Revenues - Contract Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Contract with Customer, Liability [Abstract]
Contract liability, opening balance	$ 74	$ 3	$ 2	$ 2
Revenue recognized that was included in the contract liability balance at the beginning of the period	(8)	(3)	(2)	(2)
Increases due to consideration received, excluding amounts recognized as revenue during the period	4	2	70	2
Contract liability, closing balance	$ 70	$ 2	$ 70	$ 2