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Income Taxes (Change in unrecognized tax benefits) (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]    
Beginning, January 1 $ 29 $ 26
Increases due to tax positions related to current year 1 2
Increases due to tax positions related to a prior year 2 1
Decreases due to tax positions related to a prior year (2) 0
Balance, December 31 $ 30 $ 29