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Summary of significant accounting policies - Schedule of Concentrations of Risk (Details) - Customer Concentration Risk
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue | Customer A      
Product Information [Line Items]      
Concentration risk percentage 21.00% 25.00% 28.00%
Revenue | Customer B      
Product Information [Line Items]      
Concentration risk percentage 30.00% 29.00% 30.00%
Accounts Receivable | Customer A      
Product Information [Line Items]      
Concentration risk percentage 27.00% 29.00%  
Accounts Receivable | Customer B      
Product Information [Line Items]      
Concentration risk percentage 20.00% 17.00%  
Accounts Receivable | Customer C      
Product Information [Line Items]      
Concentration risk percentage   17.00%