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Basis for preparation, consolidation and accounting policies (Details Textual) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of basis for preparation, consolidation and accounting policies [Line Items]      
Useful lives or depreciation rates, investment property, cost model 20    
Workers' profit sharing rate 8.00%    
Qualifying asset, description A qualifying asset is one whose value is greater than US$5 million and requires a longer period to 12 months to get ready for its intended use    
Applicable tax rate 29.50% 32.00% 32.00%
OSINERGMIN [Member]      
Disclosure of basis for preparation, consolidation and accounting policies [Line Items]      
Contribution rate on sales invoice value 0.15% 0.16% 0.19%
OEFA [Member]      
Disclosure of basis for preparation, consolidation and accounting policies [Line Items]      
Contribution rate on sales invoice value 0.11% 0.13% 0.15%
Administrative expense [Member]      
Disclosure of basis for preparation, consolidation and accounting policies [Line Items]      
Amount of reclassifications or changes in presentation     $ 6.3
Other property, plant and equipment [member]      
Disclosure of basis for preparation, consolidation and accounting policies [Line Items]      
Amount of reclassifications or changes in presentation   $ 4.3