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Property, plant and equipment - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
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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

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