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Effect of One Percent Change in Assumptions at Which Retirement Liabilities Could be Effectively Settled (Detail) (CAD)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]  
Effect on net benefit cost, before tax, One percent increase - Rate of return on plan assets (60)imo_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseInRateOfReturnOnPlanAssetsOnNetBenefitCost
Effect on net benefit cost, before tax, One percent increase - Discount rate (70)imo_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseInDiscountRateOnNetBenefitCost
Effect on benefit obligation, One percent increase - Discount rate (1,100)imo_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseInDiscountRateOnBenefitObligation
Effect on net benefit cost, before tax, One percent increase- Rate of pay increases 35imo_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseInRateOfPayIncreasesOnNetBenefitCost
Effect on benefit obligation, One percent increase - Rate of pay increases 190imo_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseInRateOfPayIncreasesOnBenefitObligation
Effect on net benefit cost, before tax, One percent decrease - Rate of return on plan assets 60imo_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseInRateOfReturnOnPlanAssetsOnNetBenefitCost
Effect on net benefit cost, before tax, One percent decrease - Discount rate 90imo_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseInDiscountRateOnNetBenefitCost
Effect on benefit obligation, One percent decrease - Discount rate 1,400imo_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseInDiscountRateOnBenefitObligation
Effect on net benefit cost, before tax, One percent decrease- Rate of pay increases (30)imo_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseInRateOfPayIncreasesOnPlanAssetsOnNetBenefitCost
Effect on benefit obligation, One percent decrease - Rate of pay increases (170)imo_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseInRateOfPayIncreasesOnBenefitObligation