Leases Supplemental Information Related to Leases (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating cash outflows from operating leases	$ 74	$ 82
Operating cash outflows from finance leases	10	10
Financing cash outflows from finance leases	8	6
Right-of-use assets obtained in exchange for operating lease liabilities	34	38
Right-of-use assets obtained in exchange for finance lease liabilities	$ 4	$ 4