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Property, plant and equipment - Schedule of Plant, Property and Equipment (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Property, Plant and Equipment [Line Items]    
Cost $ 9,588,813 $ 8,427,088
Accumulated depreciation 1,346,975 1,186,108
Net book value 8,241,838 7,240,980
Land    
Property, Plant and Equipment [Line Items]    
Cost 114,847 74,517
Accumulated depreciation 0 0
Net book value 114,847 74,517
Equipment    
Property, Plant and Equipment [Line Items]    
Cost 99,722 94,583
Accumulated depreciation 51,979 47,541
Net book value 47,743 47,042
Generation    
Property, Plant and Equipment [Line Items]    
Cost 2,918,692 2,816,611
Accumulated depreciation 633,210 540,118
Net book value 2,285,482 2,276,493
Generation | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 136,424 140,235
Accumulated depreciation 0 0
Net book value 136,424 140,235
Distribution and Transmission    
Property, Plant and Equipment [Line Items]    
Cost 5,766,885 4,997,613
Accumulated depreciation 661,786 598,449
Net book value 5,105,099 4,399,164
Distribution and Transmission | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 552,243 303,529
Accumulated depreciation 0 0
Net book value $ 552,243 $ 303,529