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Significant accounting policies - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Jan. 01, 2018
USD ($)
Dec. 31, 2019
USD ($)
MWh
Facility
Dec. 31, 2018
USD ($)
Significant Accounting Policies [Line Items]      
Number of electric generating facilities | Facility   3  
Number of power generating facilities | Facility   2  
Non-regulated energy sales   $ 1,624,921 $ 1,648,463
Interest expense on long-term debt and others   $ 181,488 152,118
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   $ 60,230 59,288
Long-term debt of Long Sault   21,754 22,263
Operating expenses and amortization   4,930 4,634
Interest expense on long-term debt and others   $ 2,340 2,557
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  
Accounting Standards Update 2014-09      
Significant Accounting Policies [Line Items]      
Adjustment to retained earnings, for previously deferred revenue, before tax $ 2,488    
Adjustment to retained earnings, for previously deferred revenue, net of tax $ 1,860    
Non-regulated energy sales      
Significant Accounting Policies [Line Items]      
Non-regulated energy sales   $ 246,601 235,359
Non-regulated energy sales | Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant      
Significant Accounting Policies [Line Items]      
Non-regulated energy sales   $ 17,108 $ 17,232