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Long-term debt - Schedule of Long-term Debt (Detail)
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Debt Instrument [Line Items]      
Less: current portion CAD (15,511,000)   CAD (10,075,000)
Long-term debt, excluding current portion 3,847,785,000   3,903,340,000
Senior Unsecured Notes      
Debt Instrument [Line Items]      
Long-term debt 3,863,296,000   3,913,415,000
Less: current portion (15,511,000)   (10,075,000)
Long-term debt, excluding current portion CAD 3,847,785,000   3,903,340,000
Senior Unsecured Notes | Canadian Dollar Senior Unsecured Notes      
Debt Instrument [Line Items]      
Weighted average coupon 4.61% 4.61%  
Par value CAD 785,669,000    
Long-term debt CAD 781,833,000   487,389,000
Senior Unsecured Notes | Canadian Dollar Senior Secured Project Notes      
Debt Instrument [Line Items]      
Weighted average coupon 10.27% 10.27%  
Par value CAD 33,568,000    
Long-term debt CAD 33,507,000   35,600,000
Senior Unsecured Notes | U.S. Dollar Senior Unsecured Notes      
Debt Instrument [Line Items]      
Weighted average coupon 4.09% 4.09%  
Par value | $   $ 1,225,000,000  
Long-term debt CAD 1,527,726,000   700,600,000
Senior Unsecured Notes | U.S. Dollar Senior Unsecured Utility Notes      
Debt Instrument [Line Items]      
Weighted average coupon 5.98% 5.98%  
Par value | $   $ 227,000,000  
Long-term debt CAD 309,309,000   174,206,000
Senior Unsecured Notes | U.S Dollar Senior Secured Utility Bonds      
Debt Instrument [Line Items]      
Weighted average coupon 4.95% 4.95%  
Par value | $   $ 752,500,000  
Long-term debt CAD 969,567,000   132,551,000
Revolving Credit Facility | Senior Unsecured Notes      
Debt Instrument [Line Items]      
Weighted average coupon 0.00% 0.00%  
Revolving Credit Facility | Senior Unsecured Notes | Senior Unsecured Revolving Credit Facilities (a)      
Debt Instrument [Line Items]      
Long-term debt CAD 65,017,000   242,947,000
Revolving Credit Facility | Senior Unsecured Notes | Senior Unsecured Bank Credit Facilities (b)      
Debt Instrument [Line Items]      
Long-term debt 169,343,000   2,140,122,000
Revolving Credit Facility | Senior Unsecured Notes | Commercial Paper      
Debt Instrument [Line Items]      
Long-term debt CAD 6,994,000   CAD 0