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% | |