Leases - Supplemental Information Related to Leases (Details) - CAD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating cash outflows from operating leases	$ 20	$ 66
Operating cash outflows from finance leases	5	10
Financing cash outflows from finance leases	2	4
Right-of-use assets obtained in exchange for operating lease liabilities	8	31
Right-of-use assets obtained in exchange for finance lease liabilities	$ 0	$ 4