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Significant accounting policies - Additional Information (Detail)
CAD in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
CAD
MWh
Facility
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Significant Accounting Policies [Line Items]      
Commonly Owned, Number of Electric Generating Facilities | Facility 3    
Restricted cash held for common shareholders | $     $ 1,495,774
Number of power generating facilities | Facility 2    
Non-regulated energy sales CAD 282,558 CAD 243,149  
Interest expense on long-term debt and others CAD 202,631 131,592  
Electricity generated from an eligible energy source (megawatt) | MWh 1    
Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant      
Significant Accounting Policies [Line Items]      
Generating assets of Long Sault CAD 84,550 87,189  
Long-term debt of Long Sault 35,914 40,398  
Long-term debt of Long Sault, with recourse 3,900 6,900  
Non-regulated energy sales 22,743 29,132  
Operating expenses and amortization 5,564 6,175  
Interest expense on long-term debt and others CAD 3,573 CAD 4,064  
Minimum      
Significant Accounting Policies [Line Items]      
Ownership interest in commonly owned facilities 7.52%    
Maximum      
Significant Accounting Policies [Line Items]      
Ownership interest in commonly owned facilities 60.00%    
Power sales contracts | Minimum      
Significant Accounting Policies [Line Items]      
Intangible asset, useful life 6 years    
Power sales contracts | Maximum      
Significant Accounting Policies [Line Items]      
Intangible asset, useful life 25 years    
Interconnection agreements      
Significant Accounting Policies [Line Items]      
Intangible asset, useful life 40 years    
Customer Relationships      
Significant Accounting Policies [Line Items]      
Intangible asset, useful life 40 years