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Capital Disclosure of investment grade credit rating (Details) - CAD / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investment [Line Items]      
Comparable funds from operations to adjusted interest coverage, actual 4.3 3.9  
Adjusted comparable funds from operations to adjusted net debt, actual 20.40% 16.30%  
Adjusted net debt to comparable earnings before interest, taxes, depreciation, and amortization, actual 3.6 3.8  
Percentage of dividends added to Comparable FFO before Interest to Adjusted Interest Coverage 50.00%    
Percentage of preferred dividends deducted from Adjusted Comparable FFO to Adjusted Net Debt 50.00%    
Percentage of preferred shares added to net debt in Adjusted Comparable FFO to Adjusted Net Debt 50.00%    
Annual dividend per share   CAD 0.16 CAD 0.72
Minimum      
Investment [Line Items]      
Comparable funds from operations to adjusted interest coverage, Target 4    
Adjusted comparable funds from operations to adjusted net debt, Target 20    
Adjusted net debt to comparable earnings before interest, taxes, depreciation, and amortization, Target 3.0    
Maximum      
Investment [Line Items]      
Comparable funds from operations to adjusted interest coverage, Target 5    
Adjusted comparable funds from operations to adjusted net debt, Target 25    
Adjusted net debt to comparable earnings before interest, taxes, depreciation, and amortization, Target 3.5