Income Taxes - Reconciliation of Uncertain Tax Positions Related to Unrecognized Tax Benefits (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Uncertain Tax Positions to Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits at January 1	CAD 13	CAD 15	CAD 17
Increase in unrecognized:
Tax benefits related to the current year	0	0	4
Dispositions:
Gross uncertain tax benefits related to prior years	0	(2)	(6)
Unrecognized tax benefits at December 31	CAD 13	CAD 13	CAD 15