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Common Shares - Additional Information (Detail) - shares
6 Months Ended
Jun. 30, 2018
Jun. 27, 2018
Jun. 13, 2018
Jun. 13, 2017
Class of Stock [Line Items]        
Exxon Mobil Corporation's ownership interest in Imperial 69.60%      
Normal course issuer bid share repurchase shares authorized 42,326,545 40,391,196    
Percent of total shares     5.00% 5.00%
Normal course issuer bid share repurchase term, months 12 months