDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[2] 2,161us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_GovernmentDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[2]
Significant Other Observable Inputs (Level 2) | Asset Backed Debt Securities    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 5us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[2]  
Significant Other Observable Inputs (Level 2) | Cash    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 15us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[3] 13us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
[3]
Significant Unobservable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 211us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
189us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Significant Unobservable Inputs (Level 3) | Mortgage Funds    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value   1us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
[4]
Significant Unobservable Inputs (Level 3) | Equities - Venture Capital    
Defined Benefit Plan Disclosure [Line Items]    
Total plan assets at fair value 211us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_VentureCapitalEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
[5] 188us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= imo_VentureCapitalEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
[5]
[1] For company equity securities held in the form of fund units that are redeemable at the measurement date, the unit value is treated as a Level 2 input. The fair value of the securities owned by the funds is based on observable quoted prices on active exchanges, which are Level 1 inputs.
[2] For corporate, government and asset-backed debt securities, fair value is based on observable inputs of comparable market transactions.
[3] For cash balances that are held in Level 2 funds prior to investment in those fund units, the cash value is treated as a Level 2 input.
[4] For mortgage funds, fair value represents the principal outstanding which is guaranteed by Canada Mortgage and Housing Corporation.
[5] For venture capital partnership investments, fair value is generally established by using revenue or earnings multiples or other relevant market data including Initial Public Offerings.