Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 81		$ 81		$ 78
Term for expected payments to be made			10 years
Purchased Services and Other [Member]
Site Contingency [Line Items]
Environmental remediation expense	$ 1	$ 1	$ 2	$ 2