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Property, plant and equipment - Schedule of Plant, Property and Equipment (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]    
Cost $ 7,406,319 $ 7,125,369
Accumulated depreciation 1,012,761 820,472
Net book value 6,393,558 6,304,897
Land    
Property, Plant and Equipment [Line Items]    
Cost 73,773 71,689
Accumulated depreciation 0 0
Net book value 73,773 71,689
Equipment and other    
Property, Plant and Equipment [Line Items]    
Cost 88,757 91,233
Accumulated depreciation 41,295 37,104
Net book value 47,462 54,129
Generation    
Property, Plant and Equipment [Line Items]    
Cost 2,470,279 2,382,279
Accumulated depreciation 450,230 394,509
Net book value 2,020,049 1,987,770
Generation | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 104,996 209,979
Accumulated depreciation 0 0
Net book value 104,996 209,979
Liberty Utilities Group    
Property, Plant and Equipment [Line Items]    
Cost 4,455,935 4,205,823
Accumulated depreciation 521,236 388,859
Net book value 3,934,699 3,816,964
Liberty Utilities Group | Construction in progress    
Property, Plant and Equipment [Line Items]    
Cost 212,579 164,366
Accumulated depreciation 0 0
Net book value $ 212,579 $ 164,366