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Property, plant and equipment - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of plant in service $ 546,332 $ 493,570
Accumulated depreciation related to commonly owned facilities 42,476 8,578
Expenditures 75,427 79,657
Contribution received 6,057 12,742
Generation    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 104,107 113,822
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 34,916 34,908
Depreciation expense 1,987 1,633
Distribution    
Property, Plant, and Equipment Disclosure [Line Items]    
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE 1,383,960 1,341,716
Renewable generation assets related to facilities under capital lease and owned by consolidated VIE, accumulated depreciation 69,960 28,809
Liberty Utilities Group    
Property, Plant, and Equipment Disclosure [Line Items]    
Cost of distribution assets 3,076 3,076
Accumulated depreciation 669 $ 336
Liberty Utilities Group    
Property, Plant, and Equipment Disclosure [Line Items]    
Expansion costs $ 1,000