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Employee Benefit Plans (Accumulated Benefit Obligation (ABO)) (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Plans with Accumulated Benefit Obligation ("ABO") in Excess of Plan Assets    
ABO for the defined benefit pension plans $ 2,639 $ 2,687
Defined benefit pension plans    
Plans with Accumulated Benefit Obligation ("ABO") in Excess of Plan Assets    
ABO 1,519 2,665
Fair value of plan assets 1,419 2,557
Funded Status $ (100) $ (108)