Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 82		$ 82		$ 77
Term for expected payments to be made			10 years
Purchased services and other
Site Contingency [Line Items]
Environmental remediation expense	$ 3	$ 1	$ 4	$ 2