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Significant accounting policies - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
USD ($)
facility
Dec. 31, 2020
USD ($)
Significant Accounting Policies [Line Items]    
Number of electric generating facilities | facility 3  
Number of power generating facilities | facility 2  
Generating assets of Long Sault $ 16,785,836 $ 13,224,149
Long-term debt of Long Sault 9,084,231 7,234,784
Non-regulated energy sales 2,285,479 1,676,991
Interest expense on long-term debt and others 209,554 181,934
Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant    
Significant Accounting Policies [Line Items]    
Generating assets of Long Sault 59,877 59,521
Long-term debt of Long Sault 18,344 20,328
Operating expenses and amortization 5,410 5,400
Interest expense on long-term debt and others $ 2,055 2,119
Minimum    
Significant Accounting Policies [Line Items]    
Ownership interest in commonly owned facilities 7.52%  
Lease renewal term 1 year  
Maximum    
Significant Accounting Policies [Line Items]    
Ownership interest in commonly owned facilities 60.00%  
Lease renewal term 5 years  
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 | Minimum    
Significant Accounting Policies [Line Items]    
Intangible asset, useful life 25 years  
Customer Relationships | Maximum    
Significant Accounting Policies [Line Items]    
Intangible asset, useful life 40 years  
Non-regulated energy sales    
Significant Accounting Policies [Line Items]    
Non-regulated energy sales $ 267,970 255,955
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 $ 16,772 $ 17,116