| Gross finance lease obligations | en | http://www.xbrl.org/2003/role/label | http://www.xbrl.org/2003/role/link |
| Concept not required as it has an expiry date of 01-01-2019 | en | http://www.xbrl.org/2009/role/deprecatedLabel | http://www.xbrl.org/2003/role/link |
| 2019-01-01 | en | http://www.xbrl.org/2009/role/deprecatedDateLabel | http://www.xbrl.org/2003/role/link |
| The amount of contractual undiscounted cash flows in relation to finance lease obligations before deducting finance charges. | en | http://www.xbrl.org/2003/role/documentation | http://www.xbrl.org/2003/role/link |