Contingencies - Environmental Liabilities (Details) - CAD CAD in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Site Contingency [Line Items]
Total amount provided for provisions for environmental remediation costs	CAD 89		CAD 85
Term for expected payments to be made	10 years
Purchased Services and Other [Member]
Site Contingency [Line Items]
Environmental remediation expense	CAD 1	CAD 1