Common Shares - Additional Information (Detail) - shares	9 Months Ended
	Sep. 30, 2018	Jun. 27, 2018	Jun. 13, 2018
Class of Stock [Line Items]
Normal course issuer bid share repurchase shares authorized		40,391,196
Percent of total shares			5.00%
Exxon Mobil Corporation's ownership interest in Imperial	69.60%
Normal course issuer bid share repurchase term, months	12 months