Contingencies - Environmental Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	$ 80		$ 78
Term for expected payments to be made	10 years
Purchased Services and Other [Member]
Site Contingency [Line Items]
Environmental remediation expense	$ 1	$ 1