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Acquisitions and divestments - 2017 Divestments (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Nov. 30, 2017
Jun. 30, 2017
Dec. 31, 2017
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Selkirk Cogen Partners, LP      
Divestments      
Sale of ownership percentage 17.70%    
Proceeds from disposal $ 1.0    
Equity investment after impairment write down   $ 0.0  
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Selkirk Cogen Partners, LP | Earnings from unconsolidated affiliates      
Divestments      
Gain on sale of equity method investment     $ 1.0
Selkirk Cogen Partners, LP | Earnings from unconsolidated affiliates      
Divestments      
Impairment charge on pending sale   $ 10.6 10.6
Gain on sale of equity method investment     $ 1.0
Selkirk Cogen Partners, LP | Disposal Group, Disposed of by Sale, Not Discontinued Operations      
Divestments      
Sale of ownership percentage 17.70%