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Gains and Losses on Asset Sales (Parenthetical) (Detail) (CAD)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Investment And Other Income [Line Items]      
Gain/(loss) on asset sales, before tax 696us-gaap_GainLossOnDispositionOfAssets1 [1],[2] 150us-gaap_GainLossOnDispositionOfAssets1 [1],[2] 94us-gaap_GainLossOnDispositionOfAssets1 [1],[2]
Gain/(loss) on asset sales, after tax 526imo_NetGainLossOnAssetSalesAfterTax [1],[2] 120imo_NetGainLossOnAssetSalesAfterTax [1],[2] 72imo_NetGainLossOnAssetSalesAfterTax [1],[2]
Conventional Assets      
Investment And Other Income [Line Items]      
Gain/(loss) on asset sales, before tax 638us-gaap_GainLossOnDispositionOfAssets1
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= imo_ConventionalAssetsMember		    
Gain/(loss) on asset sales, after tax 478imo_NetGainLossOnAssetSalesAfterTax
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= imo_ConventionalAssetsMember		    
Non-Operating Assets      
Investment And Other Income [Line Items]      
Gain/(loss) on asset sales, before tax 85us-gaap_GainLossOnDispositionOfAssets1
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= imo_NonOperatingAssetsMember		    
Gain/(loss) on asset sales, after tax 73imo_NetGainLossOnAssetSalesAfterTax
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= imo_NonOperatingAssetsMember		    
[1] 2014 included a gain of $638 million ($478 million, after tax) for the sale of the company's interest in producing conventional assets located in Boundary Lake, Cynthia/West Pembina and Rocky Mountain House.
[2] 2013 included a gain of $85 million ($73 million, after tax) for the sale of non-operating assets.