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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
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Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 6,807 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |  | 5,872 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
Canadian Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 460 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_CanadianEquitySecuritiesMember |  | 932 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_CanadianEquitySecuritiesMember |
Non-Canadian Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 2,153 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_NonCanadianEquitySecuritiesMember |  | 1,911 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_NonCanadianEquitySecuritiesMember |
Corporate Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 922 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 654 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember |
Government Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 3,033 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_GovernmentDebtSecuritiesMember |  | 2,161 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_GovernmentDebtSecuritiesMember |
Asset Backed Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AssetBackedSecuritiesMember |  |  |
Mortgage Funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value |  |  | 1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember |
Equities - Venture Capital |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 211 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_VentureCapitalEquitySecuritiesMember |  | 188 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_VentureCapitalEquitySecuritiesMember |
Cash |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total plan assets at fair value | 23 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember |  | 25 us-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 | 8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 12 us-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 | 8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 12 us-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 | 6,588 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 5,671 us-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 | 460 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_CanadianEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [1] | 932 us-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 | 2,153 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_NonCanadianEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [1] | 1,911 us-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 | 922 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [2] | 654 us-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 | 3,033 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_GovernmentDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [2] | 2,161 us-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 | 5 us-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 | 15 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [3] | 13 us-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 | 211 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 189 us-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 |  |  | 1 us-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 | 211 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = imo_VentureCapitalEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | [5] | 188 us-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.

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
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