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Property, plant and equipment - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of plant in service $ 514,709 $ 503,664
Accumulated depreciation related to commonly owned facilities 31,349 21,697
Expenditures 69,210 75,427
Contribution received 7,137 6,057
Generation    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 109,653 104,107
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 39,638 34,916
Depreciation expense 1,615 1,987
Distribution    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 1,450,946 1,383,960
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 97,080 69,960
Regulated Services Group    
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of distribution assets 3,076 3,076
Accumulated depreciation 1,003 $ 669
Regulated Services Group    
Property, Plant, and Equipment Disclosure [Line Items]    
Expansion costs $ 1,000