Leases Maturities of Lease Liabilities (Details) $ in Millions	Dec. 31, 2020 CAD ($)
Leases [Abstract]
Finance Leases, Payments due 2021	$ 11
Finance Leases, Payments due 2022	107
Finance Leases, Payments due 2023	9
Finance Leases, Payments due 2024	8
Finance Leases, Payments due 2025	8
Finance Leases, Payments due thereafter	12
Finance Leases, Total lease payments	155
Finance Leases, Imputed interest	(12)
Finance Leases, Present value of lease payments	143
Operating Leases, Payments due 2021	71
Operating Leases, Payments due 2022	59
Operating Leases, Payments due 2023	53
Operating Leases, Payments due 2024	42
Operating Leases, Payments due 2025	34
Operating Leases, Payments due thereafter	88
Operating Leases, Total lease payments	347
Operating Leases, Imputed interest	(36)
Operating Leases, Present value of lease payments	$ 311