Leases - Supplemental Information Related to Leases (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating cash outflows from operating leases	$ 18	$ 46
Operating cash outflows from finance leases	2	5
Financing cash outflows from finance leases	1	2
Right-of-use assets obtained in exchange for operating lease liabilities	14	23
Right-of-Use asset obtained in exchange for finance lease liabilities	$ 4	$ 4