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Employee Future Benefits (Details)
CAD in Millions
 12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2017
CAD
Dec. 31, 2017
CAD
age
Dec. 31, 2017
CAD
year
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Disclosure of defined benefit plans [line items]            
Administration expenses CAD 2.0       CAD 2.0 CAD 2.0
Weighted average duration of defined benefit obligation | year       14.6    
Minimum            
Disclosure of defined benefit plans [line items]            
Employer contribution (as percent)   500.00%        
Maximum            
Disclosure of defined benefit plans [line items]            
Employer contribution (as percent)   1000.00%        
Registered Pension Plan            
Disclosure of defined benefit plans [line items]            
Administration expenses 2.0       2.0 2.0
Registered Pension Plan | Letter of credit            
Disclosure of defined benefit plans [line items]            
Letter of credit, posted 77.0 CAD 77.0 CAD 77.0 CAD 77.0    
Supplemental Pension Plan            
Disclosure of defined benefit plans [line items]            
Administration expenses 0.0       0.0 0.0
Other Post-Employment Benefit Plans            
Disclosure of defined benefit plans [line items]            
Maximum age disabled and retired members eligible for benefits   65 65      
Administration expenses 0.0       0.0 CAD 0.0
Related parties [member] | Registered Pension Plan            
Disclosure of defined benefit plans [line items]            
Administration expenses CAD 0.1       CAD 0.1