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Property, plant and equipment - Schedule of Plant, Property and Equipment (Detail) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Property, Plant and Equipment [Line Items]    
Cost $ 12,630,666 $ 9,588,813
Accumulated depreciation 1,588,220 1,346,975
Net book value 11,042,446 8,241,838
Land    
Property, Plant and Equipment [Line Items]    
Cost 114,821 114,847
Accumulated depreciation 0 0
Net book value 114,821 114,847
Equipment    
Property, Plant and Equipment [Line Items]    
Cost 101,971 99,722
Accumulated depreciation 56,464 51,979
Net book value 45,507 47,743
Generation    
Property, Plant and Equipment [Line Items]    
Cost 4,187,197 2,918,692
Accumulated depreciation 751,219 633,210
Net book value 3,435,978 2,285,482
Generation | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 148,302 136,424
Accumulated depreciation 0 0
Net book value 148,302 136,424
Distribution and transmission    
Property, Plant and Equipment [Line Items]    
Cost 7,468,236 5,766,885
Accumulated depreciation 780,537 661,786
Net book value 6,687,699 5,105,099
Distribution and transmission | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 610,139 552,243
Accumulated depreciation 0 0
Net book value $ 610,139 $ 552,243