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Significant accounting policies - Estimated And Weighted Average Useful Lives of Depreciable Assets (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Generation | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 3 years 3 years
Generation | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 60 years 60 years
Generation | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 33 years 32 years
Distribution | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 5 years 5 years
Distribution | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 100 years 100 years
Distribution | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 40 years 41 years
Equipment and other | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 5 years 5 years
Equipment and other | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 50 years 50 years
Equipment and other | Weighted Average    
Public Utility, Property, Plant and Equipment [Line Items]    
Estimated useful lives 13 years 11 years