Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 79		$ 79		$ 82
Term for expected payments to be made			10 years
Purchased services and other
Site Contingency [Line Items]
Environmental remediation expense	$ 1	$ 1	$ 2	$ 2