Summary of significant accounting policies - Property and equipment (Details) |
12 Months Ended |
|---|---|
Dec. 31, 2017 | |
| Software | Minimum | |
| Property and equipment | |
| Estimated useful lives | 2 years |
| Software | Maximum | |
| Property and equipment | |
| Estimated useful lives | 5 years |
| Computer equipment | Minimum | |
| Property and equipment | |
| Estimated useful lives | 3 years |
| Computer equipment | Maximum | |
| Property and equipment | |
| Estimated useful lives | 5 years |
| Furniture, fixtures and office equipment | Minimum | |
| Property and equipment | |
| Estimated useful lives | 3 years |
| Furniture, fixtures and office equipment | Maximum | |
| Property and equipment | |
| Estimated useful lives | 5 years |
| Leasehold improvements | Minimum | |
| Property and equipment | |
| Estimated useful lives | 2 years |
| Leasehold improvements | Maximum | |
| Property and equipment | |
| Estimated useful lives | 18 years |