Leases - Components of Lease Expense (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating lease cost	$ 23	$ 45
Short-term lease cost	1	2
Variable lease cost	5	6
Sublease income	(1)	(1)
Finance lease cost, Amortization of right-of-use assets	3	5
Finance lease cost, Interest on lease liabilities	2	5
Total lease costs	$ 33	$ 62