Leases Components of Lease Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease cost	$ 83	$ 89
Short-term lease cost	10	10
Variable lease cost	13	13
Sublease Income	(3)	(3)
Finance lease, Amortization of right-of-use assets	9	9
Finance lease, Interest on lease liabilities	11	11	$ 11
Total lease costs	$ 123	$ 129