Significant accounting policies - Employee Benefits (Details) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Significant accounting policies | ||
| Percentage of bonus paid immediately around year-end, Senior Management Bonus Scheme | 50.00% | 50.00% |
| Percentage of deferred bonus paid for three years, Senior Management Bonus Scheme | 50.00% | 50.00% |
| Term of deferrment for the remaining 50% bonus payment, Senior Management Bonus Scheme | 3 years | 3 years |
| Share price below threshold in which the deferred bonus will be adjusted and the remainder will be forfeited | 10.00% | 10.00% |
| Share price above threshold in which the deferred bonus will be forfeited | 10.00% | 10.00% |