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Risk Management Activities - Sensitivity Analysis For Management Assessment (Details) - Currency risk - CAD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Sensitivity analysis, Net earnings increase (decrease) $ (20,000,000) $ (12,000,000) $ (12,000,000)
Sensitivity analysis, OCI gain 0 0 0
Sensitivity analysis, management assessment 0.04 0.04 0.04
U.S. Dollars      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Sensitivity analysis, Net earnings increase (decrease) (13,000,000) (5,000,000) (5,000,000)
Sensitivity analysis, OCI gain 0 0 0
Australia, Dollars      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Sensitivity analysis, Net earnings increase (decrease) (7,000,000) (7,000,000) (7,000,000)
Sensitivity analysis, OCI gain $ 0 $ 0 $ 0