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% | |