Summary of significant accounting policies - Concentrations of credit risk (Details) - Customer - item	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total Revenue | Customer Group
Concentrations of credit risk
Number of customer group	3	4	2
Concentration risk percentage	37.00%	44.00%	23.00%
Total accounts receivable | Customer Group
Concentrations of credit risk
Number of customer group	4	3
Total accounts receivable | Customer Group One
Concentrations of credit risk
Concentration risk percentage	20.00%	19.00%
Total accounts receivable | Customer Group Two
Concentrations of credit risk
Concentration risk percentage	19.00%	17.00%
Total accounts receivable | Customer Group Three
Concentrations of credit risk
Concentration risk percentage	17.00%	13.00%
Total accounts receivable | Customer Group Four
Concentrations of credit risk
Concentration risk percentage	13.00%