Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 83		$ 77
Term for expected payments to be made	10 years
Purchased services and other
Site Contingency [Line Items]
Environmental remediation expense	$ 1	$ 1