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Significant accounting policies - Estimated And Weighted Average Useful Lives of Depreciable Assets (Detail)
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Generation | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 33 years 33 years
Generation | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives   3 years
Generation | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives   60 years
Distribution | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 39 years 40 years
Distribution | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives   1 year
Distribution | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives   100 years
Equipment | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 11 years 11 years
Equipment | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 5 years 5 years
Equipment | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 54 years 50 years