Miscellaneous Financial Information - Additional Information (Detail) - CAD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Additional Financial Information [Line Items]
Decrease in Long Term Purchase Commitment Amount		$ 2,900
Inventory write-down to current market value			$ (281)	$ 281
Liabilities	$ 18,170				$ 18,170		$ 16,613
Upstream [Member]
Additional Financial Information [Line Items]
Non-cash impairment of goodwill				20
Inventory write-down to current market value				229
Downstream [Member]
Additional Financial Information [Line Items]
Inventory write-down to current market value				$ 52
Accounts payable and accrued liabilities
Additional Financial Information [Line Items]
Liabilities		$ 62