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Preferred Shares - Characteristics Specific to Series of Preferred Shares (Details) - CAD / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of classes of share capital [line items]      
Annual dividend rate per share   CAD 0.16 CAD 0.72
Series A      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 0.67725    
Series A | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 2.03%    
Series B      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 0.7255    
Series B | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 2.03%    
Series C      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 1.00675    
Series C | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.10%    
Series D      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 0.0000    
Series D | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.10%    
Series E      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 1.29850    
Series E | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.65%    
Series F      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 0.0000    
Series F | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.65%    
Series G      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 1.325    
Series G | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.80%    
Series H      
Disclosure of classes of share capital [line items]      
Annual dividend rate per share CAD 0.0000    
Series H | Benchmark      
Disclosure of classes of share capital [line items]      
Rate spread over Benchmark 3.80%