v3.6.0.2
Summary of significant accounting policies - Concentrations of credit risk (Details) - Customer - item
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Total Revenue | Customer Group      
Concentrations of credit risk      
Concentration risk percentage 23.00% 17.00% 15.00%
Number of customer group 2 1 1
Total accounts receivable | Customer Group      
Concentrations of credit risk      
Number of customer group 3 4  
Total accounts receivable | Customer Group One      
Concentrations of credit risk      
Concentration risk percentage   28.00%  
Total accounts receivable | Customer Group Two      
Concentrations of credit risk      
Concentration risk percentage 26.00% 19.00%  
Total accounts receivable | Customer Group Three      
Concentrations of credit risk      
Concentration risk percentage 18.00% 19.00%  
Total accounts receivable | Customer Group Four      
Concentrations of credit risk      
Concentration risk percentage 17.00% 10.00%