Summary of significant accounting policies - Concentrations of credit risk (Details) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Customer A | |||
| Concentration risk (as a percent) | 44.00% | ||
| Customer A | Maximum | |||
| Concentration risk (as a percent) | 10.00% | 10.00% | |
| Customer B | |||
| Concentration risk (as a percent) | 15.00% | ||
| Customer B | Maximum | |||
| Concentration risk (as a percent) | 10.00% | ||
| Customer C | |||
| Concentration risk (as a percent) | 13.00% | ||
| Customer C | Maximum | |||
| Concentration risk (as a percent) | 10.00% | 10.00% | |
| Revenue | Customer Concentration Risk | |||
| Concentration risk (as a percent) | 10.00% | ||