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Share-Based Incentive Compensation Programs - Additional Information (Detail) (CAD) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Compensation expense charged against income | 90 us-gaap_AllocatedShareBasedCompensationExpense | 92 us-gaap_AllocatedShareBasedCompensationExpense | 58 us-gaap_AllocatedShareBasedCompensationExpense
Income tax benefit recognized in income related to compensation expense | 31 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 33 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 20 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Cash payment for compensation expense | 94 us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards | 88 us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards | 97 us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
Restricted Stock Unit Plan 1 And 2 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of consecutive trading days of share price to average prior to exercise date | 5 days |  |
Restricted Stock Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Before-tax unrecognized compensation expense related to non-vested restricted stock | 191 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember |  |
Weighted average vesting period of nonvested restricted stock | 3 years 6 months |  |
Restricted Stock Units | Restricted Stock Unit Plan 1 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Percentage restricted stock units exercisable three years following grant date | 50.00%imo_PercentageOfSharesThatBecomeExercisableInThreeYear / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_StatementEquityComponentsAxis = imo_RestrictedStockUnitPlanOneMember |  |
Percentage restricted stock units exercisable seven years following grant date | 50.00%imo_PercentageOfSharesThatBecomeExercisableInSevenYear / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_StatementEquityComponentsAxis = imo_RestrictedStockUnitPlanOneMember |  |
Restricted Stock Units | Restricted Stock Unit Plan 2 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Percentage restricted stock units exercisable five years following grant date | 50.00%imo_PercentageRestrictedStockUnitsExercisableInFiveYears / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_StatementEquityComponentsAxis = imo_RestrictedStockUnitPlanTwoMember |  |
Percentage restricted stock units exercisable ten years following grant date | 50.00%imo_PercentageRestrictedStockUnitsExercisablePeriodTenYearsOrRetirement / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_StatementEquityComponentsAxis = imo_RestrictedStockUnitPlanTwoMember |  |

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- Definition Number of consecutive trading days of share price to average prior to exercise date + References No definition available. + Details
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- Definition Percentage of restricted stock units that can be exercised in seven years following the grant date. + References No definition available. + Details
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- Definition Percentage of the restricted stock unites that are exercisable three years after the grant date (remainder are exercisable seven years after the grant date). + References No definition available. + Details
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- Definition Percentage of restricted stock units that can be exercised in five years following the grant date. + References No definition available. + Details
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- Definition Percentage of restricted stock units that become exercisable in the later of ten years following the grant date or the retirement date of the recipient. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (k) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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