Leases Supplemental Balance Sheet Information (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease, ROU asset	$ 316	$ 358
Finance lease, ROU asset	171	177
Finance lease, Liability, Current	8	7
Finance lease, Liability, Non-current	135	144
Other assets
Lessee, Lease, Description [Line Items]
Operating lease, ROU asset	316	358
Accounts payable and accrued liabilities
Lessee, Lease, Description [Line Items]
Operating lease, Liability, Current	63	69
Other long-term liabilities
Lessee, Lease, Description [Line Items]
Operating lease, Liability, Non-current	$ 248	$ 285