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Risk Management Activities - Sensitivity Analysis For Management Assessment (Details) - Currency risk - CAD
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Sensitivity analysis, Net earnings increase (decrease) CAD (12,000,000) CAD (12,000,000) CAD (1,000,000)
Sensitivity analysis, OCI gain 0 0 5,000,000
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) (5,000,000) (5,000,000) 2,000,000
Sensitivity analysis, OCI gain 0 0 5,000,000
Australia, Dollars      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Sensitivity analysis, Net earnings increase (decrease) (7,000,000) (7,000,000) (3,000,000)
Sensitivity analysis, OCI gain CAD 0 CAD 0 CAD 0