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Long-term investments - Additional Information (Detail)
CAD / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Mar. 14, 2017
CAD
Dec. 20, 2016
CAD
shareholder
CAD / shares
MW
Oct. 12, 2016
USD ($)
Sep. 15, 2016
CAD
party
MW
Sep. 15, 2016
USD ($)
party
MW
Apr. 12, 2016
Oct. 19, 2015
USD ($)
member
MW
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Dec. 31, 2017
CAD
MW
Dec. 31, 2016
CAD
May 10, 2017
CAD
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Red Lily I Project, wind energy facility, capacity (megawatt) | MW                   26.4    
Equity-method investees               CAD 40,479,000 CAD 64,419,000 CAD 40,479,000 CAD 64,419,000  
Proceeds from issuance of debt                   1,838,035,000 2,399,009,000  
Number of parties included in development loan | party       2 2              
Notes Receivable                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Notes receivable               41,873,000 38,183,000 41,873,000 38,183,000  
Development loans | Notes Receivable                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Notes receivable               37,710,000 32,125,000 CAD 37,710,000 32,125,000  
Interest in Red LIly I Partnership                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity interest           100.00%            
75% Interest in Red Lily I Partnership                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Income (loss) from equity method investments               2,776,000 1,288,000      
75% Interest in Red Lily I Partnership | Maximum                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Option to subscribe equity interest in partnership, percentage           75.00%            
50% interest in Odell and Deerfield Wind Projects | Development loans                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Letter of credit fee percent                   2.00%    
50% Interest in Deerfield Wind Project                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity-method investees               0 34,727,000 CAD 0 34,727,000  
Equity interest             50.00%          
Project power capacity (megawatt) | MW             150          
Number of members | member             2          
Contributions of capital | $     $ 262,900                  
Interest capitalized on non-regulated property                   6,072,000    
50% Interest in Deerfield Wind Project | Deerfield SponsorCo                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Contributions of capital | $     $ 19,891       $ 1,000          
50% interest in Odell Wind Project                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity interest       50.00% 50.00%              
Project power capacity (megawatt) | MW       200 200              
Interest in Amherst Wind Project                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity-method investees               11,191,000 558,000 11,191,000 558,000  
Interest capitalized on non-regulated property                   1,447,000 491,000  
Deerfield SponsorCo                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity interest 50.00%           50.00%          
Business combination, purchase price CAD 21,585,000                      
Tax equity funding received                       CAD 166,595,000
Interest in natural gas pipeline development                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Equity method investment, realized gain (loss) on disposal                   6,367,000    
Amherst and Deerfield Wind Projects                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Accrued interest revenue               0   0    
Odell SponsorCo                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Business acquisition, percentage of voting interest acquired       50.00% 50.00%              
Payments to acquired businesses | $         $ 26,500              
Business combination, step acquisition, equity interest in acquiree, remeasurement gain       CAD 0                
Variable Interest Entity, Not Primary Beneficiary                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount               289,374,000   289,374,000    
Variable interest entity, number of shareholders with joint control | shareholder   2                    
Net gain on acquisition   CAD 0                    
Variable Interest Entity, Not Primary Beneficiary | Windlectric                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Letters of Credit and Guarantees of Obligations               72,068,000   72,068,000    
Guarantee Obligation Liability, Noncurrent               2,449,000   2,449,000    
Variable Interest Entity, Not Primary Beneficiary | Windlectric | Notes Receivable                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Notes receivable               CAD 37,710,000 CAD 29,723,000 CAD 37,710,000 CAD 29,723,000  
Windlectric                        
Accounts, Notes, Loans and Financing Receivable [Line Items]                        
Project power capacity (megawatt) | MW   75                    
Sale of stock, percentage of commons shares sold   50.00%                    
Sale of stock, price per share (CAD per share) | CAD / shares   CAD 50