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Property, plant and equipment - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about property, plant and equipment [line items]    
Cash additions $ 9,854,000 $ 3,068,000
Adjustment for asset retirement obligations 171,000 0
Impairment loss   284,000
Property, plant and equipment, net realizable value   50,000
Cost    
Disclosure of detailed information about property, plant and equipment [line items]    
Disposals 2,712,000 $ 1,589,000
Power Manager Business | Cost    
Disclosure of detailed information about property, plant and equipment [line items]    
Disposals $ 495,000