v3.8.0.1
Summary of significant accounting policies - Concentrations of credit risk (Details) - Customer - item
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Total Revenue | Customer Group      
Concentrations of credit risk      
Concentration risk percentage 44.00% 23.00% 17.00%
Number of customer group 4 2 1
Total accounts receivable | Customer Group      
Concentrations of credit risk      
Number of customer group 3 3  
Total accounts receivable | Customer Group One      
Concentrations of credit risk      
Concentration risk percentage 19.00% 26.00%  
Total accounts receivable | Customer Group Two      
Concentrations of credit risk      
Concentration risk percentage 17.00% 18.00%  
Total accounts receivable | Customer Group Three      
Concentrations of credit risk      
Concentration risk percentage 13.00% 17.00%