Miscellaneous Financial Information - Additional Information (Detail) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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	0	$ 281
Liabilities	17,271		$ 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