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Property, plant and equipment - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of plant in service $ 531,191 $ 514,709
Accumulated depreciation related to commonly owned facilities 50,919 31,349
Expenditures 61,827 69,210
Contribution received 4,214 7,137
Generation    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 111,806 109,653
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 43,444 39,638
Depreciation expense 1,708 1,615
Distribution and Transmission    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 885,087 1,125,062
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 28,779 81,480
Regulated Services Group    
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of distribution assets 3,076 3,076
Accumulated depreciation 1,321 1,003
Expansion costs $ 1,000 $ 1,000