Other Long-Term Obligations (Parenthetical) (Detail) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Schedule of Other Liabilities [Line Items]
Employee retirement benefit obligations in current liabilities	$ 58	$ 58
Asset retirement obligations and other environmental liabilities in current liabilities	100	100
Undiscounted Commitments	30	27
Accounts Payable and Accrued Liabilities [Member]
Schedule of Other Liabilities [Line Items]
Lease liability due within one year, Operating leases	$ 82	$ 97