Net Interest Expense (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Income (Expense), Net [Abstract]
Interest cost	CAD 491	CAD 497	CAD 409
Interest capitalized to Properties	(16)	(25)	(14)
Interest expense	475	472	395
Interest income	(2)	(1)	(1)
Net interest expense	CAD 473	CAD 471	CAD 394