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Risk Management Activities - Impacts of currency rate risk (Details) - Currency risk - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of credit risk exposure [line items]      
Sensitivity analysis, Net earnings increase (decrease) $ (12) $ (24) $ (20)
Sensitivity analysis, OCI gain 1 2 0
U.S. Dollars      
Disclosure of credit risk exposure [line items]      
Sensitivity analysis, Net earnings increase (decrease) (8) (18) (13)
Sensitivity analysis, OCI gain 1 2 0
Australia, Dollars      
Disclosure of credit risk exposure [line items]      
Sensitivity analysis, Net earnings increase (decrease) (4) (6) (7)
Sensitivity analysis, OCI gain $ 0 $ 0 $ 0