Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 81		$ 80
Term for expected payments to be made	10 years
Purchased services and other
Site Contingency [Line Items]
Environmental remediation expense	$ 2	$ 1