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Segment information (Details 2)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Supplier A      
Supplier accounting for more than 5% of Group's purchases 45% 63% 39%
Supplier B      
Supplier accounting for more than 5% of Group's purchases 10% 7% 11%
Supplier C      
Supplier accounting for more than 5% of Group's purchases 9% 5% 6%
Supplier D      
Supplier accounting for more than 5% of Group's purchases 4% 5% 5%